Presence of cell nucleus was reported in 1831 by Robert brown. Its early discovery was undoubtedly due to its prominence in many cells, where it stands out as slightly darker than the surrounding cytoplasm. It controls the life and activities of the cell. In animal cells, it generally occupies the central space, while in the case of plant cells it is pushed towards periphery due to the presence of a large vacuole. Nucleus may be irregular or spherical in shape. Generally, the cells have one nucleus and are called mononucleate. On the other hand, the cells with two nuclei are binucleate and with more than two as multinucleate.
Nucleus is only visible when the cell is in non-dividing stage. It contains chromatin network and soluble sap called nucleoplasm. In dividing cells, the nucleus disappears and the chromatin material in it is replaced by chromosomes. The heredity material is in the form of chromosomes, which controls all the activaties of the cell. DNA, RNA and proteins including enzymes form the chemical composition of the nucleus. Nucleus consists of nuclear membrane, nucleoli, nucleoplasm and chromosomes.
Nuclear membrane
Nucleus is suttounded by a nuclear membrane which separates the nuclear material from the cytoplasm. The nuclear membrane is actually a nuclear envelope composed of two membranes. The outer membrane is at places continuous with the endoplasmic reticulum, while the inner membrane encloses the nuclear content. The outer and the inner membranes are continuous at certain points resulting in the formation of pores, the nuclear pores. The nuclear pores allow the exchange of materials between the nucleus and the cytoplasm. The number of nuclear pores is highly variable. The undifferentiated cells (such as eggs) have numerous pores (about 30,000 per nucleus), whereas differentiated cells such aserythocytes have only 3 or 4 pores/nucleus. Each pore has a definite structure which controls the traffic of substances passing through them (Figs. 4.7, 4.10 and 4.16).
Nucleolus
It is darkly stained body within the nucleus, and is without any membranous boundary to separate it from the rest of the nuclear material. There may be one or more nucleoli in the nucleus. The ribosomal RNA (rRNA) is synthesized and stored in the nucleolus. It is composed of two regions, the peripheral granular area composed of precursors of ribosomal subunits and the central fibril area consisting of large molecular weight RNA and rDNA. It is the nucleolus where ribosomes are assembled and are then exported to the cytoplasm.
Chromosomes
Nucleus is often deeply stained with basic dyes because of the chromatin material. During cell division chromatin material is converted into darkly stained thread like structures known are chromosomes. Under a compound microscope, chromosomes appear to be made of arms and centromeres. Centromere is the place on the chromosome where spindle fibers are attached during cell division. Each chromosome consists of two inentical chromatids at the beginning of cell division (chromatid is exact replica of the chromosome) which are held together at centromere (fig. 4.15).
A chromosome is composed of DNA and proteins. All the information necessary to control the activities of the cell is located on the chromosomes in the form of genes, which spindle fibers are attached during cell division. Each chromosome consists of two the other. The number of chromosomes in all individuals of the same species remains constant generation after generation. In man, each cell contains 46 chromosomes, frog cell has 26 and chimpanzee has 48 chromosomes. There are 8 chromosomes in the fruit fly, drosophila melanogaster, 16 chromosomes in onion, 48 in potato, and 14 in garden pea. The number of chromosomes in normal body cells is diploid (2n), whereas haploid chromosome number (n) is present in germ cells, e.g. human sperms and eggs have 23, while those of drosophila have 4 chromosomes.
Fig. 4.15. :structure of chromosome and its shape.
Fig. 4.16structure of nucleus