travel outside of the nucleus

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Alberts B, Johnson A, Lewis J, et al. Molecular Biology of the Cell. 4th edition. New York: Garland Science; 2002.

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Molecular Biology of the Cell. 4th edition.

The transport of molecules between the nucleus and the cytosol.

The nuclear envelope encloses the DNA and defines the nuclear compartment . This envelope consists of two concentric membranes that are penetrated by nuclear pore complexes ( Figure 12-9 ). Although the inner and outer nuclear membranes are continuous, they maintain distinct protein compositions. The inner nuclear membrane contains specific proteins that act as binding sites for chromatin and for the protein meshwork of the nuclear lamina that provides structural support for this membrane . The inner membrane is surrounded by the outer nuclear membrane , which is continuous with the membrane of the ER . Like the membrane of the ER that will be described later in this chapter, the outer nuclear membrane is studded with ribosomes engaged in protein synthesis. The proteins made on these ribosomes are transported into the space between the inner and outer nuclear membranes (the perinuclear space), which is continuous with the ER lumen (see Figure 12-9 ).

Figure 12-9

The nuclear envelope. The double-membrane envelope is penetrated by nuclear pore complexes and is continuous with the endoplasmic reticulum. The ribosomes that are normally bound to the cytosolic surface of the ER membrane and outer nuclear membrane are (more...)

Bidirectional traffic occurs continuously between the cytosol and the nucleus . The many proteins that function in the nucleus—including histones, DNA and RNA polymerases, gene regulatory proteins, and RNA-processing proteins—are selectively imported into the nuclear compartment from the cytosol, where they are made. At the same time, tRNAs and mRNAs are synthesized in the nuclear compartment and then exported to the cytosol. Like the import process, the export process is selective; mRNAs, for example, are exported only after they have been properly modified by RNA-processing reactions in the nucleus. In some cases the transport process is complex : ribosomal proteins, for instance, are made in the cytosol, imported into the nucleus—where they assemble with newly made ribosomal RNA into particles—and are then exported again to the cytosol as part of a ribosomal subunit . Each of these steps requires selective transport across the nuclear envelope .

• Nuclear Pore Complexes Perforate the Nuclear Envelope

The nuclear envelope of all eucaryotes is perforated by large, elaborate structures known as nuclear pore complexes . In animal cells, each complex has an estimated molecular mass of about 125 million and is thought to be composed of more than 50 different proteins, called nucleoporins , that are arranged with a striking octagonal symmetry ( Figure 12-10 ).

Figure 12-10

The arrangement of nuclear pore complexes in the nuclear envelope. (A) A small region of the nuclear envelope. In cross section, a nuclear pore complex seems to have four structural building blocks: column subunits, which form the bulk of the pore wall; (more...)

In general, the more active the nucleus is in transcription, the greater the number of pore complexes its envelope contains. The nuclear envelope of a typical mammalian cell contains 3000–4000 pore complexes. If the cell is synthesizing DNA , it needs to import about 10 6 histone molecules from the cytosol every 3 minutes to package the newly made DNA into chromatin , which means that, on average, each pore complex needs to transport about 100 histone molecules per minute. If the cell is growing rapidly, each complex also needs to transport about 6 newly assembled large and small ribosomal subunits per minute from the nucleus, where they are produced, to the cytosol, where they are used. And that is only a very small part of the total traffic that passes through the pore complexes.

Each pore complex contains one or more open aqueous channels through which small water-soluble molecules can passively diffuse. The effective size of these channels has been determined by injecting labeled water-soluble molecules of different sizes into the cytosol and then measuring their rate of diffusion into the nucleus . Small molecules (5000 daltons or less) diffuse in so fast that the nuclear envelope can be considered to be freely permeable to them. A protein of 17,000 daltons takes 2 minutes to equilibrate between the cytosol and the nucleus, whereas proteins larger than 60,000 daltons are hardly able to enter the nucleus at all. A quantitative analysis of such data suggests that the nuclear pore complex contains a pathway for free diffusion equivalent to a water-filled cylindrical channel about 9 nm in diameter and 15 nm long; such a channel would occupy only a small fraction of the total volume of the pore complex ( Figure 12-11 ).

Figure 12-11

Possible paths for free diffusion through the nuclear pore complex. This drawing shows a hypothetical diaphragm (gray) inserted into the pore to restrict the size of the open channel to 9 nm, the pore size estimated from diffusion measurements. Nine nanometers (more...)

Because many cell proteins are too large to pass by diffusion through the nuclear pore complexes, the nuclear envelope enables the nuclear compartment and the cytosol to maintain different complements of proteins. Mature cytosolic ribosomes, for example, are about 30 nm in diameter and thus cannot diffuse through the 9 nm channels; their exclusion from the nucleus ensures that protein synthesis is confined to the cytosol. But how does the nucleus export newly made ribosomal subunits or import large molecules, such as DNA and RNA polymerases, which have subunit molecular weights of 100,000–200,000 daltons? As we discuss next, these and many other protein and RNA molecules bind to specific receptor proteins that ferry them actively through nuclear pore complexes.

• Nuclear Localization Signals Direct Nuclear Proteins to the Nucleus

When proteins are experimentally extracted from the nucleus and reintroduced into the cytosol (e.g., through experimentally induced perforations in the plasma membrane ), even the very large ones reaccumulate efficiently in the nucleus. The selectivity of this nuclear import process resides in nuclear localization signals (NLSs) , which are present only in nuclear proteins. The signals have been precisely defined in numerous nuclear proteins by using recombinant DNA technology ( Figure 12-12 ). As mentioned earlier, they can be either signal sequences or signal patches. In many nuclear proteins they consist of one or two short sequences that are rich in the positively charged amino acids lysine and arginine (see Table 12-3 , p. 667), the precise sequence varying for different nuclear proteins. Other nuclear proteins contain different signals, some of which are not yet characterized.

Figure 12-12

The function of a nuclear localization signal. Immunofluorescence micrographs showing the cellular location of SV40 virus T-antigen containing or lacking a short peptide that serves as a nuclear localization signal. (A) The normal T-antigen protein contains (more...)

The signals characterized this far can be located almost anywhere in the amino acid sequence and are thought to form loops or patches on the protein surface. Many function even when linked as short peptides to lysine side chains on the surface of a cytosolic protein, suggesting that the precise location of the signal within the amino acid sequence of a nuclear protein is not important.

The transport of nuclear proteins through nuclear pore complexes can be directly visualized by coating gold particles with a nuclear localization signal , injecting the particles into the cytosol , and then following their fate by electron microscopy ( Figure 12-13 ). Studies with various sizes of gold beads indicate that the opening can dilate up to about 26 nm in diameter during the transport process. A structure in the center of the nuclear pore complex seems to function like a close-fitting diaphragm that opens just the right amount to let transport substrates pass (see Figure 12-11 ). The molecular basis of the gating mechanism remains a mystery.

Figure 12-13

Visualizing active import through nuclear pores. This series of electron micrographs shows colloidal gold spheres (arrowheads) coated with peptides containing nuclear localization signals entering the nucleus by means of nuclear pore complexes. Gold particles (more...)

The mechanism of macromolecular transport across nuclear pore complexes is fundamentally different from the transport mechanisms involved in protein transfer across the membranes of other organelles, because it occurs through a large aqueous pore rather than through a protein transporter spanning one or more lipid bilayers. For this reason, nuclear proteins can be transported through a pore complex while they are in a fully folded conformation . Likewise, a newly formed ribosomal subunit is transported out of the nucleus as an assembled particle. By contrast, proteins have to be extensively unfolded during their transport into most other organelles, as we discuss later. In the electron microscope , however, very large particles traversing the pore seem to become constricted as they squeeze through the nuclear pore complex , indicating that at least some of them must undergo restructuring during transport. This has been most extensively studied for the export of some very large mRNAs, as discussed in Chapter 6 (see Figure 6-39 ).

• Nuclear Import Receptors Bind Nuclear Localization Signals and Nucleoporins

To initiate nuclear import, most nuclear localization signals must be recognized by nuclear import receptors , which are encoded by a family of related genes. Each family member encodes a receptor protein that is specialized for the transport of a group of nuclear proteins sharing structurally similar nuclear localization signals ( Figure 12-14A ).

Figure 12-14

Nuclear import receptors. (A) Many nuclear import receptors bind both to nucleoporins and to a nuclear localization signal on the cargo proteins they transport. Cargo proteins 1, 2, and 3 in this example contain different nuclear localization signals, (more...)

The import receptors are soluble cytosolic proteins that bind both to the nuclear localization signal on the protein to be transported and to nucleoporins, some of which form the tentaclelike fibrils that extend into the cytosol from the rim of the nuclear pore complexes. The fibrils and many other nucleoporins contain a large number of short amino- acid repeats that contain phenylalanine and glycine and are therefore called FG-repeats (named after the one-letter code for amino acids, discussed in Chapter 5). FG-repeats serve as binding sites for the import receptors. They are thought to line the path through the nuclear pore complexes taken by the import receptors and their bound cargo proteins. These protein complexes move along the path by repeatedly binding, dissociating, and then re-binding to adjacent repeat sequences. Once in the nucleus , the import receptors dissociate from their cargo and are returned to the cytosol.

Nuclear import receptors do not always bind to nuclear proteins directly. Additional adaptor proteins are sometimes used that bridge between the import receptors and the nuclear localization signals on the proteins to be transported. Surprisingly, the adaptor proteins are structurally related to nuclear import receptors, suggesting a common evolutionary origin. The combined use of import receptors and adaptors allows a cell to recognize the broad repertoire of nuclear localization signals that are displayed on nuclear proteins.

• Nuclear Export Works Like Nuclear Import, But in Reverse

The nuclear export of large molecules, such as new ribosomal subunits and RNA molecules, also occurs through nuclear pore complexes and depends on a selective transport system. The transport system relies on nuclear export signals on the macromolecules to be exported, as well as on complementary nuclear export receptors . These receptors bind both the export signal and nucleoporins to guide their cargo through the pore complex to the cytosol .

