Describe the structure and function of the nucleolus;How is it formed? How many types of proteins and nucleic acids are found in the nucleus? Discuss the cytochemistry of nucleus?
In this article we will discuss about the nucleus:-Explanation of the nucleus, shape and size, morphology, structure.
(2) Shape and Size of the nucleus.
(3) Morphology of the nucleus:
(b). Binucleate cells
(c). Polynucleate cells
(4) Structure of the nucleus:
Nucleopores and nucleo cytoplasmic traffic-
*structure of nucleopores
*Numbes of nucleopores (poredensity)
*Arrangements of nucleopores on nuclear envelope
*Rate of the transport
(c) Chromatine fibres:
*ultrastructure and function
The nucleus is the heart of the cell. It is here that almost all of the cell’s DNA is confined, replicated and transcribed.The nucleus,thus,controls different metabolic as well as hereditary activities of the cell. A synonymous term for this organelle is the Greek word karyon. Nucleus serves as the main distinguishing feature of eukaryotic calls, i. e.. this is the true nucleus as opposed to the nuclear region, prokaryon or nucleoid of the prokaryotic cells. The following statement of Vincent Alfrey (1968) completely qualifies the central position of the nucleus in the affairs of an eukaryotic cell:The cell nucleus, central and commanding, is essential for the biosynthetic events that characterite cell type and cell fraction, it is a wait of genetic information encoding the past history and future prospects of the cell, an organelle silmerged and deceptively serene in its sea of turbulent cytoplason, a firm and purposeful guide, u baromeier exquivitely sensitive to the changing demands of the organism and its environment. This is our subject to be examined in terms of its ultrastructure, composition and function.Nuclei were first discovered and named by Robert Brown in 1833 in the plant cells and were quickly recognized as a constant feature of all animal and plant cells.
The nucleus is found in all the eukaryotic cells of the plants and animals. However, certain eukaryotic cells such as the mature sieve tubes of higher plants and mammalian erythrocytes contain nucleus. In such cells nuclei are present during the early stages of development. Since mature malian 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 true nucleus, Le, the single, circular and large ONA molecule remains in direct contact with the cytoplasm. The position or location of the nucleus cell is usually the characteristic of the cell type and it is often variable. Usually the nucleus remains pated in the centre. But its position may change from time to time according to the metabolic states be cell. For example, in the embryonic cells the nucleus generally occupies the geometric centre of de 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.
Shape & size of Nucleus:-
The shape of the nucleus normally remains related with the shape of the cell, but certain nuclei are almost irregular in shape. The spheroid, cuboid or polyhedral cells (isodiametrical cells) cons the spheroid nuclei. The nuclei of the cylindrical, prismatic or fusiform cells are ellipsoid in shape The cells of the squamous epithelium contain the discoidal nuclei. The leukocytes, certain infusoria glandular cells of some insects and spermatozoa contain the irregular shaped nuclei. Nuclei of cells of silk glands of silk worm have finger-like extensions that greatly increase their surface area.
Nucleus occupies about 10 per cent of the total cell volume. Nuclei vary in size from about 3 um to 25 um in diameter, depending on cell type and contain diploid set of chromosome The size of the nucleus is directly proportional to that of the cytoplasm.
Morphology of nucleus:-
The cells contain single nucleus but the number of the nucleus may vary from cell to cell. According to the number of the nuclei following types of cells have been recognised:
(1) Mononucleate cells:
Most plant and animal cells contain single nucleus, such cells are know as mononucleate cells.
(2) Binucleate cells:
The cells which contain two nucleic are know as binucleate cells. Such cells two nuclei are know as binucleate cells. Such cells occur in certain protozoans such as paramecium and cells of cartilage and liver.
The cells which contain many (from 3 to 100) nuclei are know as polynucleate cells. The polynucleate cells of the animals are termed as syncytial cells, while the polynucleate cells of the syncytial cells, while the polynucleate cells of the plants are know as coenocytes. The most common example of the syncytial cells are the osteoblast (polykaryocytes of the bone marrow) which contain about 100 nuclei per cell and striated muscle fibers each of which contains many hundred nuclei.
Structure of the nucleus:-
The nucleus is composed of the following structure:-
(1) The nuclear membrane or karyotheca or nuclear envelope.
(2) The nuclear sap or nucleoplasm.
(3) The chromatine fibre.
(4) The nucleolus.
The nuclear envelope (perinuclear cisterna) enclosed the DNA and defines the nuclear compartment of interphase and prophase nuclei.
