6.3 Meiosis

In the sexually reproducing organisms, two important phenomena regulate the number of chromosomes in the life cycle. These are meiosis and fertilization. Meiosis is the reduction division in which the diploid (2n) number of chromosomes is reduced to haploid (n) during gamete formation (or spore formation). Whereas, in fertilization, the two haploid gametes fuse to form a diploid zygote. In this way, the diploid condition is restored again in the life cycle.

Definition : "Meiosis is a special type of division characteristic of reproductive cells in which the diploid number of chromosomes is reduced to haploid in the daughter cells. In meiosis, chromosomes divide once while the nucleus (and in some cases the cytoplasm also) divides twice. Four haploid daughter cells result from one diploid mother cell. These differ from each other as well as from the mother cell."

Homologous chromosomes : Sexually reproducing diploid organism develops from a diploid zygote (2n). The zygote is formed when haploid (n) male and haploid (n) female gametes fuse at the time of fertilization. Thus, the diploid individual receives two sets of chromosomes; one through the male gamete (paternal set) and the other through the female gamete (maternal set). For every chromosome in the paternal set, there is a similar looking chromosome present in the maternal set. Such similar chromosomes from paternal and maternal sets called homologous chromosomes. In this way, in both the sets, all chromosomes have their homologues.

The meiotic cell cycle : It consists of the interphase, karyokinesis and cytokinesis.

Interphase : It consists of G1, S and G2 phases and involves changes as described earlier in this chapter.

The interphase nucleus : The nucleus enlarges during interphase. The chromosomes are not clearly visible being very thin and long threads. However, nucleus envelope and the nuclear are prominent.

Karyokinesis in meiosis : It consists of two complete nuclear divisions : Meiosis-I and Meiosis-II. The time interval between M-I and M-II is called interkinesis.

It is a reduction division during which a diploid nucleus divides into two haploid nuclei through separation of homologous chromosomes. Chromosomes are not duplicated in M-I.

Table of Contents

6.0 - Introduction
6.1 The Cell Cycle
6.2 Mitosis
6.3 Meiosis
6.4 Comparison between Mitosis & Meiosis

Chapter 7