Nuclear export receptors are structurally related to nuclear import receptors, and they are encoded by the same gene family of nuclear transport receptors , or karyopherins. In yeast , there are 14 genes encoding members of this family; in animal cells the number is significantly larger. From their amino acid sequence alone, it is often not possible to distinguish whether a particular family member works as a nuclear import or nuclear export receptor . It comes as no surprise, therefore, that the import and export transport systems work in similar ways but in opposite directions: the import receptors bind their cargo molecules in the cytosol , release them in the nucleus , and are then exported to the cytosol for reuse, while the export receptors function in reverse.

If gold spheres similar to those used in the experiments shown in Figure 12-13 are coated with small RNA molecules ( tRNA or ribosomal 5S RNA) and injected into the nucleus of a cultured cell, they are rapidly transported through the nuclear pore complexes into the cytosol . Using two sizes of gold particles, one coated with RNA and injected into the nucleus and the other coated with nuclear localization signals and injected into the cytosol, it can be shown that a single pore complex conducts traffic in both directions. How a pore complex coordinates the bidirectional flow of macromolecules to avoid congestion and head-on collisions is not known.

• The Ran GTPase Drives Directional Transport Through Nuclear Pore Complexes

The import of nuclear proteins through the pore complex concentrates specific proteins in the nucleus , thereby increasing order in the cell, which must consume energy (discussed in Chapter 2). The energy is thought to be provided by the hydrolysis of GTP by the monomeric GTPase Ran . Ran is found in both the cytosol and the nucleus, and it is required for both the nuclear import and export systems.

Like other GTPases, Ran is a molecular switch that can exist in two conformational states, depending on whether GDP or GTP is bound (discussed in Chapter 3). Conversion between the two states is triggered by two Ran-specific regulatory proteins: a cytosolic GTPase-activating protein (GAP) that triggers GTP hydrolysis and thus converts Ran-GTP to Ran-GDP, and a nuclear guanine exchange factor ( GEF ) that promotes the exchange of GDP for GTP and thus converts Ran-GDP to Ran-GTP. Because Ran-GAP is located in the cytosol and Ran-GEF is located in the nucleus , the cytosol primarily contains Ran-GDP, and the nucleus primarily contains Ran-GTP ( Figure 12-15 ).

Figure 12-15

The compartmentalization of Ran-GDP and Ran-GTP. Localization of Ran-GDP to the cytosol and Ran-GTP to the nucleus results from the localization of two Ran regulatory proteins: Ran GTPase-activating protein (Ran-GAP) is located in the cytosol and Ran (more...)

This gradient of the two conformational forms of Ran drives nuclear transport in the appropriate direction ( Figure 12-16 ). Docking of nuclear import receptors to FG-repeats on the cytosolic side of the nuclear pore complex , for example, occurs only when these receptors are loaded with an appropriate cargo. The import receptors with their bound cargo then move along tracks lined by FG-repeat sequences until they reach the nuclear side of the pore complex, where Ran-GTP binding causes the import receptors to release their cargo ( Figure 12-17 ). By favoring cargo-dependent loading of import receptors onto the FG-repeat track in the cytosol and Ran-GTP-dependent cargo release in the nucleus , the nuclear localization of Ran-GTP imposes directionality.

Figure 12-16

A model for how GTP hydrolysis by Ran provides directionality for nuclear transport. Movement through the pore complex of loaded nuclear transport receptors may occur by guided diffusion along the FG-repeats displayed by nucleoporins. The differential (more...)

Figure 12-17

A model for how the binding of Ran-GTP might cause nuclear import receptors to release their cargo. (A) Nuclear transport receptors are composed of repeated α-helical motifs that stack into either large arches or snail-shaped coils, depending (more...)

Having discharged its cargo in the nucleus , the empty import receptor with Ran -GTP bound is transported back through the pore complex to the cytosol . There, two cytosolic proteins, Ran Binding Protein and Ran- GAP collaborate to convert Ran-GTP to Ran-GDP. The Ran Binding Protein first displaces Ran-GTP from the import receptor, which allows Ran-GAP to trigger Ran to hydrolyze its bound GTP. The Ran-GDP then dissociates from the Ran Binding Protein and is reimported into the nucleus, thereby completing the cycle.

Nuclear export occurs by a similar mechanism, except that Ran -GTP in the nucleus promotes cargo binding to the export receptor and the binding of the loaded receptor to the nuclear side of the pore complex . Once in the cytosol , Ran encounters Ran- GAP and Ran Binding Protein and hydrolyses its bound GTP. The export receptor then releases both its cargo and Ran-GDP in the cytosol and dissociates from the pore complex, and free export receptors are returned to the nucleus to complete the cycle (see Figure 12-16 ).

• Transport Between the Nucleus and Cytosol Can Be Regulated by Controlling Access to the Transport Machinery

Some proteins, such as those that bind newly made mRNAs in the nucleus , contain both nuclear localization and nuclear export signals. These proteins continually shuttle between the nucleus and the cytosol . The steady-state localization of such shuttling proteins is determined by the relative rates of their import and export. If the rate of import exceeds the rate of export, a protein will be located primarily in the nucleus. Conversely, if the rate of export exceeds the rate of import, a protein will be located primarily in the cytosol. Thus, changing the rate of import, export, or both, can change the location of a protein.

Some shuttling proteins move continuously in and out of the nucleus . In other cases, however, the transport is stringently controlled. As discussed in Chapter 7, the activity of some gene regulatory proteins is controlled by keeping them out of the nuclear compartment until they are needed there ( Figure 12-18 ). In many cases, this control depends on the regulation of nuclear localization and export signals; these can be turned on or off, often by phosphorylation of adjacent amino acids ( Figure 12-19 ).

Figure 12-18

The control of fly embryo development by nuclear transport. The gene regulatory protein dorsal is expressed uniformly throughout this early Drosophila embryo, which is shown in cross section. It is active only in cells at the ventral side (bottom) of (more...)

Figure 12-19

The control of nuclear import during T-cell activation. The nuclear factor of activated T cells (NF-AT) is a gene regulatory protein that, in the resting T cell, is found in the cytosol in a phosphorylated state. When T cells are activated, the intracellular (more...)

Other gene regulatory proteins are bound to inhibitory cytosolic proteins that either anchor them in the cytosol (through interactions with the cytoskeleton or with specific organelles), or mask their nuclear localization signals so that they are unable to interact with nuclear import receptors. When the cell receives an appropriate stimulus, the gene regulatory protein is released from its cytosolic anchor or mask and is transported into the nucleus . One important example is the latent gene regulatory protein that controls the expression of proteins involved in cholesterol metabolism . The protein is made and stored in an inactive form as a transmembrane protein in the ER . When deprived of cholesterol, the cell activates specific proteases that cleave the protein, releasing its cytosolic domain . This domain is then imported into the nucleus, where it activates the transcription of genes required for cholesterol import and synthesis.

Cells control the export of RNA from the nucleus in a similar way. Messenger RNAs become bound to proteins that are loaded onto the RNA as transcription and splicing proceed. These proteins contain nuclear export signals that are recognized by export receptors that guide the RNA out of the nucleus through nuclear pore complexes. Upon entry into the cytosol , the proteins coating the RNA are stripped off and rapidly returned to the nucleus. Other RNAs, such as snRNAs and tRNAs, are exported by different sets of nuclear export receptors.

Incompletely processed pre-mRNAs are actively retained in the nucleus , anchored to the nuclear transcription and splicing machinery, which releases an RNA molecule only after its processing is completed. Genetic studies in yeast show that a mutant pre- mRNA that cannot properly engage with the splicing machinery is improperly exported as an unspliced molecule.

• The Nuclear Envelope Is Disassembled During Mitosis

The nuclear lamina is a meshwork of interconnected protein subunits called nuclear lamins . The lamins are a special class of intermediate filament proteins (discussed in Chapter 16) that polymerize into a two-dimensional lattice ( Figure 12-20 ). The nuclear lamina gives shape and stability to the nuclear envelope , to which it is anchored by attachment to both the nuclear pore complexes and integral membrane proteins of the inner nuclear membrane . The lamina also interacts directly with chromatin , which itself interacts with the integral membrane proteins of the inner nuclear membrane. Together with the lamina, these membrane proteins provide structural links between the DNA and the nuclear envelope.

Figure 12-20

The nuclear lamina. An electron micrograph of a portion of the nuclear lamina in a Xenopus oocyte prepared by freeze-drying and metal shadowing. The lamina is formed by a regular lattice of specialized intermediate filaments. (Courtesy of Ueli Aebi.) (more...)

When a nucleus disassembles during mitosis , the nuclear lamina depolymerizes. The disassembly is at least partly a consequence of direct phosphorylation of the nuclear lamins by the cyclin -dependent kinase activated at the onset of mitosis (discussed in Chapter 17). At the same time, proteins of the inner nuclear membrane are phosphorylated, and the nuclear pore complexes disassemble and disperse in the cytosol . Nuclear envelope membrane proteins—no longer tethered to the pore complexes, lamina, or chromatin —diffuse throughout the ER membrane. Together, these events break down the barriers that normally separate the nucleus and cytosol, and these nuclear proteins that are not bound to membranes or chromosomes intermix completely with the cytosol of the dividing cell ( Figure 12-21 ).

Figure 12-21

The breakdown and re-formation of the nuclear envelope during mitosis. The phosphorylation of the lamins is thought to trigger the disassembly of the nuclear lamina, which in turn causes the nuclear envelope to break up. Dephosphorylation of the lamins (more...)

Later in mitosis (in late anaphase ), the nuclear envelope reassembles on the surface of the chromosomes, as inner nuclear membrane proteins and dephosphorylated lamins rebind to chromatin . ER membranes wrap around groups of chromosomes and continue fusing until a sealed nuclear envelope is reformed. During this process, the nuclear pore complexes also reassemble and start actively reimporting proteins that contain nuclear localization signals. Because the nuclear envelope is initially closely applied to the surface of the chromosomes, the newly formed nucleus excludes all proteins except those initially bound to the mitotic chromosomes and those that are selectively imported through nuclear pore complexes. In this way, all other large proteins are kept out of the newly assembled nucleus.

