MEIOSIS

       Meiosis is derived from the Greek work “ meioum” which means “to diminish”. Since in this division the original chromosome number (e.g., 2n = 46) present in the parents is reduced to half (e.g., n = 23) in the daughter cells, it’s also known as reductional division. Another special feature of this type of division is the separation of homologous chromosomes or sister chromosomes and not their halves (i.e., chromatids). Thirdly, meiosis is observed in sexually reproducing organisms. Since, sexual reproduction involves union of male gametes (spores in plants or sperms in animals) and female gametes (ovules in plants and ova in animals), the chromosome number is doubled and will continue to duplicate every time the cell undergoes meiosis. But, this does not happen in nature! Because, the chromosome number is halved during meiosis to keep it constant in a particular species or race.

   It involves two divisions – (a) Meiosis I, and (b) Meiosis II

   Meiosis shows many such unique features which are summarized in Fig. B.2.1.

Meiosis I

Prophase I

          It is far more complex than mitosis and is a long drawn process. In some, it may last for days (e.g., human male -24 days) or even months or several years (e.g., human female till puberty 15-16 years). It is further subdivided into five stages characterized by the following features:

1.Leptotene (Gr. “leptos” meaning thin, “nema” – meaning thread)

(a)The chromosomes become more apparent under microscope as thin, elongated threads (hence the name) aligned very close to each other. They show bead-like thickenings at regular intervals known as chromomeres.

 (b)Nucleus increases in size.

2.Zygotene (Gr. “zygon” – adjoining)

Homologous chromosomes, which are sister chromosomes, one coming from the male parent and other from the female parent, are aligned and undergo pairing. The pairing is known as synapsis. It can begin at scattered points but progresses in such a fashion that the homologous chromosomes are exactly paired chromomere –by-chromomere and gene-by-gene. The pairing of homologous chromosomes is known as synapsis forming synaptonemal complex.

3. Pachytene (Gr. “pachus”- thick)

(a) By the mid-stage, the nucleus contains half the number of chromosomes, because each unit is now a bivalent (“bi” meaning “two” referring to two homologous chromosomes) or a tetrad (“tetra “means “four” referring to four chromatids). Thus the two non-sister homologous chromosomes with their four arms or chromatids are intimately paired during this stage.

(b) Another important development which occurs during this phase and has far-reaching effects with respect to formation of recombinants is “crossing-over”. The two non-sister homologous chromosomes exchange segments containing genetic material, resulting in new combinations.