Physical Basis of Heredity 2.  Cell Division (Mitosis and Meiosis)
Significance of meiosis
Reproductive cycles
Stages of meiosis
Synaptonemal complex
Recombination nodules
Comparison of meiosis and mitosis

Significance of meiosis
It is obvious from the study of mitosis that daughter cells resulting from a mitotic division have same chromosome number as the parent cell had. This is necessary for growth and reproduction, if only asexual method is present. However, in sexual method of reproduction, fusion of sex cells (male and female) is required. If sex cells have the same chromosome number as the somatic cells have, the zygote will have double the chromosome number. For instance, if an individual has four chromosomes in each cell including the sex cells, the zygote resulting from fusion of the two sex cell will have eight chromosomes and an individual having eight chromosomes in each cell would result. This cycle will repeat in each generation and with each cycle of sex cells fusion, chromosome number will be doubled again and again. This will be a difficult situation.
It is also known that despite sexual reproduction involving fusion of sex cells, chromosome number remains constant from one generation to the other. This is possible only when the chromosome number in the sex cells is half that in the somatic cells. It would mean that during formation of sex cells, reduction of chromosome number takes place. This reduced chromosome number, whether reduced at the time of formation of the sex cells (higher plants and animals) or at any other time during life cycle (formation of spores in Bryophytes and Pteridophytes), is called haploid condition. When two haploid cells fuse, diploid condition will be restored (Figs. 7.5 and 7.6).

Sexual reproduction cycle in Chlamydomonas.
Fig. 7.5. Sexual reproduction cycle in Chlamydomonas.
Alternation of generations in Bryophytes.
Fig. 7.6. Alternation of generations in Bryophytes.