Nuclear localization signals are not cleaved off after transport into the nucleus . This is presumably because nuclear proteins need to be imported repeatedly, once after every cell division . In contrast, once a protein molecule has been imported into any of the other membrane -enclosed organelles, it is passed on from generation to generation within that compartment and need never be translocated again; the signal sequence on these molecules is often removed after protein translocation .

The nuclear envelope consists of an inner and an outer nuclear membrane . The outer membrane is continuous with the ER membrane, and the space between it and the inner membrane is continuous with the ER lumen . RNA molecules, which are made in the nucleus , and ribosomal subunits, which are assembled there, are exported to the cytosol , while all the proteins that function in the nucleus are synthesized in the cytosol and are then imported. The extensive traffic of materials between the nucleus and cytosol occurs through nuclear pore complexes, which provide a direct passageway across the nuclear envelope.

Proteins containing nuclear localization signals are actively transported inward through the nuclear pore complexes, while RNA molecules and newly made ribosomal subunits contain nuclear export signals that direct their active transport outward through the pore complexes. Some proteins, including nuclear import and export receptors, continually shuttle between the cytosol and nucleus . The GTPase Ran , provides directionality for nuclear transport . The transport of nuclear proteins and RNA molecules through the pore complexes can be regulated by denying these molecules access to the transport machinery. Because nuclear localization signals are not removed, nuclear proteins can be imported repeatedly, as is required each time that the nucleus reassembles after mitosis .

• Cite this Page Alberts B, Johnson A, Lewis J, et al. Molecular Biology of the Cell. 4th edition. New York: Garland Science; 2002. The Transport of Molecules between the Nucleus and the Cytosol.
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Cell Nucleus: Definition, Structure, and Functions

What is a nucleus.

The nucleus is a double membrane-bound organelle located centrally only in a eukaryotic cell , enclosing the   DNA, the genetic material. It is the most important and defining feature of all higher organisms, including plant and animal cells, whose main function is to control and coordinate the functioning of the entire cell. 

The word ‘nucleus’ (plural: nuclei) is derived from the Latin word ‘ nucleus ‘, meaning ‘kernel’ or ‘ seed ’.

Structure and Characteristics

The largest and most prominent organelle in the cell, the nucleus, accounts for almost 10% of the volume of the entire cell. In mammalian cells, the average diameter of the nucleus is approximately 6 µm in size. Mostly the shape of the nucleus is found to be either spherical or oblong. Eukaryotes usually contain a single nucleus, however erythrocytes and platelets are without a nucleus and osteoclasts of bones have many of them. The color of the nucleus is usually grayish but can differ depending on the type of the cell.

Parts and Their Functions

Anatomically, the nucleus of all plant and animal cell is made up of several components that are listed below. All of these components work together in order for the nucleus to accomplish its purpose as the ‘controlling center’ of the cell.

1) Nuclear Envelope and Nuclear Pores

Surrounding the nucleus, the nuclear envelope is made of a phospholipid bilayer, similar to cell membranes, and contains tiny openings called nuclear pores over them. The two membranes are often referred to as the inner and outer nuclear membranes with a fluid-filled region called perinuclear space in between. The perinuclear space has a thickness of 20 to 40 nm. The outer membrane is attached to ribosomes and is continuous with the cell’s endoplasmic reticulum , a system that helps to package, transport, and export substances outside the cell.

• Nuclear envelope separates the nuclear content from the cytoplasm an is selectively permeable in nature
• Nuclear pores regulate the flow of molecules into and out of the nucleus

2) Nuclear Lamina

They are meshwork of protein filaments organized in a net-like fashion that line below the inner nuclear membrane. The proteins that make up the nuclear lamina are known as lamins, which are intermediate filament proteins.

• Supports the nuclear envelope, maintaining the overall shape and structure of the nucleus

3) Chromatin

It is a complex of genetic material (DNA or RNA) and proteins found in a resting or non-dividing cell nucleus. The chromatin is classified into two types, heterochromatin and euchromatin, based on functions. The heterochromatin is a functionally inactive form of chromatin, found near the nuclear envelope. On the contrary, euchromatin is a mild, less condensed form that is in functionally active state. An organized chromatin material that is highly condensed and paired is known as the  chromosome . 

• Contains hereditary information and instructions necessary for controlling processes such as metabolism, cell growth, and cell division
• Helps in gene expression where DNA molecules make an RNA copy, a process called transcription which is later converted to proteins by a process called translation

4) Nucleoplasm

Also known as karyoplasm, it is found inside the nucleus, and is a gelatinous substance similar to the cytoplasm, being composed mainly of water with dissolved salts, enzymes, and suspended organic molecules.

• Protects the nuclear content by providing a cushion around the nucleolus and the chromosome
• Supports the nucleus to hold its shape
• Provides a medium through which enzymes and fragments of genetic materials (DNA or RNA), can be transported throughout the nucleus

5) Nucleolus

It looks like a dark spot within the nucleus and is a dense, membrane-less structure composed of RNA and proteins along with granules and fibers that remain attached to chromatin. The nucleolus contains multipleregions called nucleolar organizers that are the segments of chromosomes that contain the genes for ribosomal RNA. The nucleolus disappears when a cell undergoes division and is reformed after the completion of cell division.

• Synthesize ribosomes that are involved in protein synthesis

Ans. The nucleus was the first organelle to be discovered by Antonie van Leeuwenhoek during his study involving microorganisms, which was further described in detail by Robert Brown in 1831.

Ans . Prokaryotic cells, including bacteria and archaea , do not have a true nucleus; instead, they have a membrane-less nucleoid region that holds their free-floating DNA.

Ans . Both archaebacteria and eubacteria being prokaryotic organisms lack all membrane-bound organelles, including the nucleus.

Ans . Like all eukaryotic cells, protists have a characteristic central compartment called the nucleus, which houses their genetic material.

Ans . Being eukaryotes, fungi, and amoeba has a membrane-bound nucleus within their cell.

Ans . White blood cells also known as leucocytes have a distinct nucleus that differentiates them from other blood cells.

Ans . The DNA, which is the genetic material of the cell, is a polymer of nucleotides, found within the nucleus.

Ans . Eukaryotic DNA never leaves the nucleus but is copied into RNA molecules, which may then travel out of the nucleus.

Ans . A nucleus is a membrane-bound organelle that houses DNA, the genetic material of eukaryotes whereas nucleoid is an irregularly shaped region that houses the genetic material of prokaryotes.

Ans . Nucleus is a membrane-bound organelle that houses DNA, the genetic material of a eukaryotic cell whereas nucleolus is a sub-organelle found within the nucleus containing RNA and is responsible for ribosome synthesis.

• Nucleus – Britannica.com
• Nucleus- Definition, Structure, Functions and Diagram – Microbenotes.com
• The Cell Nucleus – Thoughtco.com
• Cell Nucleus – Kenhub.com
• Nucleus – Biologyonline.com
• The Structure and Functions of a Cell Nucleus Explained – Biologywise.com

Article was last reviewed on Thursday, February 4, 2021

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Genes and Chromosomes

• Chromosomes |
• Genetic Disorders |

Genes are segments of deoxyribonucleic acid (DNA) that contain the code for a specific protein that functions in one or more types of cells in the body or the code for functional ribonucleic acid (RNA) molecules. Chromosomes are structures within cells that contain a person's genes.

Genes are contained in chromosomes, which are in the cell nucleus.

A chromosome contains hundreds to thousands of genes.

Every normal human cell contains 23 pairs of chromosomes, for a total of 46 chromosomes.

A trait is any gene-determined characteristic and is often determined by more than one gene.

Some traits are caused by mutated genes that are inherited or that are the result of a new gene mutation.

Proteins are probably the most important class of material in the body. Proteins are not just building blocks for muscles, connective tissues, skin, and other structures. They also are needed to make enzymes. Enzymes are complex proteins that control and carry out nearly all chemical processes and reactions within the body. The body produces thousands of different enzymes. Thus, the entire structure and function of the body is governed by the types and amounts of proteins the body synthesizes. Protein synthesis is controlled by genes, which are contained on chromosomes.

The genotype (or genome) is a person’s unique combination of genes or genetic makeup. Thus, the genotype is a complete set of instructions on how that person’s body synthesizes proteins and thus how that body is supposed to be built and function.

The phenotype is the actual structure and function of a person’s body. The phenotype is how the genotype manifests in a person—not all the instructions in the genotype may be carried out (or expressed). Whether and how a gene is expressed is determined by a complex interaction of multiple factors including genotype, gene expression, environmental factors (including illnesses and diet), and other factors, some of which are unknown.

A karyotype is a picture of the full set of chromosomes in a person’s cells.

Humans have about 20,000 to 23,000 genes.

Genes consist of deoxyribonucleic acid (DNA). DNA contains the code, or blueprint, used to synthesize a protein or a ribonucleic acid (RNA) molecule. Genes vary in size, depending on the sizes of the proteins or RNA for which they code.

Each DNA molecule is a long double helix that resembles a spiral staircase containing millions of steps. The steps of the staircase consist of pairs of four types of molecules called bases (nucleotides). In each step, the base adenine (A) is paired with the base thymine (T), or the base guanine (G) is paired with the base cytosine (C). Each extremely long DNA molecule is coiled up inside one of the chromosomes .

Structure of DNA

Synthesizing proteins.

Proteins are composed of a long chain of amino acids linked together one after another. There are 20 different amino acids that can be used in protein synthesis—some must come from the diet (essential amino acids), and some are made by enzymes in the body. As a chain of amino acids is put together, it folds upon itself to create a complex three-dimensional structure. It is the shape of the folded structure that determines its function in the body. Because the folding is determined by the precise sequence of amino acids, each different sequence results in a different protein. Some proteins (such as hemoglobin) contain several different folded chains. Instructions for synthesizing proteins are coded within the DNA.

