Cells multiply by cell division. Each dividing cell passes through a cell cycle consisting of interphase and divisional phase. Interphase includes the DNA synthetic phase(S) which is preceded by G1 and succeeded by G2. During G1 and G2, RNA and proteins are synthesized.
Somatic cells divide by mitosis. Mitosis includes both nuclear division (karyokinests) and division of cytoplasm (cytokinesis). During karyokinesis the chromosomes become free due to membrane disintegraion. Mitotic spindle is formed and the chromosomes remain at the equator. Each chromosome splits into a pair of chromatids which are joined at centromere. Subsequently, the two chromatids separate from one another and pass to opposite poles and become chromosomes of daughter cells.
Reconstitution of nucleus and division of cytoplasm (cytokinesis) take place. In animal cells cytokinesis occurs by formation and extension of cleavage furrow and in plants there is cell plate formation. Mitosis increases the somatic cell number with similar genetic constitution. It helps in replacement of lost cells too.
Germ cells divide by meiosis and produce gametes. The amount of DNA gets doubled during ‘S’ phase of interphase. The division constitutes meiosis-I: (heterolytic division) and meiosis-II (homolytic division). Meiosis-I is essentially the reduction division where homologous chromosomes form bivalents through synapsis. Exchange of genetic material takes place between non-sister chromatids of homologous chromosomes by formation of chiasmata and crossing over. The chromosomes then separate and move to opposite poles of the spindle, thus, forming two haploid nuclei.
The cell then enters the meiosis-II after passing through a short interkinesis. Unlike meiosis-I, meiosis-II is an equational division like mitosis. The resultant pair of haploid cells are converted into four haploids, The diploid chromosome number is restored when haploid gametes fertilize. Thus, meiosis maintains the constancy of chromosome number in organisms. It also brings about genetic variation through crossing over.