Nucleus: Structure and function
Cell Biology ( Cell Organelles)
Nucleus bears the DNA which is capable of replication on and transcription.
Due to this nucleus controls the different metabolic as well as hereditary activities of cell so also known as heart of the cell.
Firstly discovered the nucleus by Robert brown in 1831.
Nucleoli were further described by M.J. Schleiden in 1838.
The hereditary rote was described by Hammerling in 1953.
Occurance:
Nucleus is found in all cells of plants and animals.
Nuclens is absent in some parts of plants, some areas of leans of eye and mammalian erythrocytes (R.B.S.) prokaryotic cells don't have true nucleons.
The single circular and large DNA molecules remains in direct with cytoplasm.
Position of nucleus differs as the type of cell. it remains in the centre.
Position of nuclens changes according the metabolism of the cell.
In embryonic cells it always lies in geometric centre of the cells later on displaced as differentiation advances.
In glandular cells it lies at the basal region.
The shape of nuclens also varies according to the cell type. It is generally spherical or may be irregular as in leucocytes.
Nuclens is also found in horse shoe shape branched or lanceolate forms size of nucleus is also not constant but in gene a ratio exists between the nuclear volume and the volume of the cytoplasm and this is a characteristic of each cell type.
This can be expressed numerically. This can be expressed numerically in the so called nucleoplasmic index (NP).
The lack of maintenance of NP ratio appears to act as stimulus to cell division.
NP=Vn divided by Vc-Vn
Vn Nuclear volume
Vc Volume of the cytoplasm.
The size of nuclens also depends upon chromosome number.
A cell with two sets of chromosomes in the nuclens (somatic cell) is called diploid cell but when one set is present the term haploid or monoploid (sex cells) is used.
Diploid chromosome number varies from 6 to 78 in animals whom haploid number will be from 3 to 39. Structure:
Nucleus components:
2) Nuclear sap of karyolymph or nucleoplasm.
3) Chromation network
4) Nucleolus.
5) Nuclear pore
1) Nuclear membrane :
The nucleus is seperated from the cytoplasm by a limiting membrane called as nuclear membrane or karyotheca.
This membrane plays an important role for the transport between the nucleus and cytoplasm.
It has direct connections with the endoplasmic reticulum and during cell division this nuclear envelope develops from parts of endoplasmic reticulum.
Nuclear membrance is double membrance with interruptions at intervals known as nuclear pore.
The outer membrane is known as ectokaryotheca and inner one is termed as en dokaryotheca.
Each membrane is 90 A thick, and the space between the two perinuclear space being about 150-300 A wide.
The nuclear pore is about 400 Å in diameter.
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2) Nucleoplasm/ Nuclear sap :
The nucleus contains a granular homogenous matrix during interphase called as nucleoplasm.
This is composed of protenous material and is the main site for enzyme activity.
Nucleoplasm shows variations in appearance during different stages of cell division.
Spindle apparatus in plants is formed by nucleoplasm.
3) Chromation (Material) Network :
In the nucleoplasm of nucleus there appears a net like structure composed of linin or ach romation.
Associated with this linin net are granules which becomes darkly stained with basic dyes.
These are called as chromatin granules. During cell division these chromation granules become entangled with the network forming long thread like structures called as chromo somes.
These are the result of DNA replication which causes condensation of chromosomes.
Certain regions of chromosomes containing chromation mass become darkly stained than other regions.
These are known as heterochromatic regions or heterochromation and unstained parts are known as euchromation.
The darkly stained regions show numerous bead like bodies along the chromosomes called chromomeres.
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4) Nucleolus:
In higher organisms every cell nucleus has a spherical, colloidal body embeded in matrix called as nucleolus.
The number of nucleoli in nucleus depends upon the number of sets of chromosomes and species. (One set nucleolus per haploid set).
It means that there are two nucleoli in each somatic cell and one in each ovum and sperm.
But in nucleoli of amphibian oocytes hundreds of nucleoli are found.
The size of nucleolus is related to synthetic activities of the cell.
The cells not involved inprotein synthesis (eg, sperm and muscle cells) show small or absentia of nucleolus.
While the cells involved in protein synthesis show giant nucleolus (eg, oocyte, neuron and secretory cells).
In the nucleolus is present. With this interanucleolar chromation is also present which penetrates the nucleolar matrix forming intra nucleolar ramifications.
Which are the extensions of DNA from chromosomes.
5) Components of Nucleolus :
The Nucleolus generally disappears during the first stage or prophase stage of cell division but it reappears in the daughter cells.
Morphologically two components may be defined in a nucleolus.
1) Pars amorpha:
This is the component of nucleus which first disappears but reappears at the end of cell division.
2. Nucleolonema:
This is the second permanent component which doesn't disappear but remains persistent through out the cell cycle.
Chemical composition of Nucleus:
The chemical components have been analyses out from the isolated nuclear material.
1) Nucleoproteins
2) Nucleic acids
3) Enzymes
4) Lipids
1) Nucleoproteins:
The chromosomes of eukaryotic cells are made up of DNA which is made up of chromation and histone proteins.
The protein part of nucleus is very complex and has several components of these the nest known are two strongly basic and simple proteins, the protamines and the histones.
Histones are characterized by the preponderance of amino acids arginine and lysine while protamines show mainly arginine
2) Nucleic acids:
Nucleic acids are of two types:
1) Ribonucleic acid : RNA and
2) Deoxyribonuclic acid (DNA) have been observed in the nuclear region of the cell.
3) Nuclear enzymes:
These are the most important components of nucleus.
According to recent studies the nuclear enzymes fall into two classes a) Generally distributed and are related with nucleotide metabolism.
Such as adenosine diaminase.
Other enzymes are ester asses are present in varying concentration. eg. alkaline phosphatase.
Nuclear lipids :
The lipid content of the nucleus has been investigated in the isolated nuclei.
Recently it has been reported that the lipo-protein complex nuclei from ox spleen and chicken erythrocytes is approximately 10% lipid, these give positive tests for phospholipids and cholesterol.
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Functions of Nucleus :
The nucleus is a major seat of heredity and of the control over all synthetic reactions of cell. If it is removed then the cell dies.
This clearily indicates that the cell cytoplasm is dependent upon or regulated by the nucleus.
The nucleolus plays important role in mitosis.
The nucleoli help in protein synthesis by formation of ribonucleic acid.
This ribonucleic acid plays and role in formation of protein.
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