Information is coded within DNA by the sequence in which the bases (A, T, G, and C) are arranged. The code is written in triplets. That is, the bases are arranged in groups of three. Particular sequences of three bases in DNA code for specific instructions, such as the addition of one amino acid to a chain. For example, GCT (guanine, cytosine, thymine) codes for the addition of the amino acid alanine, and GTT (guanine, thymine, thymine) codes for the addition of the amino acid valine. Thus, the sequence of amino acids in a protein is determined by the order of triplet base pairs in the gene for that protein on the DNA molecule. The process of turning coded genetic information into a protein involves transcription and translation.

Transcription and translation

Transcription is the process in which information coded in DNA is transferred (transcribed) to ribonucleic acid (RNA). RNA is a long chain of bases just like a strand of DNA, except that the base uracil (U) replaces the base thymine (T). Thus, RNA contains triplet-coded information just like DNA.

When transcription is initiated, part of the DNA double helix opens and unwinds. One of the unwound strands of DNA acts as a template against which a complementary strand of RNA forms. The complementary strand of RNA is called messenger RNA (mRNA). The mRNA separates from the DNA, leaves the nucleus, and travels into the cell cytoplasm (the part of the cell outside the nucleus). There, the mRNA attaches to a ribosome, which is a tiny structure in the cell where protein synthesis occurs.

With translation, the mRNA code (from the DNA) tells the ribosome the order and type of amino acids to link together. The amino acids are brought to the ribosome by a much smaller type of RNA called transfer RNA (tRNA). Each molecule of tRNA brings one amino acid to be incorporated into the growing chain of protein, which is folded into a complex three-dimensional structure under the influence of nearby molecules called chaperone molecules.

Control of gene expression

There are many types of cells in a person’s body, such as heart cells, liver cells, and muscle cells. These cells look and act differently and produce very different chemical substances. However, every cell is the descendant of a single fertilized egg cell and as such contains essentially the same DNA. Cells acquire their very different appearances and functions because different genes are expressed in different cells (and at different times in the same cell). The information about when a gene should be expressed is also coded in the DNA. Gene expression depends on the type of tissue, the age of the person, the presence of specific chemical signals, and numerous other factors and mechanisms. Knowledge of these other factors and mechanisms that control gene expression is growing rapidly, but many of these factors and mechanisms are still poorly understood.

The mechanisms by which genes control each other are very complicated. Genes have chemical markers to indicate where transcription should begin and end. Various chemical substances (such as histones) in and around the DNA block or permit transcription. Also, a strand of RNA called antisense RNA can pair with a complementary strand of mRNA and block translation.

Replication

Cells reproduce by dividing in two. Because each new cell requires a complete set of DNA molecules, the DNA molecules in the original cell must reproduce (replicate) themselves during cell division. Replication happens in a manner similar to transcription, except that the entire double-strand DNA molecule unwinds and splits in two. After splitting, bases on each strand bind to complementary bases (A with T, and G with C) floating nearby. When this process is complete, two identical double-strand DNA molecules exist.

To prevent mistakes during replication, cells have a “proofreading” function to help ensure that bases are paired properly. There are also chemical mechanisms to repair DNA that was not copied properly. However, because of the billions of base pairs involved in, and the complexity of, the protein synthesis process, mistakes may happen. Such mistakes may occur for numerous reasons (including exposure to radiation, drugs, or viruses) or for no apparent reason. Minor variations in DNA are very common and occur in most people. Most variations do not affect subsequent copies of the gene. Mistakes that are duplicated in subsequent copies are called mutations.

Inherited mutations are those that may be passed on to offspring. Mutations can be inherited only when they affect the reproductive cells (sperm or egg). Mutations that do not affect reproductive cells affect the descendants of the mutated cell (for example, becoming a cancer) but are not passed on to offspring.

Mutations may be unique to an individual or family, and most harmful mutations are rare. Mutations that become so common that they affect more than 1% of a population are called polymorphisms (for example, the human blood types A, B, AB, and O). Most polymorphisms have little or no effect on the phenotype (the actual structure and function of a person’s body).

Mutations may involve small or large segments of DNA. Depending on its size and location, the mutation may have no apparent effect or it may alter the amino acid sequence in a protein or decrease the amount of protein produced. If the protein has a different amino acid sequence, it may function differently or not at all. An absent or nonfunctioning protein is often harmful or fatal. For example, in phenylketonuria , a mutation results in the deficiency or absence of the enzyme phenylalanine hydroxylase. This deficiency allows the amino acid phenylalanine (absorbed from the diet) to accumulate in the body, ultimately causing severe intellectual disability.

In rare cases, a mutation introduces a change that is advantageous. For example, in the case of the sickle cell gene, when a person inherits two copies of the abnormal gene, the person will develop sickle cell disease . However, when a person inherits only one copy of the sickle cell gene (called a carrier), the person develops some protection against malaria (a blood infection). Although the protection against malaria can help a carrier survive, sickle cell disease (in a person who has two copies of the gene) causes symptoms and complications that may shorten life span.

Natural selection refers to the concept that mutations that impair survival in a given environment are less likely to be passed on to offspring (and thus become less common in the population), whereas mutations that improve survival progressively become more common. Thus, beneficial mutations, although initially rare, eventually become common. The slow changes that occur over time caused by mutations and natural selection in an interbreeding population collectively are called evolution.

Did You Know...

Chromosomes.

Image courtesy of the Centers for Disease Control and Prevention Public Health Image Library and Suzanne Trusler, MPH, DrPH.

A chromosome is made of a very long strand of DNA and contains many genes (hundreds to thousands). The genes on each chromosome are arranged in a particular sequence, and each gene has a particular location on the chromosome (called its locus). The form of the gene that occupies the same locus on each chromosome of a pair (one inherited from the mother and one from the father) is called an allele. In addition to DNA, chromosomes contain other chemical components that influence gene function.

Except for certain cells (for example, sperm and egg cells or red blood cells), the nucleus of every normal human cell contains 23 pairs of chromosomes, for a total of 46 chromosomes. Normally, each pair consists of one chromosome from the mother and one from the father.

There are 22 pairs of nonsex (autosomal) chromosomes and one pair of sex chromosomes. Paired nonsex chromosomes are, for practical purposes, identical in size, shape, and position and number of genes. Because each member of a pair of nonsex chromosomes contains one of each corresponding gene, there is in a sense a backup for the genes on those chromosomes.

The 23rd pair is the sex chromosomes (X and Y).

Sex chromosomes

The pair of sex chromosomes determines whether a fetus becomes male or female. Males have one X and one Y chromosome. A male’s X comes from his mother and the Y comes from his father. Females have two X chromosomes, one from the mother and one from the father. In certain ways, sex chromosomes function differently than nonsex chromosomes.

The smaller Y chromosome carries the genes that determine male sex as well as a few other genes. The X chromosome contains many more genes than the Y chromosome, many of which have functions besides determining sex and have no counterpart on the Y chromosome. In males, because there is no second X chromosome, these extra genes on the X chromosome are not paired and virtually all of them are expressed. Genes on the X chromosome are referred to as sex-linked, or X-linked, genes.

Normally, in the nonsex chromosomes, the genes on both of the pairs of chromosomes are capable of being fully expressed. However, in females, most of the genes on one of the two X chromosomes are turned off through a process called X inactivation (except in the eggs in the ovaries). X inactivation occurs early in the life of the fetus. In some cells, the X from the father becomes inactive, and in other cells, the X from the mother becomes inactive. Thus, one cell may express a gene from the person’s mother and another cell expresses the gene from the person’s father. Because of X inactivation, the absence of one X chromosome usually results in relatively minor abnormalities (such as Turner syndrome ). Thus, missing an X chromosome is far less harmful than missing a nonsex chromosome (see Overview of Sex Chromosome Abnormalities ).

Courtesy of Drs. L. Carrell and H. Williard, Case Western Reserve University School of Medicine.

If a female has a disorder in which she has more than two X chromosomes, the extra chromosomes tend to be inactive. Thus, having one or more extra X chromosomes causes far fewer developmental abnormalities than having one or more extra nonsex chromosomes. For example, women with three X chromosomes ( triple X syndrome ) are often unaffected physically and mentally. Males who have more than one Y chromosome (see XYY Syndrome ) may be affected physically and mentally.

Chromosome abnormalities

There are several types of chromosome abnormalities . A person may have an abnormal number of chromosomes or have abnormal areas on one or more chromosomes. Many such abnormalities can be diagnosed before birth (see Testing for chromosome and gene abnormalities ).

Abnormal numbers of nonsex chromosomes usually result in severe abnormalities. For example, receiving an extra nonsex chromosome may be fatal to a fetus or lead to abnormalities such as Down syndrome , which commonly results from a person having three copies of chromosome 21. Absence of a nonsex chromosome is fatal to the fetus.

Large areas on a chromosome may be abnormal, usually because a whole section was left out (called a deletion) or mistakenly placed in another chromosome (called translocation). For example, chronic myeloid leukemia is sometimes caused by translocation of part of chromosome 9 onto chromosome 22. This abnormality can be inherited or be the result of a new mutation .

Mitochondrial chromosomes

Mitochondria are tiny structures inside cells that synthesize molecules used for energy. Each cell has 1000 to 2500 mitochondria. Unlike other structures inside cells, each mitochondrion contains its own circular chromosome. This chromosome contains DNA (mitochondrial DNA) containing 37 genes that code for 13 proteins, various RNAs, and several enzymes. Mitochondrial DNA usually comes only from the person’s mother because, in general, when an egg is fertilized, only mitochondria from the egg become part of the developing embryo. Mitochondria from the sperm usually do not become part of the developing embryo.

A trait is any gene-determined characteristic. Many traits are determined by the function of more than one gene. For example, a person's height is likely to be determined by many genes, including those affecting growth, appetite, muscle mass, and activity level. However, some traits are determined by the function of a single gene.

