Nucleus was discovered by Robert Brown (1831). Fleming later named the substance of nucleus as chromatin that takes the stain of basic dye. Nucleus (nuclei : Plural) is the single most important component of the cell. It controls all vital metabolic activities of the cell and it also contains genetic material.
All eukaryotic cells contain nucleus. In some eukaryotic cells however the nucleus disappears at a later stage, e.g. sieve tube in plant, erythrocytes in mammals. Prokaryotic cells like bacteria, blue-green algae do not possess true nucleus. Usually nucléus is located at the centre of the cell. But the position may change due to many reasons.
A cell may be mononucleate containing one nucleus per cell. Most plant and animal cells are mononucleate. Cells of certain protozoans contain two nuclei. These cells are termed as binucleate cells. Some kinds of cells also may be polynucleate (3-100 nuclei) as in the case with osteoblast cells of bone marrow, striated muscle fibre, cells of some algae, fungi etc.
Nuclei are generally spherical or elliptical. But in some types of cells nuclei other than the usual shaped one are found. For example, the nuclei of blood cells of mammals have constriction here and there, the nuclei of secretory cells of insects are branched, in some algae and unicelluar organisms the nuclei are tape shaped. Size and shape of nucleus depend upon the size and shape of the cell. Size of the nucleus is proportional to the size of the cell, quantity of cytoplasm, number of chromosomes it contains. The diploid cell possesses larger nucléus than haploid cell.
Structure of the Nucleus and functions of the Nucleus are given below:
Ultra structure of the Nucleus:-
Nucleus is composed of the following structures :
1. Nuclear membrane:
Nuclear membrane which is referred to as nuclear envelope separates the nucleoplasm from the cytoplasm of the cell. The nuclear membrane is composed of two unit membranes -an outer and the other inner.The space between these two membrane is called peinuclear space. It acts as barrier between materials inside and outside the nucleus. Each membrane is 75-90 A° thick and is composed of lipoprotein.
Ribosomes, mitochondria, Golgi complex may remain attached to nuclear membrane. Nuclear membrane also remains continuous with the endoplasmic reticulum. Moreover, the nuclear membrane has a fibrous supporting membrane of uniform thickness. This membrane is called fibrous lamina, which is nearly 300 A° thick.
2. Nuclear Pore:
Nuclear membrane is not continuous. It has a large number of pores. The number of pores varies according to the species and the types of cells. The pores which are about 600 A° thickness are enclosed by circular structures called annuli. In some cases the pores and annuli are covered by a membrane. The pores allow movement of RNA and protein in both direction.
The nucleus is filled up by a transparent, semi-solid, jelly-like substance called nucleoplasm or nuclear sap or karyolymph. Chromosomes and nucleolus remain suspended in the nucleoplasm. The nucleoplasm is composed of nucleic acid, proteins. enzymes and minerals.
DNA and RNA constitute the nucleic acids. Nucleoprotamines, nucleohistones, phosphoproteins constitute proteins of nucleoplasm. A large variety of enzymes are also found in nucleoplasm. Among the most important are DNA polymerase, RNA polymerase, NAD synthetase, nucleoside triphosphate, adenosine diaminase etc. Nucleoplasm contains many different kinds of minerals.
The nucleus contains 9-12 per cent DNA, 5 per cent RNA, 3 per cent lipid, 15 per cent basic proteins and 65 per cent other proteins. RNA moves out of the nucléus .
4. Chromatin reticulum:
During interphase stage nucleoplasm contains many thread like structure that forms a network called chromatin reticulum. Later in course of time these organised into a number of chromosomes. Chromation contains DNA and a kind of proteins called histones.
Every chromosome essentially has a primary constriction called ccntromere. Each chromosome has a disc like structure at the centromere region. This is called Kinetochore. The chromosomes can be divided into four types based upon the position of centromere.
(i) The chromosomes called metacentric when centromere is at the middle.
(ii) It is called sub-metacentric when centromere is slightly away from centre.
(iii) It is called acrocentric when centromere is situated close to its end.
(iv) The chromosome is called telocentrtic when the centromere is Situated at extreame end.
In some chromosomes some secondary constructions are found at one end which do not accept stain. These are called satellite.
5. Nucleolus (pl = Nucleoli):
Nucleus may contain one or more spherical dense bodies inside it. These bodies are called nucleoli. The nucléus is much denser than nucleus. Nucleolus is always without a membranous cover. nucléus may remain attached to a chromosome during the interphase.
Functions of the Nucleus:
(1) Nucleus is most essential for survival of the cell and hence the organism. Nucleus regulates all vital activities of the cell.
(2) It also maintains the genetic materials essential for continuity of species.
(3) Nucleus plays most essential role in cell replication.
(4) Nucleoplasm contains many essential enzymes necessary for biochemical activities of the cell.
(5) The nucleolus synthesises ribosomes and r-RNA for the cell.
(6) Cytoplasm cannot exist without nucleus. Similarly nucléus cannot exist without cytoplasm. Both of these, therefore. interact in order to maintain the integrity and activities of the cell.
In cytoplasm many living and non-living objects remain suspended. Among these non-living objects the important ones are oil-drops, yolk granules, starch, glycogen, pigments, secretory and excretory substances. The living objects which are referred to as cell organelles include mitochondria. Golgi complex, endoplasmic reticulum, lysosomes, ribosomes, plastids, ccntrosomes. microtubules, basal granules, cilia, flagella etc. nucléus may also be included as cell organelles. Because, all membrane bound living structures of cells are called cell organelles.