Definition of Nucleus
The nucleus is a double membrane-bound organelle that contains the genetic material (DNA) of eukaryotic organisms. As such, it serves to maintain the integrity of the cell by facilitating transcription and replication processes. It’s the largest organelle inside the cell taking up about a tenth of the entire cell volume.
What is cell nucleus?
The nucleus is the information center of the cell and is surrounded by a nuclear membrane in all eukaryotes. A cell normally contains only one nucleus. Under some conditions, however, the nucleus divides but the cytoplasm does not.
The nucleus is the heart of the cell. Almost all of the cell’s DNA is enclosed, replicated, and transcribed here. The nucleus thus controls various metabolic and hereditary activities of the cell. A synonym for this organelle is the Greek word karyon.
The nucleus serves as the main distinguishing feature of eukaryotic cells. i.e., This is the true nucleus as opposed to the nuclear region, the prokaryon, or the nucleoid of the prokaryotic cells. The following statement by Vincent Allfrey fully qualifies the central position of the nucleus in the affairs of a eukaryotic cell.
“The central and dominant cell nucleus is of essential importance for the biosynthetic events that characterize the cell type and cell fraction.
It is a vault of genetic information encoding the cell’s past and future prospects, an organelle sunk and deceptively calm in its sea of turbulent cytoplasm, a solid and focused guide, a barometer extremely sensitive to changing requirements the organism and its organisms react to the environment. “
The nucleus was the first organelle to be discovered. Unlike mammalian red blood cells, those of other vertebrates still contain nuclei. The nucleus was also described by Franz Bauer in 1804 and in more detail in 1831 by Scottish botanist Robert Brown in a talk at the Linnean Society of London
The nucleus was the first organelle to be discovered and named in plant cells by Robert Brown in 1833 and was quickly recognized as a constant part of both plant and animal cells. The nucleus was also described by Franz Bauer in 1804.
However, a more detailed description is given in 1831 by Scottish botanist Robert Brown in a talk at the Linnean Society of London. Nucleoli were described by M. J. Schleiden in 1838, although they were first notated by Fontana.
The term nucleolus was coined by Bowman in 1840. In 1879 W. Flemming coined the term chromatin for chromosomal meshwork. Strasburger introduced the terms cytoplasm and nucleoplasm. The existence of a membrane delimiting the nucleus was first demonstrated by O. Hertwig in 1893.
In 1934, Barbara McClintock recognized and named nucleolar organizers in chromosomes. In 1950 Callan and Tomlin first observed the nucleopores in the nuclei of amphibian oocytes.
The role of the nucleus in inheritance was firmly established through Hammerling’s transplant experiments with Acetabularia. The ultrastructure of the nuclear envelope, the pore complexes, and the nuclear lamina was worked out by Kirschner et al., Schatten and Thoman
In common with all other eukaryotes, nucleocytoplasmic interaction in plants takes the form of the traffic of signals across the nuclear envelope. The signaling molecules involved range from nuclear gene transcripts to small polypeptides often possessing specific amino acid targeting sequences
The evidence for nucleocytoplasmic communication as a factor in cell maintenance and development has been known before the rediscovery of Mendel’s “genes”. In the late nineteenth century, Verworm, Balbiani, and others showed that the following microsurgery showed that nucleated halves of various protozoa survived and grew, while the enucleated halves degenerated and died. Later, in the 1930s it was shown that insertion of nuclei into enucleated amoebae restored pseudopodial activity, feeding behavior, and growth.
The nucleus has also been shown to be essential for the growth and regeneration of the morphologically complex ciliate stentor. In a classic series of tests between 1934 and 1954 on the unicellular alga Acetabularia, Hammerling showed by means of interspecific nuclear transplants that morphological features, in particular the shape of the cap, were determined by the nucleus.
He also showed that even after removal of the nucleus, the cell was able to continue morphogenesis for a time and proposed that the cytoplasm contained a store of morphogenetic material (later on recognized as mRNA molecules) that had been produced by the nucleus.
Let’s closely examine the Hammerling’s classical nuclear transplantation experiments:
Hammerling’s experiment with the single celled green algae, Acetabularia, showed that the nucleus of a cell contains the genetic information that directs cellular development. In his experiments, Hammerling grafted the stalk of one species of Acetabularia onto the foot of another species
Hammerling’s experiment with the unicellular green alga Acetabularia showed that the cell nucleus contains the genetic information that controls cell development. In his experiments, Hammerling grafted the stem of one Acetabularia species onto the foot of another species.