Variation in some traits, such as eye color or blood type, is considered normal. Other variations, such as albinism , Marfan syndrome , and Huntington disease , harm body structure or function and are considered disorders. However, not all such gene abnormalities are uniformly harmful. For example, one copy of the sickle cell gene can provide protection against malaria, but two copies of the gene cause sickle cell disease.

Genetic Disorders

A genetic disorder is a detrimental trait caused by an abnormal gene. The abnormal gene may be inherited or may arise spontaneously as a result of a new mutation. Gene abnormalities are fairly common. Every human carries an average of 100 to 400 abnormal genes (different ones in different people). However, most of the time the corresponding gene on the other chromosome in the pair is normal and prevents any harmful effects.

In the general population, the chance of a person having two copies of the same abnormal gene (and hence a disorder) is very small. However, in children who are offspring of close blood relatives, the chances are higher. Chances are also higher among children of parents who have married within an isolated population, such as within the Amish or Mennonite communities.

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3.3 The Nucleus and DNA Replication

Learning objectives.

By the end of this section, you will be able to:

• Describe the structure and features of the nuclear membrane
• List the contents of the nucleus
• Explain the organization of the DNA molecule within the nucleus
• Describe the process of DNA replication

The nucleus is the largest and most prominent of a cell’s organelles ( Figure 3.3.1 ). The nucleus is generally considered the control center of the cell because it stores all of the genetic instructions for manufacturing proteins. Interestingly, some cells in the body, such as muscle cells, contain more than one nucleus ( Figure 3.3.2 ), which is known as multinucleated. Other cells, such as mammalian red blood cells (RBCs), do not contain nuclei at all. RBCs eject their nuclei as they mature, making space for the large numbers of hemoglobin molecules that carry oxygen throughout the body ( Figure 3.3.3 ). Without nuclei, the life span of RBCs is short, and so the body must produce new ones constantly.

External Website

View the University of Michigan WebScope at http://141.214.65.171/Histology/Basic%20Tissues/Muscle/058thin_HISTO_83X.svs/view.apml to explore the tissue sample in greater detail.

View the University of Michigan WebScope at http://virtualslides.med.umich.edu/Histology/EMsmallCharts/3%20Image%20Scope%20finals/139%20-%20Erythroblast_001.svs/view.apml to explore the tissue sample in greater detail.

Inside the nucleus lies the blueprint that dictates everything a cell will do and all of the products it will make. This information is stored within DNA. The nucleus sends “commands” to the cell via molecular messengers that translate the information from the DNA. Each cell in your body (with the exception of germ cells) contains the complete set of your DNA. When a cell divides, the DNA must be duplicated so that each new cell receives a full complement of DNA. The following section will explore the structure of the nucleus and its contents, as well as the process of DNA replication.

Organization of the Nucleus and its DNA

Like most other cellular organelles, the nucleus is surrounded by a membrane called the nuclear envelope . This membranous covering consists of two adjacent lipid bilayers with a thin fluid space in between them. Spanning these two bilayers are nuclear pores. A nuclear pore is a tiny passageway for the passage of proteins, RNA, and solutes between the nucleus and the cytoplasm. Proteins called pore complexes lining the nuclear pores regulate the passage of materials into and out of the nucleus.

Inside the nuclear envelope is a gel-like nucleoplasm with solutes that include the building blocks of nucleic acids. There also can be a dark-staining mass often visible under a simple light microscope, called a nucleolus (plural = nucleoli). The nucleolus is a region of the nucleus that is responsible for manufacturing the RNA necessary for construction of ribosomes. Once synthesized, newly made ribosomal subunits exit the cell’s nucleus through the nuclear pores.

The genetic instructions that are used to build and maintain an organism are arranged in an orderly manner in strands of DNA. Within the nucleus are threads of chromatin composed of DNA and associated proteins ( Figure 3.3.4 ). Along the chromatin threads, the DNA is wrapped around a set of histone proteins. A nucleosome is a single, wrapped DNA-histone complex. Multiple nucleosomes along the entire molecule of DNA appear like a beaded necklace, in which the string is the DNA and the beads are the associated histones. When a cell is in the process of division, the chromatin condenses into chromosomes, so that the DNA can be safely transported to the “daughter cells.” The chromosome is composed of DNA and proteins; it is the condensed form of chromatin. It is estimated that humans have almost 22,000 genes distributed on 46 chromosomes.

DNA Replication

In order for an organism to grow, develop, and maintain its health, cells must reproduce themselves by dividing to produce two new daughter cells, each with the full complement of DNA as found in the original cell. Billions of new cells are produced in an adult human every day. Only very few cell types in the body do not divide, including nerve cells, skeletal muscle fibers, and cardiac muscle cells. The division time of different cell types varies. Epithelial cells of the skin and gastrointestinal lining, for instance, divide very frequently to replace those that are constantly being rubbed off of the surface by friction.

A DNA molecule is made of two strands that “complement” each other in the sense that the molecules that compose the strands fit together and bind to each other, creating a double-stranded molecule that looks much like a long, twisted ladder. Each side rail of the DNA ladder is composed of alternating sugar and phosphate groups ( Figure 3.3.5 ). The two sides of the ladder are not identical, but are complementary. These two backbones are bonded to each other across pairs of protruding bases, each bonded pair forming one “rung,” or cross member. The four DNA bases are adenine (A), thymine (T), cytosine (C), and guanine (G). Because of their shape and charge, the two bases that compose a pair always bond together. Adenine always binds with thymine, and cytosine always binds with guanine. The particular sequence of bases along the DNA molecule determines the genetic code. Therefore, if the two complementary strands of DNA were pulled apart, you could infer the order of the bases in one strand from the bases in the other, complementary strand. For example, if one strand has a region with the sequence AGTGCCT, then the sequence of the complementary strand would be TCACGGA.

DNA replication is the copying of DNA that occurs before cell division can take place. After a great deal of debate and experimentation, the general method of DNA replication was deduced in 1958 by two scientists in California, Matthew Meselson and Franklin Stahl. This method is illustrated in Figure 3.3.6 and described below.

Stage 1: Initiation. The two complementary strands are separated, much like unzipping a zipper. Special enzymes, including helicase , untwist and separate the two strands of DNA.

Stage 2: Elongation. Each strand becomes a template along which a new complementary strand is built. DNA polymerase brings in the correct bases to complement the template strand, synthesizing a new strand base by base. A DNA polymerase is an enzyme that adds free nucleotides to the end of a chain of DNA, making a new double strand. This growing strand continues to be built until it has fully complemented the template strand.

Stage 3: Termination. Once the two original strands are bound to their own, finished, complementary strands, DNA replication is stopped and the two new identical DNA molecules are complete.

Each new DNA molecule contains one strand from the original molecule and one newly synthesized strand. The term for this mode of replication is “semiconservative,” because half of the original DNA molecule is conserved in each new DNA molecule. This process continues until the cell’s entire genome , the entire complement of an organism’s DNA, is replicated. As you might imagine, it is very important that DNA replication take place precisely so that new cells in the body contain the exact same genetic material as their parent cells. Mistakes made during DNA replication, such as the accidental addition of an inappropriate nucleotide, have the potential to render a gene dysfunctional or useless. Fortunately, there are mechanisms in place to minimize such mistakes. A DNA proofreading process enlists the help of special enzymes that scan the newly synthesized molecule for mistakes and corrects them. Once the process of DNA replication is complete, the cell is ready to divide. You will explore the process of cell division later in the chapter.

Watch this video to learn about DNA replication. DNA replication proceeds simultaneously at several sites on the same molecule. What separates the base pair at the start of DNA replication?

Chapter Review

The nucleus is the command center of the cell, containing the genetic instructions for all of the materials a cell will make (and thus all of its functions it can perform). The nucleus is encased within a membrane of two interconnected lipid bilayers, side-by-side. This nuclear envelope is studded with protein-lined pores that allow materials to be trafficked into and out of the nucleus. The nucleus contains one or more nucleoli, which serve as sites for ribosome synthesis. The nucleus houses the genetic material of the cell: DNA. DNA is normally found as a loosely contained structure called chromatin within the nucleus, where it is wound up and associated with a variety of histone proteins. When a cell is about to divide, the chromatin coils tightly and condenses to form chromosomes.

There is a pool of cells constantly dividing within your body. The result is billions of new cells being created each day. Before any cell is ready to divide, it must replicate its DNA so that each new daughter cell will receive an exact copy of the organism’s genome. A variety of enzymes are enlisted during DNA replication. These enzymes unwind the DNA molecule, separate the two strands, and assist with the building of complementary strands along each parent strand. The original DNA strands serve as templates from which the nucleotide sequence of the new strands are determined and synthesized. When replication is completed, two identical DNA molecules exist. Each one contains one original strand and one newly synthesized complementary strand.

Interactive Link Questions

Review questions, critical thinking questions.

Explain in your own words why DNA replication is said to be “semiconservative”?

DNA replication is said to be semiconservative because, after replication is complete, one of the two parent DNA strands makes up half of each new DNA molecule. The other half is a newly synthesized strand. Therefore, half (“semi”) of each daughter DNA molecule is from the parent molecule and half is a new molecule.

Why is it important that DNA replication take place before cell division? What would happen if cell division of a body cell took place without DNA replication, or when DNA replication was incomplete?

During cell division, one cell divides to produce two new cells. In order for all of the cells in your body to maintain a full genome, each cell must replicate its DNA before it divides so that a full genome can be allotted to each of its offspring cells. If DNA replication did not take place fully, or at all, the offspring cells would be missing some or all of the genome. This could be disastrous if a cell was missing genes necessary for its function and health.

This work, Anatomy & Physiology, is adapted from Anatomy & Physiology by OpenStax , licensed under CC BY . This edition, with revised content and artwork, is licensed under CC BY-SA except where otherwise noted.

Images, from Anatomy & Physiology by OpenStax , are licensed under CC BY except where otherwise noted.

Access the original for free at https://openstax.org/books/anatomy-and-physiology/pages/1-introduction .

