Meiosis

Meiosis takes its name from the Greek meioum, meaning to reduce, and it is unique among the forms of nuclear division for three linked reasons: it involves synapsis (the pairing of homologous chromosomes), homologous recombination (the exchange of genetic material between paired chromosomes, i.e. crossing over), and a reduction in chromosome number. Meiosis takes place only in the reproductive organs, and its outcome is the formation of gametes carrying half the normal chromosome number: haploid sperm are made in the testes and haploid eggs in the ovaries of animals, while in flowering plants meiosis occurs during microsporogenesis in the anther and megasporogenesis in the ovule. Unlike mitosis, which produces daughter cells that are exact genetic copies of the parent, meiosis produces cells that are not genetically identical to one another or to the parent cell - which is precisely its biological point, since this is the process that generates new combinations of genes and drives genetic variation within a species (Figure 7.8 traces the whole process from a diploid parent cell to four haploid products). Meiosis is studied as two successive divisions, meiosis I and meiosis II; as in mitosis, the cell is described as being in interphase during the periods when it is not actively dividing. Meiosis I is the …

Prophase I is the longest and most complex stage in the whole of meiosis, and it is conventionally divided into five successive substages: Leptotene, Zygotene, Pachytene, Diplotene and Diakinesis (Figure 7.7 illustrates the paired, crossed-over chromosomes characteristic of this stage). Leptotene is the substage at which chromosomes first become visible under the light microscope as condensation begins; the paired sister chromatids of each chromosome start to condense together as a single visible thread. Zygotene is when the defining event of meiosis takes place: homologous chromosomes actively pair up in a process called synapsis, physically zipped together by a protein structure called the synaptonemal complex; the paired unit formed by the two homologous chromosomes is called a bivalent, or equivalently a tetrad, since it ultimately contains four chromatids. Pachytene is the stage at which these bivalents become clearly visible as tetrads, each made up of 4 chromatids held together by 2 centromeres. Synapsis is completed during pachytene, and recombination nodules appear at the specific sites where crossing over will occur between non-sister chromatids of the two homologous chromosomes; by the end of pachytene the actual genetic recombination is complete, mediated by the enzyme recombinase, although the chromosomes remain physically linked at the crossover sites. Diplotene begins as the synaptonemal complex disassembles and dissolves; the homologous chromosomes start to separate from each other but remain attached at one or more points corresponding to where crossing over took place - these X-shaped points of attachment are called chiasmata, specialised chromosomal structures that continue to hold the two homologues together even as the rest of each chromosome pulls apart, while the two sister chromatids of each homologue stay closely associated with each other throughout. Diplotene can be unusually prolonged, lasting from days to years depending on the sex and species involved; in females, the chromosomes remain very actively transcribed during this extended diplotene arrest as the developing egg stockpiles materials it will need for later embryonic development, and in some animals the chromosomes at this stage form conspicuous loops known as lampbrush chromoso …

In metaphase I, spindle fibres attach to the centromeres of the two homologous chromosomes making up each bivalent, and the bivalents (still consisting of two homologous chromosomes, four chromatids and two centromeres each) become aligned at the equator of the cell, forming the metaphase plate. This is the key structural difference from mitotic metaphase, where individual chromosomes line up singly: here, whole paired bivalents line up together, with the two homologues of each pair facing opposite poles. Crucially, which homologue of a given bivalent faces which pole is determined purely by chance, independently for each of the many bivalents lined up along the metaphase plate; this random orientation is called independent assortment, and because it is repeated independently for every chromosome pair, it alone can generate an enormous number of differ …

In anaphase I, it is the homologous chromosomes making up each bivalent that separate from one another, drawn towards opposite poles of the cell as the spindle fibres shorten. This is the crucial point at which the actual reduction in chromosome number takes place, distinguishing anaphase I sharply from mitotic anaphase or from anaphase II later in meiosis: here, each homologous chromosome moves to its pole still carrying both of its sister chromatids and an undivided centromere, rather than the centromere splitting and single chromatids separating. Depending purely on how each bivalent happened to orient at metaphase I, the homologous chromosomes reaching a given pole may be of paternal or maternal origin in any combination - this is the physical basis of independent assortment. Throughout anaphase I, sister chromatids remain firmly attached to one anoth …

By telophase I, a full haploid set of chromosomes (each still made of two sister chromatids) has arrived at each pole of the cell. Two daughter nuclei re-assemble, each containing this haploid chromosome number: a nuclear envelope forms around the chromosomes at each pole, the chromosomes decondense into an uncoiled state, and the nucleolus reappears in each new nucleus. In plants, cytokinesis follows karyokinesis at this stage in the usual way, with a cell plate forming between the two newly separated groups of chromosomes; the resulting pair of haploid daughter cells is together referred to as a dyad of cells. Between the completion of meiosis I and the start of meiosis II lies a brief, short-lived stage called interkinesis - broadly comparable to interphase, but notably without any DNA replication, since …

Meiosis II is the equational division of meiosis, and it is sometimes called mitotic meiosis because it runs through all the same stages as an ordinary mitotic division, just starting from an already-haploid cell rather than a diploid one. Prophase II sees each chromosome, still made of two chromatids, become short, condensed, thick and clearly visible again; a new spindle develops at right angles to the axis of the previous (meiosis I) spindle, and the nuclear membrane and nucleolus disappear once more. Metaphase II brings each chromosome to the equatorial plane of this new spindle, with the spindle's microtubules attaching to the centromere of the sister chromatids of each chromosome, exactly as in mitotic metaphase. In Anaphase II, the sister chromatids of each chromosome finally separate from one another - the centromere of each chromosome splits, and the resulting daughter chromosomes are pulled towards opposite poles as the microtubules shorten, holding and moving the separating sister chromatids apart; this is the step, absent from meiosis I, that mirrors ordinary mitotic anaphase. Telophase II completes the process: four separate groups of chromosomes are organised, one at each of the four poles created across the two meiosis-II spindles, into four haploid nuclei, the spindles disappear, and the nuclear envelope and nucle …