The body of an algae Acetabularia is about six centimeters long and is differentiated into afoot, a stalk, and a cap. The cap has a characteristic shape for each species and can be easily regenerated when removed. The single nucleus is located in the rhizoid part.
Acetabularia crenulata has a cap of about 31 rays with pointed tips, but Acetabularia mediterranea has about 81 rays with rounded tips. When the cap, stem, or even the nucleated part of the rhizoid is removed, the remaining part of the algae can regenerate into a whole plant.
The enucleated part loses the regeneration capacity after a few decapitations, but the nucleated portion always maintains this ability. When the stalk of one species is grafted onto the nucleated rhizoid of the other species, an intermediate cap forms.
When decapitated, a second cap develops, similar to the cap of the species that make up the nucleus. When the nuclei of both species are present in the same cytoplasm, an intermediate cap type develops. Such experiments have clearly shown that the nucleus is the warehouse and control tower of all hereditary information.
Where is the Nucleus Located in the Cell?
The cell’s nucleus is located in the middle of the cell’s cytoplasm, the liquid that fills the cell. The nucleus may not, however, be right in the middle of the cell itself. Taking up about 10 percent of the cell’s volume, the nucleus is usually around the center of the cell itself.
The nucleus is found in all the eukaryotic cells of plants and animals. However, certain eukaryotic cells such as the mature sieve tubes of higher plants and mammalian erythrocytes contain no nucleus.
In such cell’s nuclei are present during the early stages of development. Since mature mammalian red blood cells are without any nuclei, they are called red blood “corpuscles” rather than cells (L. corpus = body, especially dead body or corpse).
The prokaryotic cells of the bacteria do not have a true nucleus, i.e., the single, circular, and large DNA molecule remains in direct contact with the cytoplasm. The position or location of the nucleus in a cell is usually the characteristic of the cell type and it is often variable.
Usually, the nucleus remains located in the center. But its position may change from time to time according to the metabolic states of the cell. For example, in embryonic cells, the nucleus generally occupies the geometric center of the cell but as the cells start to differentiate and the rate of the metabolic activities increases, the displacement in the position of the nucleus takes place. In certain cells such as the glandular cells, the nucleus remains located in the basal portion of the cell.
Structure of Nucleus
The nucleus is an organelle found in eukaryotic cells. Inside its fully enclosed nuclear membrane, it contains the majority of the cell’s genetic material. This material is organized as DNA molecules, along with a variety of proteins, to form chromosomes.
The main structures of the nucleus are the nuclear envelope, a double membrane that encloses the entire organelle and isolates its contents from the cellular cytoplasm. and the nuclear matrix (which contains the nuclear lamina), a network within the nucleus that adds mechanical support, much like the cytoskeleton supports the cell as a whole.
The nucleus is composed of the following structures:
- The nuclear membrane or karyotheca or nuclear envelope;
- The nuclear sap or nucleoplasm;
- The chromatin fibers;
- The nucleolus.
1. Nuclear Envelope
The nuclear envelope (or perinuclear cisterna) encloses the DNA and defines the nuclear compartment of interphase and prophase nuclei. It is formed from two concentric unit membranes, each 5–10 nm thick. The spherical inner nuclear membrane contains specific proteins that act as binding sites for the supporting fibrous sheath of intermediate filaments (IF), called nuclear lamina.
The nuclear lamina has contact with the chromatin (or chromosomes) and nuclear RNAs. The inner nuclear membrane is surrounded by the outer nuclear membrane, which closely resembles the membrane of the endoplasmic reticulum, which is continuous with it.
It is also surrounded by less organized intermediate filaments. Like the membrane of the rough ER, the outer surface of the outer nuclear membrane is generally studded with ribosomes engaged in protein synthesis.
The proteins made on these ribosomes are transported into space between the inner and outer nuclear membrane, called perinuclear space. The perinuclear space is a 10 to 50 nm wide fluid-filled compartment that is continuous with the ER lumen and may contain fibers, crystalline deposits, lipid droplets, or electron-dense material.