Anatomy & Physiology Copyright © 2019 by Lindsay M. Biga, Staci Bronson, Sierra Dawson, Amy Harwell, Robin Hopkins, Joel Kaufmann, Mike LeMaster, Philip Matern, Katie Morrison-Graham, Kristen Oja, Devon Quick, Jon Runyeon, OSU OERU, and OpenStax is licensed under a Creative Commons Attribution-ShareAlike 4.0 International License , except where otherwise noted.

How molecules are speedily transported into and out of the cell’s nucleus

'fuzzy' interaction makes it possible for the nuclear pore complex to rapidly and selectively move large molecules.

A cell does everything it can to protect its nucleus, where precious genetic information is stored. That includes controlling the movement of molecules in and out using gateways called nuclear pore complexes (NPCs).

Now, researchers at The Rockefeller University, Albert Einstein College of Medicine, and the New York Structural Biology Center have identified the molecular mechanism that makes both swift and cargo-specific passage through the NPC possible for large molecules. Their work appeared September 15 in eLife .

Scientists are paying close attention to this regulation since dysfunction in nuclear transport has been linked to many diseases, including cancers and developmental disorders.

While small molecules can easily pass in and out of the nucleus, the transport of large molecules such as proteins and RNA is more complex and less well understood. These are moved through the NPC rapidly, but also selectively to avoid allowing the wrong big molecules through.

It was already known that proteins called transport factors bind to large cargo and escort it through the NPC. A team led by Michael P. Rout, a professor at Rockefeller University and head of the Laboratory of Cellular and Structural Biology, and David Cowburn, a professor of biochemistry and of physiology & biophysics at Albert Einstein College of Medicine, sought to explain the speed with which transport factors ferry large molecules across the NPC, a process that lasts only a few milliseconds.

"It's understood how these transport factors selectively choose and bind to their cargo," Rout says. "However, it's been unclear how such a specific process can also shepherd molecules through the nuclear pore complex so quickly."

At the center of the NPC, the transport factors and their cargo must pass through a selectivity filter made of proteins called FG Nups. These proteins form a dense mesh that normally prevents large molecules from getting through. Using a technique known as nuclear magnetic resonance spectroscopy, the researchers collected atomic-scale information about the behavior of the FG Nups, focusing on Nsp1, the most studied representative of the FG Nups.

Normally, proteins fold into large structures. Relative to small molecules such as water, these large protein structures move very slowly. This means their interactions are correspondingly slow.

The researchers measured the physical state of FG repeats with and without transport factors bound to them. They found that rather than folding like proteins generally do, the FG Nups are loose and string-like, remaining highly dynamic and lacking a predictable structure.

"Usually, binding between traditionally folded proteins is a time consuming, cumbersome process, but because the FG Nups are unfolded, they are moving very quickly, very much like small molecules. This means their interaction is very quick," explains Rout.

The disordered structure of the FG regions is critical to the speed of transport, allowing for quick loading and unloading of cargo-carrying transport factors. At the same time, because transport factors have multiple binding sites for FG Nups, they are the only proteins that can specifically interact with them -- making transport both fast and specific.

"We observed that there is minimal creation of a static well-ordered structure in complexes of FG Nups and transport factors," says Cowburn. "Our observations are, we propose, the first case where the 'fuzzy' property of an interaction is a key part of its actual biological function."

The team hopes this discovery will lead to detailed characterizations of nuclear transport pathways and to more close studies of the NPC's function. Ultimately, a better understanding of how the NPC works will not only provide new insight into the basic biology of cells, but also have implications for health and disease.

• Cell Biology
• Molecular Biology
• Evolutionary Biology
• Biochemistry Research
• Biotechnology and Bioengineering
• Developmental Biology
• Macromolecule
• Molecular biology
• Somatic cell nuclear transfer
• Groundwater
• Genetic recombination
• Francis Crick

Story Source:

Materials provided by Rockefeller University . Note: Content may be edited for style and length.

Journal Reference :

• Loren E Hough, Kaushik Dutta, Samuel Sparks, Deniz B Temel, Alia Kamal, Jaclyn Tetenbaum-Novatt, Michael P Rout, David Cowburn. The molecular mechanism of nuclear transport revealed by atomic scale measurements . eLife , 2015; 4 DOI: 10.7554/eLife.10027

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Why do electrons not fall into the nucleus?

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• Page ID 1665

• Stephen Lower
• Simon Fraser University

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The picture of electrons "orbiting" the nucleus like planets around the sun remains an enduring one, not only in popular images of the atom but also in the minds of many of us who know better. The proposal, first made in 1913, that the centrifugal force of the revolving electron just exactly balances the attractive force of the nucleus (in analogy with the centrifugal force of the moon in its orbit exactly counteracting the pull of the Earth's gravity) is a nice picture, but is simply untenable.

One origin for this hypothesis that suggests this perspective is plausible is the similarity of the gravity and Coulombic interactions. The expression for the force of gravity between two masses (Newton's Law of gravity) is

\[F_{gravity} \propto \dfrac{m_1m_2}{r^2}\label{1}\]

• \(m_1\) and \(m_2\) representing the mass of object 1 and 2, respectively and
• \(r\) representing the distance between the objects centers

The expression for the Coulomb force between two charged species is

\[F_{Coulomb} \propto \dfrac{q_1q_2}{r^2}\label{2}\]

• \(q_1\) and \(q_2\) representing the charge of object 1 and 2, respectively and

However, an electron, unlike a planet or a satellite, is electrically charged, and it has been known since the mid-19th century that an electric charge that undergoes acceleration (changes velocity and direction) will emit electromagnetic radiation, losing energy in the process. A revolving electron would transform the atom into a miniature radio station, the energy output of which would be at the cost of the potential energy of the electron; according to classical mechanics, the electron would simply spiral into the nucleus and the atom would collapse.

Quantum theory to the Rescue!

By the 1920's, it became clear that a tiny object such as the electron cannot be treated as a classical particle having a definite position and velocity. The best we can do is specify the probability of its manifesting itself at any point in space. If you had a magic camera that could take a sequence of pictures of the electron in the 1s orbital of a hydrogen atom, and could combine the resulting dots in a single image, you would see something like this. Clearly, the electron is more likely to be found the closer we move toward the nucleus.

This is confirmed by this plot which shows the quantity of electron charge per unit volume of space at various distances from the nucleus. This is known as a probability density plot. The per unit volume of space part is very important here; as we consider radii closer to the nucleus, these volumes become very small, so the number of electrons per unit volume increases very rapidly. In this view, it appears as if the electron does fall into the nucleus!

According to classical mechanics, the electron would simply spiral into the nucleus and the atom would collapse. Quantum mechanics is a different story.

The Battle of the Infinities Saves the electron from its death spiral

As you know, the potential energy of an electron becomes more negative as it moves toward the attractive field of the nucleus; in fact, it approaches negative infinity. However, because the total energy remains constant (a hydrogen atom, sitting peacefully by itself, will neither lose nor acquire energy), the loss in potential energy is compensated for by an increase in the electron's kinetic energy (sometimes referred to in this context as "confinement" energy) which determines its momentum and its effective velocity.

So as the electron approaches the tiny volume of space occupied by the nucleus, its potential energy dives down toward minus-infinity, and its kinetic energy (momentum and velocity) shoots up toward positive-infinity. This "battle of the infinities" cannot be won by either side, so a compromise is reached in which theory tells us that the fall in potential energy is just twice the kinetic energy, and the electron dances at an average distance that corresponds to the Bohr radius.

There is still one thing wrong with this picture; according to the Heisenberg uncertainty principle (a better term would be "indeterminacy"), a particle as tiny as the electron cannot be regarded as having either a definite location or momentum. The Heisenberg principle says that either the location or the momentum of a quantum particle such as the electron can be known as precisely as desired, but as one of these quantities is specified more precisely, the value of the other becomes increasingly indeterminate. It is important to understand that this is not simply a matter of observational difficulty, but rather a fundamental property of nature.

What this means is that within the tiny confines of the atom, the electron cannot really be regarded as a "particle" having a definite energy and location, so it is somewhat misleading to talk about the electron "falling into" the nucleus.

Arthur Eddington, a famous physicist, once suggested, not entirely in jest, that a better description of the electron would be "wavicle"!

Probability Density vs. Radial probability

We can, however, talk about where the electron has the highest probability of manifesting itself— that is, where the maximum negative charge will be found.

This is just the curve labeled "probability density"; its steep climb as we approach the nucleus shows unambiguously that the electron is most likely to be found in the tiny volume element at the nucleus. But wait! Did we not just say that this does not happen? What we are forgetting here is that as we move out from the nucleus, the number of these small volume elements situated along any radius increases very rapidly with \(r\), going up by a factor of \(4πr^2\). So the probability of finding the electron somewhere on a given radius circle is found by multiplying the probability density by \(4πr^2\). This yields the curve you have probably seen elsewhere, known as the radial probability , that is shown on the right side of the above diagram. The peak of the radial probability for principal quantum number \(n = 1\) corresponds to the Bohr radius.

To sum up, the probability density and radial probability plots express two different things: the first shows the electron density at any single point in the atom, while the second, which is generally more useful to us, tells us the the relative electron density summed over all points on a circle of given radius.

• Why Doesn't the Electron Fall Into the Nucleus? Franklin Mason and Robert Richardson, J Chem. Ed. 1983 (40-42). See also the comment on this article by Werner Luck, J Chem Ed 1985 (914).
• For more detailed descriptions of these two kinds of plots, see this McMaster U. page by Richard Bader.
• The author is grateful to Robert Harrison of U. of Tennessee-Knoxville whose suggestions led to improving this article.

Contributors and Attributions

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Wynne Parry

RNA's Secret Life Outside the Cell

For decades, researchers have been finding DNA and its sister, RNA, circulating in the body, outside the safe interior of cells where these molecules do their essential work of storing and translating the code of life. The reasons for these molecular voyages have remained mysterious, but in recent years evidence has accrued that this extracellular RNA may have a different job, at least in some organisms.

