Introduction to Nucleus

Nucleus

The heart of the cell is called nucleus. It is here that almost all of the cell’s DNA is confined, replicated and transcribed. A different metabolic, as well as hereditary activity of the cell, is controlled by the nucleus. The greek word karyon is a synonymous term for this organelle. Nucleus serves as the main distinguishing feature of eukaryotic cells. Vincent Allfrey statement completely qualifies the central position of the nucleus in the affairs of a eukaryotic cell:

“The cell nucleus , central and commanding, is essential for the biosynthetic events that characterize cell type and cell fraction: it is a vault of genetic information encoding the past history and future prospects of the cell, an organelle submerged and deceptively serene in its sea of turbulent cytoplasm , a firm, and purposeful guide , a barometer exquisitely sensitive to the changing demands of the organism and its environment. This is our subject- to be examined in the terms of its ultrastructure, composition, and function”.

Historical

Robert brown in 1833 discovered first plant cell and quickly recognized the constant feature of all animal and plant cell. In 1838 M.J. Schleiden described although first noted by Fontana in 1781. Bowman in 1840 coined the term nucleolus. In 1879 the term chromatin for chromosomal meshwork was coined by W.Flemming. the term cytoplasm and nucleoplasm was first introduced by Strasburger. In 1893 O.Hertwig demonstrated the membrane delimiting the nucleus. The nucleolar organizers in the chromosomes were recognized and named by Barbara McClintock in 1934. The nucleopores in the nuclei of amphibians oocytes was first observed by Callan Tomlin in 1950. The grafting experiments of Hammerling with Acetabular firmly established the role of the nucleus in heredity in 1953. Further Kirschner, Schatten, and Thoman worked on the ultrastructure of the nuclear envelope, pore complexes, and nuclear lamina.

ULTRASTRUCTURE

The nucleus is composed of following structures:1.The nuclear membrane or karyotheca or nuclear envelope;2.The nuclear sap or nucleoplasm;3.The chromatin fibres;and 4.The nucleolus.

1. Nuclear Envelope

The nuclear envelope 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,called nuclear lamina.The proteins made on these ribosomes are transported into space between the inner and outer nuclear membrane,called perinuclear space.

Nuclear lamina. It is also called fibrous lamina,zonula nucleus,internal dense lamella , nuclear cortex and lamina dense.The nuclear lamina is a protein meshwork that is 50 to 80 nm thick 0r 10 to 20 nm thick.The lines from dimmers that have a rod-like domain and two globular heads at one end.Under appropriate conditions of Ph and ionic strength, the dimmers 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 lamina causes the lamina to reversibly disassemble into tetramers of hypopneas-populated lamin A and lamin C and membrane-associated lamin B.Lamin B seems to remain associated with membrane vesicles during mitosis and these vesicles, in turn, remain as a distinct subset of membranes components from which nuclear envelope is assembled at telophase.Nuclear pores and nucleocytoplasmic traffic.The nuclear envelope in all eukaryotic forms,from yeasts to humans,is perforated by nuclear pores that have the following structure and function:

1.Structures of nuclear pores.

Nuclear pores appear circular in surface view and have a diameter between 10nm to 100nm. Recent electron microscopic studies have found that a nuclear pore has the far more complex structure it is called nuclear pore.Each pore complex has an estimated molecular weight of 50 to 100 million Daltons.More recent computerized image processing techniques of Unwin and Mulligan have shown that the pore complex consists of two “rings”at its periphery with an inside diameter of 80 nm, a large particle that forms a central plug and radial ‘spokes that extend from the plug to the rings.The protein may involve in the transport of materials through the nuclear pores.

2.A number of nuclear pores(Pore density). In nuclei of mammals, it has been calculated that nuclear pores account for 5 to 15 percent of the surface area of the nuclear membrane.The number of pores in the nuclear envelope or pore density seems to correlate with the transcription activity of the cell.The cells are highly differentiated but metabolically inactive and they are non-proliferating cells.

3.The 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 range from rows to Clusters.

4.Nucleocytoplasmic 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 ; MRNA, tRNA, and ribosomes subunits are exported to the cytosol.The lyophilic 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.

5.The rate of transport through the nuclear pores. The nuclear envelope of a typical mammalian cell contains 3000 to 4000 pores.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 that means an average each pore need to transport about 100 histone molecules per minute.Similarly,mRNA molecules complexed with special proteins to form ribonucleoprotein particles,are thought to be actively exported from the nucleus.

