The cell division that takes place during growth and development of an organism is known as mitosis. It also occurs during asexual reproduction. Mitosis takes place in somatic cells, body cells that are not involved in the production of gametes.
The somatic cell of a particular kind of organism has a characteristics number of chromosomes in its nucleus. These chromosomes occur in pairs, I.e. there are two sets of chromosomes. The members of each pair are called homologous chromosomes. For example, each human somatic cell has 46 chromosomes. These are present ad 23 psjrs of homologous chromosomes. The number of chromosomes in each somatic cell of an organism is called diploid number (2n).
Each chromosomes is made up of protein unit with a strand of DNA wound round them. Along its length are genes which are actually DNA segments. Genes are discrete units of inheritance which determine hereditary traits such ad type of hair, skin colour and shape of the ear. They control the functioning of the cell by instructing it to make a relevant enzymes.
Just before cell division, chromosomes appear clearly in the nucleus of a cell ad a thread like structures. This is because, at this time, the chromosomes becomes better shorter and thicker. Each chromosomes makes an exact copy or replica of itself so that it is made up of two parallel strands called chromatids. These are held together at a region called the centromere. When the 'parent' cell divides, one chromatids from each chromosomes foes in to teach daughter cell and becomes a chromosome. This ensures that each daughter cell has the exactly the same set of genes, ad the parent cell. In the cells of animals and lower plants, Centrioles are present outside the necleus. These are involved in spindle formation.
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