Original story * reprinted with permission from Quanta Magazine , an editorially independent division of SimonsFoundation.org whose mission is to enhance public understanding of science by covering research developments and trends in mathematics and the physical and life sciences.*RNA, best known to basic biology students for its role in translating genes into proteins, has turned out to be a surprisingly versatile and cosmopolitan molecule. Plants, roundworms, flatworms and insects use RNA to carry signals through their tissues, and perhaps further. Inspired by laboratory studies hinting that RNA may play a role in interactions between organisms, and even different species, Eric Miska , a molecular geneticist at the University of Cambridge, coined the term “social RNA” to describe the molecule’s apparent role in communication both inside and outside organisms.

Plants and the pests seeking to infect them can deploy RNA against one another. In a paper published in Science in October, researchers describe how a fungus — one responsible for both destroying crops with gray mold and producing the noble rot that flavors dessert wines — protects itself by using its own small RNA molecules to hijack the plants’ RNA defense machinery, silencing genes that would normally fight fungal infections. Discoveries like this point to a role for RNA in the arms race between plants and parasites, one of the potential instances of social RNA, Miska said. “I think it’s quite exciting, but it is early days,” Miska said. “A lot of things need to be discovered yet.”

Hailing Jin, Arne Weiberg, and Ming Wang at the University of California, Riverside, revealed that a fungus silences plant immunity genes by hijacking the plants’ RNA defense system.

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While RNA’s role in signaling in plants and invertebrates is not fully understood, that role is clearly established. This is not the case for RNA in mammals, including humans. In these species, scientists know these molecules are traveling outside cells, but it is not yet clear whether or not they are a form of communication.

RNA has been found in a panoply of human body fluids: blood, urine, tears, cerebrospinal fluid, breast milk, amniotic fluid, seminal fluid and others. Moreover, scientists have discovered that small bits of circulating RNA can reflect particular conditions, such as the presence of a cancerous tumor or pregnancy-related disorders. “It’s like opening up a Pandora’s box,” said Xandra Breakefield , a neurogeneticist at Massachusetts General Hospital, of the discovery of circulating RNA. “We didn’t realize all these things were out there.”

While some remain skeptical that extracellular RNA and DNA are anything more than debris, Breakefield and others see a much more exciting prospect: that these might be a newly discovered form of communication among cells that plays a role in human health. For example, some studies suggest that small RNAs act as instructions that help coordinate an immune response or prep cancer cells to invade healthy tissue.

A Silencing Signal

Beginning in the late 1950s, RNA (ribonucleic acid) was cast as a servant to its higher-profile sister DNA (deoxyribonucleic acid), a role that turned out to involve transcribing the genetic code and assembling it into the proteins that build cells and enable them to function. In recent decades, however, RNA’s job description has expanded: It can kick-start chemical reactions, regulate the activity of genes within a cell and now, some suggest, serve as a signal that allows one cell to influence the behavior of others.

A vesicle buds off a mouse cell. Vesicles vary in size and content and can include RNA and other molecules.

About 15 years ago, researchers figured out they could make the roundworm Caenorhabditis elegans twitch by injecting it with complementary strands of RNA that matched the sequence of a gene responsible for a protein in muscle fiber. The arrival of this double-stranded RNA sets off a process that effectively turns off the target gene and, in this case, damages the worm’s muscles.

Scientists have since discovered this type of RNA silencing in many organisms. They believe it helps defend against infection by shutting down the activity of invading viruses, which can temporarily exist as double-stranded RNA. When this double-stranded RNA pops up inside a worm cell, the worm’s molecular machinery uses it as a guide to shut off the viral genes that produced it. This process is called RNA interference, and it also generates an RNA silencing signal that spreads through the worm via a molecular channel. Similar signals have been shown to spread through the bodies of insects, flatworms and plants.

Viral Invasion

Plants and invertebrates respond to a potential viral invasion by shutting down the viral genes using a process called RNA interference (RNAi). Mammals, including humans, have the molecular machinery to produce an RNAi response, but they do not appear to use it to defend themselves, relying instead on other defense mechanisms. However, two studies published Oct. 11 in the journal Science suggest mammals can fight viruses with RNAi. In one case, researchers took away a virus’s defense against RNAi, which it was known to use when infecting fruit flies. Normally, the virus kills young mice. But the mice could clear the infection with crippled virus, presumably thanks to RNAi. In the other study, researchers altered mouse embryonic stem cells so they could not produce an enzyme necessary for RNAi. As a result, the cells no longer produced RNA molecules implicated in an RNAi response. However, scientists say this is likely a minor antiviral mechanism in mammals. In plants and invertebrates, the gene silencing signal produced by RNAi can spread from cell to cell. There is no evidence this occurs in mammals.

Evidence for social RNA in plants and invertebrates inevitably raises the question: What about us? Like plants and invertebrates, mammals are capable of silencing genes through RNA interference, but this system does not appear to play a major role in our immune system. So far, there is no evidence that mammalian cells can broadcast an RNA silencing signal as worm cells do. But some suspect a separate type of RNA, called microRNA, plays a similar social role in mammals.

The microRNA pathway is related to the RNA interference pathway, but microRNAs differ from the molecules involved in RNA interference in a couple of significant ways: MicroRNAs are encoded in the genome and regulate other genes in the same organism. Unlike RNA interference, which silences the genes of an infecting virus, microRNAs turn down expression of genes within the cell in which they are produced.

While the role microRNAs play inside cells is well understood, it is not clear why they are floating around outside them. Some mammalian cells spit out intercellular packages, called vesicles, that are taken up by other cells. In 2007, researchers discovered that mammalian cells can insert RNA , including microRNAs, into these packages. The findings suggest a novel way for one cell to influence the activity of another.

“We know that some cells put a lot of specific RNAs into these vesicles,” Breakefield said. “They are definitely just gobbled up [by other cells], so there is the potential to transfer information in this way.”

It has since turned out that a menagerie of RNAs, other molecules and even pieces of DNA can be found tucked into vesicles, and that vesicles are not microRNA’s only ride. The molecule can circulate through the body bound to proteins, which protect it from the hostile environment outside the cell, and by other means as well.

Evidence and Uncertainty

To understand what circulating microRNAs are up to, scientists must confirm that these molecules are indeed transferred from one cell to another. Because cells produce many microRNAs, it can be difficult to determine where a given microRNA originated. To solve this problem, D. Michiel Pegtel , a cell biologist at VU University Medical Center in Amsterdam, and colleagues turned to a virus, Epstein-Barr. The virus forces infected cells to produce viral microRNAs that help the virus replicate. Since no normal cell would produce viral microRNAs, these are relatively easy to track.

To demonstrate the transfer of gene-regulating RNA from cell to cell, researchers exposed dendritic cells, a type of immune cell shown here, to RNA-filled vesicles.

Pegtel and colleagues started with two types of immune cells; B cells, a type of white blood cell, infected with the virus, and dendritic cells, which sense viral invaders and alert other immune cells. The two were separated by a membrane with pores small enough to allow only vesicles to pass through.

The dendritic cells were genetically engineered to glow until microRNAs that the virus had forced the B cells to produce traveled across the barrier and quieted the glowing genes. The results, published in the Proceedings of the National Academy of Sciences in 2010, show that the transfer of the vesicles across the membrane does indeed dim the glowing cells.

However, not everyone is convinced. The results from this and other RNA transfer experiments likely have other explanations, said Thomas Tuschl , a nucleic acid chemist and biochemist at Rockefeller University. The fusion of the vesicle with the cell resembles a viral infection. So Tuschl suspects that something about the fusion process, or perhaps something inside the vesicle, which can carry many different types of molecules, could trigger an immune response within the cell. This in turn could spark changes in the cells that resemble the supposed effect of the arriving RNA, Tuschl said.

Pegtel said that is unlikely. An extra test showed that the viral RNAs would target one of the virus’s own genes if they were placed in the dendritic cell. What’s more, the degree of dimming in the glowing dendritic cells corresponded to the amount of viral RNA-bearing vesicles that bombarded them, he said. Vesicles lacking viral microRNA did not show the dimming effect.

Nonetheless, Tuschl is skeptical of microRNA’s role in intercellular signaling in mammals for other reasons as well. These small RNAs are present at low concentrations, and mammals, unlike plants and invertebrates, have no significant mechanism to amplify an RNA signal. “In general, there is too little of everything to make this an effective signaling mechanism,” Tuschl said.

The vesicles, which were dyed green, were taken up by the dendritic cells, turning them green. The nuclei of the dendritic cells appear in red.

Others are skeptical too. Mark Kay , a geneticist at Stanford School of Medicine, doesn’t dismiss the possibility that extracellular microRNA serves this purpose, but he is not ready to embrace it. “I try to keep an open mind, but I don’t think it is convincing at this point that the signaling is occurring in mammalian systems,” Kay said.

Even Pegtel is cautious, saying that scientists have a way to go before they can state definitively that circulating RNA causes specific changes upon arriving in cells. Most of the mammalian studies to date have been done in cells growing in test tubes rather than in living mammals. As Pegtel pointed out, these experiments rely on unnatural conditions, such as highly concentrated doses of vesicles and microRNA. He said, “That effect is very artificial.”

The next step, he said, will be to try to show that vesicle-borne RNA has a meaningful effect within the immense complexity of living mammals. “Time will tell.”

A new round of experiments could help to answer the questions and to clarify the role of circulating RNA in human health and disease. The National Institutes of Health announced in August $17 million in funds for 24 research projects focused on understanding extracellular RNA, including microRNA, and using these molecules to diagnose and treat disease.

Breakefield, who received one of the grants, is examining how RNA released by glioblastoma, a highly aggressive form of brain cancer, manipulates surrounding cells to support its own growth. Tuschl, also a grantee, is exploring RNA’s potential use as a marker for autoimmune disease. Through a separate grant, he also hopes to study a potential alternative explanation for the changes in cells that follow the arrival of the RNA-bearing vesicles.