It is also called the fibrous lamina, zonula nucleum limitans, internal dense lamella, nuclear cortex, and lamina densa. The nuclear lamina is a protein meshwork that is 50 to 80 nm thick or 10 to 20 nm thick.
It lines the inside surface of the inner nuclear membrane, except the areas of nucleopores, and consists of a square lattice of intermediate filaments. In mammals, these intermediate filaments are of three types: lamins A, B, and C having M.W. 74,000, 72,000, and 62,000 daltons, respectively.
The lamins form dimers that have a rod-like domain and two globular heads at one end. Under appropriate conditions of pH and ionic strength, the dimers spontaneously associate into filaments that have a diameter and repeating structure similar to those of cytoplasmic filaments.
The nuclear lamina is a very dynamic structure. In mammalian cells undergoing mitosis, the transient phosphorylation of several serine residues on the lamins causes the lamina to reversibly disassemble into tetramers of hypophosphorylated lamin A and lamin C and membrane-associated lamin B.
As a result, lamin A and C become entirely soluble during mitosis, and at telophase, they become dephosphorylated again and polymerize around chromatin. Lamin B seems to remain associated with membrane vesicles during mitosis, and these vesicles in turn remain as a distinct subset of membrane components from which the nuclear envelope is reassembled at telophase.
Inside an interphase nucleus, chromatin binds strongly to the inner part of the nuclear lamina which is believed to interfere with chromosome condensation. In fact, during meiotic chromosome condensation, the nuclear lamina completely disappears by the pachytene stage of prophase and reappears later during diplotene in oocytes, but does not reappear at all in spermatocytes.
The lamins may play a crucial role in the assembly of interphase nuclei. For example, when cells are left for a long time in colchicine (a drug that arrests cells in metaphase), the lamins assemble around individual chromosomes, which are then surrounded by nuclear envelopes give rise to micronuclei containing only one chromosome.
A similar phenomenon occurs during normal amphibian development. In the first few cleavages of amphibian development, the nuclear envelope initially forms around individual chromosomes, forming several vesicles that then fuse together to form a single nucleus.
This suggests that chromatin is the nucleating center for the deposition of nuclear lamina and envelope. Nuclear pores and nucleocytoplasmic traffic. The nuclear envelope in all eukaryotic forms, from yeasts to humans, is perforated by nuclear pores which have the following structure and function:
1. Structure of nuclear pores.
Nuclear pores appear circular in surface view and have a diameter between 10nm to 100 nm. Previously it was believed that a diaphragm made of amorphous to fibrillar material extends across each pore limiting the free transfer of material.
Such a diaphragm called annulus has been observed in animal cells, but lacking in plant cells. Recent electron microscopic studies have found that a nuclear pore has a far more complex structure, so it is called the nuclear pore complex.
Each pore complex has an estimated molecular weight of 50 to 100 million daltons. Negative staining techniques have demonstrated that pore complexes have an eight-fold or octagonal symmetry.
More recent computerized image-processing techniques of Unwin and Mulligan have shown that the pore complex consists of two “rings” (R or annuli) at its periphery with an inside diameter of 80 nm, a large particle that forms a central plug (C) and radial ‘spokes’ (S) that extends from the plug to the rings. Particles (P) are anchored to cytoplasmic rings and are thought to be inactive ribosomes.
The ‘hole’ in the center of the pore complex is an aqueous channel through which water-soluble molecules shuttle between the nucleus and the cytoplasm. This hole often appears to be plugged by a large central granule (central plug) which is believed to consist of newly made ribosomes or other particles caught in transit.
The pore complex perforates the nuclear envelope bringing the lipid bilayers of the inner and outer nuclear membrane together around the margins of each pore. Despite this continuity, which would seem to provide a pathway for the diffusion of membrane components between the inner and outer membranes, the two membranes remain chemically distinct.
Quite recently, the following two proteins have been found to be associated with nuclear pores:
- One is an integral membrane protein, a glycoprotein of 120,000 daltons that may anchor the annuli to the lipid bilayer.
- The second protein is a 63,000-dalton protein (that has covalently bound acetyl neuraminic acid) located on the cytoplasmic side of the electron-dense material that occludes the nuclear pores. This protein may be involved in the transport of materials through the nuclear pores.