It is formed from two concentric unit membrane,each 5-10mm thick. The spherical inner unclear membrane contains specific proteins that act as binding sites for the supporting fibrous sheat of intermediate filaments (IF), called nuclear lamina.Nuclear lamina has contact with the chromatine and nuclear RNAs.
Nuclear pores and Nucleo cytoplaamic 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 ex tends across each pore limiting free transfer of material. Such a diaphragm called annulus has been observed in animal cells, but lacking in plant cells. Recent electron microscopic stud ies have found that a nuclear pore has far more complex structure, so it is called nuclear pore complex. Each pore complex has an estimated molecular weight of 50 to 100 million daltons.
(2) Number of nuclear pores (Pore density) :
In nuclei of mammals it has been calculated that nuclear pores account for 5 to 15 per cent 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 per cent. The number of pores in the nuclear envelope or pore density seems to correlate with the transcriptional activity of the cell.
(3) Arrangement of nuclear pores on nuclear envelope :
In somatic cells, the nuclear pores are ly or randomly distributed over the surface of nuclear envelope.
(4) Nucleo-cytoplasmic traffic :
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, RNA and ribosome subunits are exported to the cytosol (cytoplaamic matrix).One of the main functions of the nuclear envelope is to prevent the entrance of active ribosomes into the nucleus.
(5) Rate of transport through the nuclear pores:
The nuclear envelope of a typical mammalian cell contains 3000 to 4000 pores (about 11 pores/ um of membrane area). If the cell is synthesizing DNA, it needs to import about 10 histone molecules from the cytoplasm every 3 minutes in order to package newly made DNA into chromatin, which means that on an average each pore needs to transport about 100 histone molecules per minute.
Nuclear pores are not the only avenues for nucleocytoplasmic exchanges .Small molecules and ions readily permeate both nuclear membranes. Larger molecules and particles may pass through the membrane by 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 or the matrix known as the nuclear por nucleoplasm of karyolymph.The nucleoplasm has a complex chemical composition.
It is composed of mainly the nucleopro teins but it also contains other inorganic and organic substances, viz., nucleic acids, proteins, enzymes and minerals.
The most common nucleic acids of the nucleoplasm are the 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 following two types :
*Non-histone or Acidic proteins
The proteins which take basic stain are known as the basic proteins. The most important basic proteins of the nucleus are nucleoprotamines and the nucleohistones.
e.g.-histones rich in lysine, histones with arginine and histones with poor amount of the lysine.
*Non histones 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 which are necessary for the synthesis of the 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, guanase, aldolase, enolase, 3-phosphoglyceraldehyde dehydrogenase and pyruvate kinase. The nucleoplasm also contains certain cofactors and coenzymes such as ATP and acetyl CoA.
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 large amount of these minerals than the nucleoplasm.
3. Chromatin Fibres:-
The nucleoplasm contains many thread-like, coiled and much elongated structures which take readily the basic stains such as the basic fuchsin. These thread-like structures are known as the chromatin (Gr., chrome= colour) substance or chromatin fibres.
Two types of chromatine material have been recognised, e.g., (A)Heterochromatin and (B)Euchromatin.
(A) Heterochromatin :-
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 suppet to be metabolically and genetically inert because it contains comparatively small amout of the DNA and 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.
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 info to be related with the synthetic activity of the cell. Therefore, the cells with little or no synthetic activities, e.g., sperm cells, blastomeres, muscle cell, etc.
(A)Chemical composition of nucleolus:
Nucleolus is not bounded by any limiting membrane;calcium ions are supposed to maintain its intact organization. Chemically,nucleolus contain DNA of nucleolar organizer, four types of rRNAS, 70 types of ribosomal proteins, RNA binding proteins (e.g. nucleolin) and RNA splicing nucleoproteins.
(B)Ultrastructure and function of nucleolus:
Nucleolus are the sites where biogenesis of ribosomal subunits(i.e.40S and 60S) takes place. In it three types of rR NAS, namely 18S, 5.88 and 28S r RNAs, are transcribed as parts of a much longer precursor molecule (45S transcript) which undergoes processing (RNA splicing, for ex. ) by the help of two types of proteins such as nucleolin and U3 sn RNP (U3 is a 250 nucleotide containing RNA, sn RNP represents small nuclear ribo nucleoprotein). The 5S r RNA is the chromosome existing outside the nucleolus and the 70 types of ribosomal proteins are synthesized in the cytoplasm.
(C)Mitotic cycle of nucleolus:
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 location of the ribosomal RNA genes(NOs).