In this article we will discuss about Nucleus on Cell Biology:- 1. Introduction to Nucleus 2. Definition of Nucleus 3. Number 4. Shape and Size.
Introduction to Nucleus:
The nucleus is an important cell organelle. It is just like the central nervous system of our body. So the nucleus is simply called the brain of the cell or ‘controlling centre of the cell’ because it controls all activities of an individual cell.
A nucleus contains many things of which the most important component is the chromosome which contains large amount of DNA and genes. Due to the presence of genes, the nucleus controls the various cellular activities and decides what to do, how to do and when to do.
Literally the nucleus is present in all kinds of living cells, but a well-organised nucleus with a definite nuclear membrane that separates the DNA from cytoplasm, is absent in bacteria, PPLO (Pleuropneumonia like organisms), Spirochaeta, blue-green algae etc.
On the basis of presence or absence of true nucleus, living organisms are divided sharply into two groups— prokaryotes, which do not have true nucleus, and eukaryotes, which have a well-organised nucleus.
The membrane less nuclear area which contains the most important genetic material, DNA, within a cell of prokaryotic organism is termed as ‘prokaryon’ or nucleoid, rather than a true nucleus.
The prokaryon or nucleoid is the nucleus-like nuclear region which is visibly distinct from cytoplasm but virtually remains in direct contact with the rest of protoplasm due to lack of nuclear membrane. Functionally it is now known to play a similar role like that of true nucleus.
The nucleoid does not have a true chromosome, and does not divide by regular mitosis. On the other hand, eukaryotic cells have a true nucleus with an elaborate nuclear membrane through which nucleocytoplasmic interchange takes place. True nucleus have chromosomes and divide by regular mitosis. Therefore, the membrane-bound true nucleus is the most important distinguishing feature of the eukaryotic cell.
Although Fontana (1781) first noticed the existence of nucleus inside the cell it was Robert Brown, in 1831, who first described the nucleus from somatic cell of Tradescantia (a plant of the family Commelinaceae) and showed its presence in all cells.
Definition of Nucleus:
The nucleus is a membrane-enclosed cell organelle containing the genetic material and various proteins involved in its replication, transcription and it controls the various metabolic functions of the cell and heredity.
Number of Nucleus:
Generally, one cell contains only one nucleus (mononucleate condition) but cell with more than one nucleus is very often found in plant and animal cells. Cell of Paramoecium, liver cells and cartilage cells are bi-nucleated [Fig. 9.1(a),(b),(c)].
In Paramoecium, one of the nuclei serves as a source of hereditary information for the next generation, the other governs the day-to-day metabolic activities of the cell. Multinucleated protoplasmic mass (polynucleate conditions) in case of animal is known as Syncytium, and in plant it is known as Cenocytes.
Syncitial cells are found in bone marrow cells (Polykaryocytes of bone marrow have up to 100 nuclei) and coenocytes are found in fungus, Vaucheria (algae) and endosperm cells of higher plant [Fig. 9.2(a),(b),(c) and (d)].
The multinucleate condition is result of either repeated division of nucleus without cytokinesis or due to fusion of cells. Nucleus is not found in mature liging sieve tube in plants and mature RBC of the mammals.
True nucleus is absent in prokaryotic cell and totally absent in dead cell like tracheid and trachea.
Shape and Size of Nucleus:
The shape of the nucleus normally remains related with the shape of the cell. The nucleus is generally a round body. In plant cells, however, where a large vacuole is present at the centre of the cell, the nucleus may be pushed against the cell wall, causing it to assume a lens shape.
In case of other types of cell, the nucleus may be spherical, oval (columnar cell), disc-like (squamous epithelium), bi- or multi-lobed (WBC), C-shaped (Vorticella), Pyriform (Paramoecium), moniliform (Spirostomum) or irregular shaped [Fig. 9.3(a),(b),(c) and (d)].
The spheroid, cuboid or polyhedral cells contains spheroid nucleus. The nucleus of the cylindrical, prismatic or fusiform cells are ellipsoid in shape. The cells of the squamous epithelium contain the discoidal nuclei. The shape of the nucleus generally is proportional to the cytoplasm.
The size of the nucleus is generally 5-7 µm in diameter. But this size is not always fixed. It may be greater or lesser than the above mentioned value. Small and large nuclei are respectively known as micro- and macro-nuclei. The size of the nucleus in every cell is directly proportional to that of the cytoplasm.
The size of the nucleus of a cell is determined by R. Hertwig’s formula:
The number of chromosomes is sometimes related with the size of nucleus. For example, polyploid cell may contain larger nuclei than the diploid cells. Size of the nucleus may vary depending upon the activity of cell. The nucleus will be larger in an active cell but will be smaller in a resting cell.