Jeff Kerr; Week 4 MED1011; Anatomy
Meiosis is 2 rounds of cell division to sort and exchange genes between maternal and paternal chromosomes. In female meiosis occurs in oocytes but is halted. In males it occurs in spermatocytes after puberty.
The principle of meiosis is replication of diploid cells from chromosomes (chain) to chromatids (X shape), which split to a diploid cell of sister chromatids then to a haploid cell in the second meiotic division. In late prophase when spindles attach the sister chromatids are lined up to become a bivalent where genes are switched between homologous chromosomes. Kinetochore is the protein structure on chromatids where the microtubules attach that pulls sister chromatids apart. It is located at the centromere.
In prophase there is 5 phases: leptotene (duplicated chromosomes start to condense), zygotene (synapsis commences, bivalent forming), pachytene (synapsis complete, crossing over occurs at synaptonemal complex), diplotene (chiasmata visible) and diakinesis (ready for metaphase). Crossing over occurs at chiasmata. Kinetochores of sister chromatids face opposite directions to allow splitting.
Oocytes complete moving out of diplotene section of prophase of meiosis I just before ovulating, complete meiosis II from metaphase when they are fertilised. Primordial follicles can stay at resting state for 50 years. Completion of division I stimulates conversion to secondary follicle and fomation of first polar body, and ovulation occurs. Meiosis II completes with second polar body.
Stem cell spermatogonia can divide to type A dark and type A pale. Type A pale can form type B cells which divide to produce primary spermatocytes. Spermatids are result of meiosis of primary spermatocytes. They have 23 chromosomes each.