2. Number of nuclear pores (Pore density).
In the nuclei of mammals, it has been calculated that nuclear pores account for 5 to 15 % of the surface area of the nuclear membrane. In amphibian oocytes, certain plant cells, and protozoa, the surface occupied by the nuclear pores may be as high as 20 to 36%.
The number of pores in the nuclear envelope or pore density seems to correlate with the transcriptional activity of the cell. Thus, pore densities as low as ~3 pores/μm2 are seen in nucleated red blood cells and lymphocytes (which are inactive in transcription).
These cells are highly differentiated but metabolically inactive and they are non-proliferating cells. The majority of proliferating cells have pore densities between 7 and 12 pores/μm2. Among cells of a third type, differentiated but highly active, pore densities are often 15 to 20 pores/μm2.
The liver, kidney, and brain cells fall into this category. Still higher pore densities are found in specialized cells, such as salivary gland cells (~40 pores/μm2) and the oocytes from Xenopus laevis (~50 pores/μm2), both of which are very active in transcription.
3. Arrangement of nuclear pores on nuclear envelope.
In somatic cells, the nuclear pores are evenly or randomly distributed over the surface of the nuclear envelope. However, pore arrangement in other cell types is not random but rather ranges from rows (e.g., spores of Eqisetum) to Clusters (e.g., oocytes of Xenopus laevis) to hexagonal (e.g., Malpighian tubules of leaf hoppers) pecking order.
4. Nucleo-cytoplasmic traffic.
Quite evidently there is considerable trafficking across the nuclear envelope during interphase. Ions, nucleotides, and structural, catalytic, and regulatory proteins are imported from the cytosol (cytoplasmic matrix); mRNA, tRNA, and ribosome subunits are exported to the cytosol (cytoplasmic matrix).
However, one of the main functions of the nuclear envelope is to prevent the entrance of active ribosomes into the nucleus. The pore appears to function like a close-fitting diaphragm that opens to just the right extent when activated by a signal on an appropriate large protein (having a diameter up to 20 nm).
Recently, it has been investigated that the nuclear-specific proteins (called karyophilic proteins) have in their molecular structure some type of signals, called karyophilic signals or nuclear import signals, that enable them to accumulate selectively in the nucleus.
For example, nucleoplasmin is an abundant, pentameric nuclear protein having distinct head and tail domains. Nucleoplasmins are actively transported through the nuclear pores, probably while still in their folded form.
The karyophilic signal for such a nuclear import apparently resides in the tail domains and such an active nuclear transport requires energy which is derived from ATP hydrolysis. Similar signals are also noted in a short sequence (126-132 amino acids) of simian virus 40 T antigen molecule.
These short sequences when attached to bigger molecules (even to metal particles such as gold) allow these bigger molecules to enter the nucleus via the nuclear pores.
5. Rate of transport through the nuclear pores.
As we have already described, the nuclear envelope of a typical mammalian cell contains 3000 to 4000 pores (about 11 pores/ μm2 of membrane area). If the cell is synthesizing DNA, it needs to import about 106 histone molecules from the cytoplasm every 3 minutes in order to package newly made DNA into chromatin, which means that on average each pore needs to transport about 100 histone molecules per minute.
Further, if the cell is growing rapidly, each nuclear pore needs to export about three newly assembled ribosomes per minute to the cytoplasm, since ribosomes are produced in the nucleus but function in the cytoplasm.
The export of new ribosomal subunits is particularly problematic since these particles are about 15 nm in diameter and are much too large to pass through the 9 nm channels of nuclear pores, it is believed that they are specifically exported through the nuclear pores by an active transport system.
Similarly, mRNA molecules complexed with special proteins to form ribonucleoprotein particles, are thought to be actively exported from the nucleus.
Lastly, nuclear pores are not the only avenues for nucleocytoplasmic exchanges. For example, small molecules and ions readily permeate both nuclear membranes. Larger molecules and particles may pass through the membrane by the formation of small pockets and vesicles that traverse the envelope and empty on the other side.
The space between the nuclear envelope and the nucleolus is filled by a transparent, semi-solid, granular, and slightly acidophilic ground substance of the matrix known as the nuclear sap or nucleoplasm or karyolymph.
The nuclear components such as the chromatin threads and the nucleolus remain suspended in the nucleoplasm. The nucleoplasm has a complex chemical composition. It is composed of main nucleoproteins but it also contains other inorganic and organic substances, viz., nucleic acids, proteins, enzymes, and minerals.
