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We now give a brief description of meiosis - the process that takes place in sexually reproducing organisms which takes (certain) diploid cells, that is cells with two copies of each chromosome, and leads to haploid cells, which have only one copy of each chromosome. In animals this is the formation of gametes (sperm, eggs), while in fungi, yeast and others, meiosis typically follows immediately after the zygote formation, resulting in the formation of free-living haploid cells. In many cases (e.g. male mammals) we begin with a single diploid cell and get 4 haploid (sperm) cells; in other cases (e.g. female mammals) we end up with only one haploid (egg) cell per meiosis.

You can read the details in any genetics book, but we need some terminology, and so a quick run through is desirable. Meiosis consists of two cell divisions, which we term meiosis I and II. The sequence of events during meiosis I is significantly different from that during meiosis II, although, as with mitosis, both are subdivided into prophase, metaphase, anaphase, and telophase. The most complex of these is prophase I, which is divided up into the five stages below. Preceding prophase I, most of the DNA synthesis (i.e. chromosome duplication) takes place, although some also occurs during it.

Prophase I

Metaphase I

Homologs line up on equatorial plane; the two centromeres of a homologous chromosome pair attach to spindle fibres from opposite poles.

Anaphase I

Chromosomes move to poles, one from each homologous pair to each pole.

Telophase I

This is quite variable.

This concludes the first or reduction division, so called because the number of chromosomes is reduced by one half. The second division is also called the equational division.

Prophase II

Contracted chromosomes show haploid humber.

Metaphase II

Chromosomes arrange on equatorial plane.

Anaphase II

Centromeres split and chromatids are pulled to opposite poles.

Telophase II

Nuclei reform.

next up previous
Next: Chance mechanisms during Up: Statistics in Genetics Previous: Some of Morgan's

Simon Cawley
Mon Apr 20 19:50:16 PDT 1998