MEIOSIS. 10.1 HL. Meiosis . Meiosis has a number of overall functions Reduce the chromosome number by half from diploid(2n) to haploid(n). Each gamete nuclei will contain one chromosome from every homologous pair. Increase genetic diversity.
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Meiosis has a number of overall functions
In the sexually mature human male the process of meiosis and the production of mature motile sperm cells (haploid) takes over a month. In the human female the process begins during embryonic development but then undergoes a period of 'dormancy'. The cell still diploid is held at prophase I until ovulation many years later. The completion of meiosis does not actually end until fertilisation and then in humans this results in a single gametic cell and not the four as described above. (see reproduction unit). Other species have refined this process in different ways as an adaptation to their own ecology.
Before crossing over, two gametes would have contained the alleles for H and E (e.g. brown hair and brown eyes) and the other two would have contained genetic information h and e (e.g. blond hair and blue eyes)
After crossing over is complete, each gamete would contain:
Gamete a: H and E ( brown hair, brown eyes)
Gamete b: H and e (brown hair, blue eyes)
Gamete c: h and E ( blond hair, brown eyes)
Gamete d: h and e ( blond hair, blue eyes)
Gametes b and c are therefore new combinations. They are called recombinants.
10.1.3.Explain how meiosis results in an effectively infinite genetic variety in gametes through crossing over in prophase I and random orientation in metaphase I
Crossing over takes place during prophase I. the number of chiasmata may differ. Chiasmata can occur between any non-sister chromatids. It is even possible to have multiple chiasmata on two non sister chromatids.
At Metaphase I, homologous pairs aligned on the equatorial plate of the dividing cell. The diploid cells are the centre row. They will divide, as for anaphase I,vertically.