1). Nucleic acids.
The most common nucleic acids of the nucleoplasm are DNA and RNA. Both may occur in the macromolecular state or in the form of their monomer nucleotides.
The nucleoplasm contains many types of complex proteins. The nucleoproteins can be categorized into the following two types:
(i) Basic proteins.
The proteins which take basic stains are known as the basic proteins. The most important basic proteins of the nucleus are nucleoprotamines and nucleohistones. The nucleoprotamines are simple and basic proteins having a very low molecular weight (about 4000 daltons).
The most abundant amino acid of these proteins is arginine (pH 10 to 11). The protamines usually remain bounded with the DNA molecules by the salt linkage. The protamines occur in the spermatozoa of certain fishes.
The nucleohistones have a high molecular weight, e.g., 10,000 to 18,000 daltons. The histones are composed of basic amino acids such as arginine, lysine, and histidine. The histone proteins remain associated with the DNA by the ionic bonds and they occur in the nuclei of most organisms.
According to the composition of the amino acids following types of histone proteins have been recognized, e.g., histones rich in lysine, histones with arginine, and histones with a poor amount of lysine.
(ii) Non-histone or Acidic proteins.
The acidic proteins either occur in the nucleoplasm or in the chromatin. The most abundant acidic proteins of the euchromatin (a type of chromatin) are the phosphoproteins.
The nucleoplasm contains many enzymes that are necessary for the synthesis of DNA and RNA. Most of the nuclear enzymes are composed of non-histone (acidic) proteins. The most important nuclear enzymes are the
- DNA polymerase,
- RNA polymerase,
- NAD synthetase,
- nucleoside triphosphatase,
- adenosine diaminase,
- nucleoside phosphorylase,
- 3-phosphoglyceraldehyde dehydrogenase
- pyruvate kinase.
The nucleoplasm also contains certain cofactors and coenzymes such as ATP and acetyl CoA.
According to Stoneburg and Dounce, the nucleoplasm contains small lipid content.
The nucleoplasm also contains several inorganic compounds such as phosphorus, potassium, sodium, calcium, and magnesium. The chromatin comparatively contains a large amount of these minerals than the nucleoplasm.
3. Chromatin Fibers
The nucleoplasm contains many thread-like, coiled, and many elongated structures that take readily the basic stains such as the basic fuchsin. These thread-like structures are known as the chromatin (Gr., chrome=color) substance or chromatin fibers.
Such chromatin fibers are observed only in the interphase nucleus. During the cell division (mitosis and meiosis) chromatin fibers become thick ribbon-like structures which are known as the chromosomes.
Chemically, chromatin consists of DNA and proteins. Small quantity of RNA may also be present but the RNA rarely accounts for more than about 5 % of the total chromatin present. Most of the protein of chromatin is histone, but “nonhistone” proteins are also present.
The protein: DNA weight ratio averages about 1:1. Histones are constituents of the chromatin of all eukaryotes except fungi, which, therefore, resemble prokaryotes in this respect.
The fibers of the chromatin are twisted, finely anastomosed and uniformly distributed in the nucleoplasm. Two types of chromatin material have been recognized, e.g., heterochromatin and euchromatin.
The darkly stained, condensed region of the chromatin is known as heterochromatin. The condensed portions of the nucleus are known as chromocenters or karyosomes or false nucleoli. The heterochromatin occurs around the nucleolus and at the periphery.
It is supposed to be metabolically and genetically inert because it contains a comparatively small amount of the DNA and a large amount of the RNA.
The light stained and diffused region of the chromatin is known as the euchromatin. The euchromatin contains comparatively large amount of DNA.
Most cells contain in their nuclei one or more prominent spherical colloidal acidophilic bodies, called nucleoli. However, cells of bacteria and yeast lack nucleolus. The size of the nucleolus is found to be related to the synthetic activity of the cell.
Therefore, the cells with little or no synthetic activities, e.g., sperm cells, blastomeres, muscle cell, etc., are found to contain smaller or no nucleoli, while the oocytes, neurons and secretory cells which synthesize the proteins or other substances contain comparatively large-sized nucleoli.