The space between the nuclear envelope and the nucleolus is filled by a transparent,semi-solid,granular and slightly acidophilic ground substance or the matrix is called the nuclear sap or nucleoplasm or karyolymph.The nucleoplasm has a complex composition.It is composed of mainly the nucleoplasm but it also contains other inorganic and organic substances,viz.,nucleic acid,proteins,enzymes and minerals.

1.Nucleic acids. The most common nucleic acids of the nucleoplasm are the DNA and RNA.Both occur in the macromolecular state or in the form of their monomer nucleotides.

2.Proteins.The nucleoplasm contains many types of complex proteins.The nucleoproteins can be categorized into two types they are;

1. Basic proteins. The proteins that take basic stain are called basic proteins.The most important basic proteins of the nucleus are nucleoproteins and the nuclear- histones. 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 the poor amount of the lysine.
2. Nonhistone or Acidic proteins. The acidic proteins either occur in the nucleoplasm or in the chromatin.The most abundant acidic proteins of the euchromatin are the phosphoproteins.
3. The nucleoplasm contains many enzymes which are necessary for the synthesis of the DNA and RNA.The most important nuclear enzymes are the DNA polymerase,RNA polymerase,NAD synthetase,nucleoside triphosphatase,adenosine deaminase,nucleoside phosphorylate,guanase,aldolase,enolase,3-phosphoglyceraldehyde dehydrogenase and pyruvate kinase.
4. According to Stoneburg and Dounce,the nucleoplasm contains small lipid content.
5. 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 Fibres

The nucleoplasm contains much thread like,coiled and much-elongated structures which take readily the basic stains such as the fuchsin.These thread-like structures called chromatin substance or chromatin fibers.Chemically chromatin consists of DNA and Proteins.Most of the protein of chromatin is histone,but “nonhistone”proteins are also present.The protein :DNA weight ratio averages about 1:1.Two types of chromatin material have been recognized ,e.g.,heterochromatin and euchromatin.

1. The darkly stained,condensed region of the chromatin is called heterochromatin.It is supposed to be metabolically and genetically inert because it contains a comparatively small amount of the DNA and large amount of the RNA.
2. The light stained and diffused region of the chromatin is called euchromatin.The euchromatin contains the comparatively large amount of DNA.

4.Nucleolus

Most cells contain in their nuclei one or more prominent spherical colloidal acidophilic bodies called nucleoli.A nucleus is often associated with the nucleolar organizer which represents the secondary constriction of the nucleolar organizing chromosomes and are 10 in number in human beings.

1.The chemical composition of the nucleolus.The 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, and RNA splicing proteins.RNA methylase enzyme transfers methyl groups to the nitrogen bases occur in the nucleus of some cells.

2.Ultrastructure and function of the nucleolus.The nucleolus is the sites where biogenesis of ribosomal subunits takes place.All of these components of the ribosomes migrate to the nucleolus,where they are assembled into two types of ribosomal subunits transported back to the cytoplasm.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 center to periphery.Following 3 regions have been identified in the nucleolus:

1. Fibrillar centre.This pale staining part represents the innermost region of the nucleolus.The RNA genes of nucleolar organizer of chromosomes are located in this region.The transcription ribosomal RNA synthesis of this genes is initiated in this region.
2. Dense fibrillar component.This region surrounds the fibrillar centre and RNA synthesis progresses in this region.The 70 ribosomal proteins also blind to the transcripts in this region.

• Cortical granular components.This is the outermost region of the nucleus where processing and maturation of pre-ribosomal particles occur.

3. The mitotic cycle of the nucleolus. The appearance of 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 as the chromosomes condense and all RNA synthesis stops,so that generally there is no nucleolus in metaphase cell.

References

Dhami, P S, and J K Dhami. A textbook of zoology Vol. II and Vol.III. Latest edition. New Delhi: Pradeep publication, n.d.

Kotpal, R L. Modern textbook of Zoology. Meerut, India: Rastogi Publication, n.d.

Rastogi, S C. Cell, and Molecular biology. New Delhi: New Age International (P) Limited, 2001.

Verma, P S, and V K Agrawal. cell biology,Genetics,Molecular Biology,Evolution, and Ecology. New Dehli, India: S. Chand and company Ltd., 2012.