From the NIH’s perspective, evidence already suggests this RNA can act as a signal. But even if traveling RNAs are only debris, they might still have uses as markers for disease and as a means to conscript the vesicles that carry them to deliver drugs to hard-to-reach places, said Danilo Tagle, associate director for special initiatives at the National Center for Advancing Translational Sciences, which is involved in the NIH’s extracellular RNA program.

The implications for cell biology and medicine are overarching, Tagle said. “In a sense we are opening up a new area of research,” he said.

Original story * reprinted with permission from Quanta Magazine , an editorially independent division of SimonsFoundation.org whose mission is to enhance public understanding of science by covering research developments and trends in mathematics and the physical and life sciences.*

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10.7: RNA and Ribosome Export from the Nucleus

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• Gerald Bergtrom
• University of Wisconsin-Milwaukee

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A. rRNA and Ribosomes

The synthesis and processing of rRNAs are coincident with the assembly of the ribosomal subunits, as shown below.

The 45S pre-rRNAs initially bind to ribosomal proteins in the nucleolus (that big nuclear body!) to initiate assembly and then and serve as a scaffold for the continued addition of ribosomal proteins to both the small and large ribosomal subunits. After the 5S rRNA added to the nascent large ribosomal subunit, processing (cleavage) of 45S rRNA is completed and the subunits are separated. The separated ribosomal subunits exit the nucleus o the cytoplasm where they will associate with mRNAs to translate new proteins. To better understand what is going on, try summarizing what you see here in the correct order of steps. You can also see this process animated at this link: here .

The 5’ methyl guanosine cap and the poly(A) tail collaborate to facilitate exit of mRNAs from the nucleus into the cytoplasm. We now understand that proteins in the nucleus participate in the export process. A nuclear transport receptor binds along the mature (or maturing) mRNA, a poly-A-binding protein binds along the poly-A tail of the message, and another protein binds at or near the methyl guanosine CAP itself. These interactions enable transport of the mRNA through nuclear pores. After the mRNA is in the cytoplasm, the nuclear transport receptor re-cycles back into the nucleus while a translation initiation factor replaces the protein bound to the CAP. The nuclear transport process is summarized in the illustration below.

See a more detailed description of mRNA transport from the nucleus at this link: here . The mature mRNA, now in the cytoplasm, is ready for translation. Translation is the process of protein synthesis mediated by ribosomes and a host of translation factors (including the initiation factor in the illustration above. The genetic code directs polypeptide synthesis during translation. Details of translation will be discussed shortly.

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Biology library

Course: biology library   >   unit 33, anatomy of a neuron, overview of neuron structure and function.

• The membrane potential
• Electrotonic and action potentials
• Saltatory conduction in neurons
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How do you know where you are right now?

The human nervous system.

• The central nervous system ( CNS ) consists of the brain and the spinal cord. It is in the CNS that all of the analysis of information takes place.
• The peripheral nervous system ( PNS ), which consists of the neurons and parts of neurons found outside of the CNS, includes sensory neurons and motor neurons. Sensory neurons bring signals into the CNS, and motor neurons carry signals out of the CNS.

Classes of neurons

Sensory neurons, motor neurons, interneurons, the basic functions of a neuron.

• Receive signals (or information).
• Integrate incoming signals (to determine whether or not the information should be passed along).
• Communicate signals to target cells (other neurons or muscles or glands).
• The dendrites tend to taper and are often covered with little bumps called spines. In contrast, the axon tends to stay the same diameter for most of its length and doesn't have spines.
• The axon arises from the cell body at a specialized area called the axon hillock .
• Finally, many axons are covered with a special insulating substance called myelin , which helps them convey the nerve impulse rapidly. Myelin is never found on dendrites.

Variations on the neuronal theme

Neurons form networks, the knee-jerk reflex, glial cells, types of glia and their functions, references:, want to join the conversation.

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Scientists call the region of space influenced by the Sun the heliosphere – but without an interstellar probe, they don’t know much about its shape

Research Fellow in Climate and Space Sciences and Engineering, University of Michigan

Disclosure statement

Sarah A. Spitzer works as a research fellow for the University of Michigan department of Climate and Space Sciences and Engineering. She receives funding from the the University of Michigan and from grants supported by organizations such as NASA. She is affiliated with the University of Michigan and the Interstellar Probe Study Team.

University of Michigan provides funding as a founding partner of The Conversation US.

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The Sun warms the Earth, making it habitable for people and animals. But that’s not all it does, and it affects a much larger area of space. The heliosphere , the area of space influenced by the Sun, is over a hundred times larger than the distance from the Sun to the Earth.

The Sun is a star that constantly emits a steady stream of plasma – highly energized ionized gas – called the solar wind. In addition to the constant solar wind , the Sun also occasionally releases eruptions of plasma called coronal mass ejections , which can contribute to the aurora , and bursts of light and energy, called flares .

The plasma coming off the Sun expands through space, along with the Sun’s magnetic field. Together they form the heliosphere within the surrounding local interstellar medium – the plasma, neutral particles and dust that fill the space between stars and their respective astrospheres. Heliophysicists like me want to understand the heliosphere and how it interacts with the interstellar medium.

The eight known planets in the solar system, the asteroid belt between Mars and Jupiter, and the Kuiper Belt – the band of celestial objects beyond Neptune that includes the planetoid Pluto – all reside within the heliosphere. The heliosphere is so large that objects in the Kuiper Belt orbit closer to the Sun than to the closest boundary of the heliosphere .

Heliosphere protection

As distant stars explode, they expel large amounts of radiation into interstellar space in the form of highly energized particles known as cosmic rays . These cosmic rays can be dangerous for living organisms and can damage electronic devices and spacecraft.

Earth’s atmosphere protects life on the planet from the effects of cosmic radiation, but, even before that, the heliosphere itself acts as a cosmic shield from most interstellar radiation.

In addition to cosmic radiation, neutral particles and dust stream steadily into the heliosphere from the local interstellar medium. These particles can affect the space around Earth and may even alter how the solar wind reaches the Earth .

Supernovae and the interstellar medium may have also influenced the origins of life and the evolution of humans on Earth. Some researchers predict that millions of years ago, the heliosphere came into contact with a cold, dense particle cloud in the interstellar medium that caused the heliosphere to shrink , exposing the Earth to the local interstellar medium.

An unknown shape

But scientists don’t really know what the heliosphere’s shape is. Models range in shape from spherical to cometlike to croissant-shaped. These predictions vary in size by hundreds to thousands of times the distance from the Sun to the Earth.

Scientists have, however, defined the direction that the Sun is moving as the “nose” direction and the opposing direction as the “tail” direction. The nose direction should have the shortest distance to the heliopause – the boundary between the heliosphere and the local interstellar medium.

No probe has ever gotten a good look at the heliosphere from the outside or properly sampled the local interstellar medium. Doing so could tell scientists more about the heliosphere’s shape and its interaction with the local interstellar medium, the space environment beyond the heliosphere.

Crossing the heliopause with Voyager

In 1977, NASA launched the Voyager mission : Its two spacecraft flew past Jupiter, Saturn, Uranus and Neptune in the outer solar system. Scientists have determined that after observing these gas giants, the probes separately crossed the heliopause and into interstellar space in 2012 and 2018, respectively.

While Voyager 1 and 2 are the only probes to have ever potentially crossed the heliopause, they are well beyond their intended mission lifetimes. They can no longer return the necessary data as their instruments slowly fail or power down.

These spacecraft were designed to study planets, not the interstellar medium. This means they don’t have the right instruments to take all the measurements of the interstellar medium or the heliosphere that scientists need.

That’s where a potential interstellar probe mission could come in. A probe designed to fly beyond the heliopause would help scientists understand the heliosphere by observing it from the outside.

An interstellar probe

Since the heliosphere is so large, it would take a probe decades to reach the boundary , even using a gravity assist from a massive planet like Jupiter.

The Voyager spacecraft will no longer be able to provide data from interstellar space long before an interstellar probe exits the heliosphere. And once the probe is launched, depending on the trajectory, it will take about 50 or more years to reach the interstellar medium. This means that the longer NASA waits to launch a probe, the longer scientists will be left with no missions operating in the outer heliosphere or the local interstellar medium.

NASA is considering developing an interstellar probe . This probe would take measurements of the plasma and magnetic fields in the interstellar medium and image the heliosphere from the outside. To prepare, NASA asked for input from more than 1,000 scientists on a mission concept.

The initial report recommended the probe travel on a trajectory that is about 45 degrees away from the heliosphere’s nose direction. This trajectory would retrace part of Voyager’s path, while reaching some new regions of space. This way, scientists could study new regions and revisit some partly known regions of space.

This path would give the probe only a partly angled view of the heliosphere, and it wouldn’t be able to see the heliotail, the region scientists know the least about.

In the heliotail, scientists predict that the plasma that makes up the heliosphere mixes with the plasma that makes up the interstellar medium. This happens through a process called magnetic reconnection , which allows charged particles to stream from the local interstellar medium into the heliosphere. Just like the neutral particles entering through the nose, these particles affect the space environment within the heliosphere.

In this case, however, the particles have a charge and can interact with solar and planetary magnetic fields. While these interactions occur at the boundaries of the heliosphere, very far from Earth, they affect the makeup of the heliosphere’s interior.

In a new study published in Frontiers in Astronomy and Space Sciences, my colleagues and I evaluated six potential launch directions ranging from the nose to the tail. We found that rather than exiting close to the nose direction, a trajectory intersecting the heliosphere’s flank toward the tail direction would give the best perspective on the heliosphere’s shape.

A trajectory along this direction would present scientists with a unique opportunity to study a completely new region of space within the heliosphere. When the probe exits the heliosphere into interstellar space, it would get a view of the heliosphere from the outside at an angle that would give scientists a more detailed idea of its shape – especially in the disputed tail region.

In the end, whichever direction an interstellar probe launches, the science it returns will be invaluable and quite literally astronomical.

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