Meiosis I

Meiosis is the special type of cell division, restricted to the germ cells that give rise to gametes, in which a single diploid parent cell ultimately produces four haploid daughter cells through two successive divisions -- meiosis I and meiosis II -- without any further round of DNA replication between them. Meiosis I is the first of these two divisions, and it is meiosis I, not meiosis II, that is properly called the reductional division, because it is during meiosis I that the chromosome number is actually halved, from diploid (2n) to haploid (n).

What makes meiosis I so distinctive, and so much more elaborate than an ordinary mitotic division, is its prophase, which is dramatically longer and is itself conventionally divided into five recognisable sub-stages, named leptotene, zygotene, pachytene, diplotene and diakinesis.

Leptotene is the first sub-stage, during which the chromosomes -- already duplicated into two sister chromatids back in the preceding interphase, though this is not yet visible -- begin to condense and become gradually visible under the microscope as long, extremely thin, individually distinguishable threads. At this stage the two members of each pair of homologous chromosomes (one inherited from each parent) are still separate from one another, showing no sign yet of coming together.

Zygotene follows, and is defined by the beginning of synapsis -- the precise, point-for-point pairing of each homologous chromosome with its partner, running along their entire length. This intimate pairing is stabilised by a protein scaffold called the synaptonemal complex, which zips the two homologues together. Each such paired unit of two homologous chromosomes is called a bivalent, or, since it in fact contains four chromatids in total (two sister chromatids from each of the two homologues), a tetrad.

Pachytene is the sub-stage at which the bivalents, now fully and firmly synapsed, appear visibly thickened and shortened. It is during pachytene that the single most genetically significant event of meiosis takes place: crossing over, the reciprocal exchange of corresponding segments of genetic material between non-sister chromatids belonging to the two different homologous chromosomes of a bivalent. Crossing over is carried out with the help of an enzyme complex assembled at localised sites along the bivalent called recombination nodules, and it is this exchange that is chiefly responsible for producing new combinations of alleles on a chromosome that were not present together on either of the two original parental chromosomes.

Diplotene begins once crossing over is complete, as the synaptonemal complex that had held the two homologues tightly together starts to dissolve, and the two homologous chromosomes of each bivalent begin to separate and pull apart from one another. However, they do not separate completely: at each point along the bivalent where crossing over occurred, the two homologues remain physically joined by a cross-shaped connection called a chiasma (plural: chiasmata), which becomes clearly visible under the microscope at this stage and provides direct cytological evidence that crossing over has taken place at that location.

Diakinesis is the fifth and final sub-stage of prophase I, marked by maximum condensation of the chromosomes, which now appear as compact, darkly staining bodies. The chiasmata, rather than remaining fixed at their original positions, tend to move progressively toward the ends of the chromosome arms, a process called terminalisation. By the close of diakinesis the nucleolus has disappeared and the nuclear envelope has begun to break down, exactly as at the end of an ordinary mitotic prophase, leaving the cell ready to proceed into metaphase I. …