Meiotic Cell Division–Why and How?

Abstract

Most somatic cells are diploid with 2 sets of chromosomes. These cells divide at regular intervals. At the time of division, called ‘mitosis’, each chromosome carries two chromatids. As they divide, the daughter cells get one chromatid from each chromosome. Thus though the cells retain their chromosome number, their DNA content gets half. Prior to the next division, these cells pass through a phase of DNA synthesis that recovers the lost DNA and adds a chromatid to each chromosome. ‘Meiosis’ is a division of cells that leads to the reduction in their chromosome number as well as the amount of DNA to haploidy (1 set of chromosomes and DNA). It is achieved by the cell undergoing two successive divisions without the intervening S phase. These haploid cells differentiate into gametes which when fused with another gamete make a diploid zygote, the progenitor of an organism. In higher organisms, cells specialised to undergo meiosis are germ cells. In several single-celled organisms too meiosis takes place. Essentially the process is conserved throughout the living world. In this article, we try to understand under what conditions and situations mitotically dividing cells initiate meiosis. We also unravel the genetic regulation of its initiation as well as the process that follows. In the unicellular mould, yeast, under conditions of stress (nutritional, environmental) a set of IME (inducers of meiosis) genes (IME1, IME2, IME4) get activated. These genes comprise regulators in their promoter regions that inhibit their expression in haploid and unstressed conditions. When activated, IME genes initiate cells into meiosis and induce a cascade of genes that steer the cells towards homologous pairing, recombination, and chromosomal segregation followed by another division without the S phase. We will also try to understand the mechanisms of other phenomena unique to meiosis viz., segregation of chromosomes instead of chromatid at anaphase I, entry into meiosis II and its execution without passing through the S phase.