The number of the nucleoli in the nucleus depends on the species and the number of the chromosomes. The number of the nucleoli in the cells may be one, two or four. The position of the nucleolus in the nucleus is eccentric.
A nucleolus is often associated with the nucleolar organizer (NO) which represents the secondary constriction of the nucleolar organizing chromosomes, and are 10 in number in human beings. In corn, Zea mays chromosome 9 and 6 contain ‘darkly staining knobs’ or nucleolar organizers.
Nucleolar organizer consists of the genes for 18S, 5.8S and 28S rRNAs. The genes for fourth type of r RNA, i.e., 5S rRNA occur outside the nucleolar organizer.
1. Chemical composition of nucleolus.
Nucleolus is not bounded by any limiting membrane; calcium ions are supposed to maintain its intact organization. Chemically, nucleolus contains DNA of nucleolar organizer, four types of rRNAs, 70 types of ribosomal proteins, RNA binding proteins (e.g., nucleolin) and RNA splicing nucleoproteins (U1, U2……U12).
It also contains phospholipids, orthophosphates, and Ca2+ ions. Nucleolus also contains some enzymes such as acid phosphatase, nucleoside phosphorylase, and NAD+ synthesizing enzymes for the synthesis of some coenzymes, nucleotides, and ribosomal RNA.
RNA methylase enzyme which transfers methyl groups to the nitrogen bases occurs in the nucleolus of some cells.
2. Structure and Function of Nucleolus.
The nucleolus is the site where the biogenesis of ribosomal subunits (i.e., the 40S and 60S) takes place. In it, three types of rRNAs, namely 18S, 5.8S, and 28S rRNAs, are transcribed as parts of a much longer precursor molecule (45S transcript) which undergoes processing (RNA splicing, for example) with the help of two types of proteins such as nucleolin and U3 snRNP (U3 is a 250-nucleotide containing RNA, snRNP represents small nuclear ribonucleoprotein).
The 5S rRNA is transcribed on the chromosome existing outside the nucleolus and the 70 types of ribosomal proteins are synthesized in the cytoplasm. All of these components of the ribosomes migrate to the nucleolus, where they are assembled into two types of ribosomal subunits which are transported back to the cytoplasm.
The smaller (40S) ribosomal subunits are formed and migrate to the cytoplasm much earlier than larger (60S) ribosomal subunits; therefore, nucleolus contains many more incomplete 60S ribosomal subunits than the 40S ribosomal subunits.
Such a time lag in the migration of 60S and 40S ribosomal subunits, prevents functional ribosomes from gaining access to the incompletely processed heterogeneous RNA (hn RNA; the precursor of m RNA) molecule inside the nucleus.
Different stages of formation of ribosomes are completed in three distinct regions of the nucleolus. Thus, their initiation, production and maturation seem to progress from centre to periphery. Following three regions have been identified in the nucleolus:
(i) Fibrillar center. This pale-staining part represents the innermost region of the nucleolus. The RNA genes of the nucleolar organizer of chromosomes are located in this region. The transcription (i.e., ribosomal RNA synthesis) of these genes is also initiated in this region.
(iii) Cortical granular components. This is the outermost region of the nucleus where processing and maturation of pre-ribosomal particles occur.
3. Mitotic cycle of nucleolus.
The appearance of the nucleolus changes dramatically during the cell cycle. During meiosis as well as during mitosis the nucleolus disappears during prophase. As the cell. approaches mitosis, the nucleolus first decreases in size and then disappears as the chromosomes condense and all RNA synthesis stops so that generally there is no nucleolus in a metaphase cell.
When ribosomal RNA synthesis restarts at the end of mitosis (in telophase), tiny nucleoli reappear at the chromosomal locations of the ribosomal RNA genes (NOs). For example, in humans, the r RNA genes are located near the tips of each of the 5 different chromosomes.
Accordingly, 10 small nucleoli are formed after mitosis in a human diploid cell, although they are rarely seen as separate entities because they quickly grow and fuse to form the single large nucleolus typical of many interphase cells.
Now let us see what happens to the RNA and protein components of the disintegrated nucleolus during mitosis?
It seems that at least some of them become distributed over the surface of all of the metaphase chromosomes and are carried as cargo to each of the two daughter cell nuclei. As the chromosomes decondensed at telophase, these “old” nucleolar components help reestablish the newly emerging nucleoli.