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Meiosis
- In spermatogenesis leads to four equipotent sperm.
- In oogenesis leads to 1 gamete and 3 polar bodies which degenerate.
- Two divisions occur reductional (meiosis I) and equatorial (meiosis II)
- Prophase of meiosis one extended and divided into 5 phases
- Leptotene: Chromosomes become visible
- Zygotene: Homologous chromosomes pair and form synaptonemal complex
- Pachytene: chromosomes condense, individual chromatids become visible,
crossing over occurs
- Diplotene Further condensing of chromatids, chiasmata occur
- Diakinesis Chromosomes reach maximum condensation nucleolus disappears
and nuclear envelope disintegrates.
- Metaphase I: Meiotic spindle forms with one of each homologous pair lying
either side of the equatorial plate
- Anaphase I: Pairs separate and one of member of each goes to each pole
- Telophase I: Nuclear envelope reconstitutes
- Each resulting daughter cell contains a haploid number of chromosomes but
diploid DNA
- Meiosis II quickly entered
1.10 MEIOSIS [Cell Biol]
| Aim is to create haploid gametes. Principal
effect of sexual cycle is random recombination of genetic information
stored in chromosomes and genes. Meiotic divisions I and II. |
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| Replication of chromosomes into sister
chromatids. Homologous chromosomes pair (®
bivalents) at synaptonemal complexes. Crossing over to exchange maternal
and paternal segments. |
Pachytene stage. |
| Homologues move apart, but held together at
chiasmata. Condensation of chromatids, with active RNA synthesis. |
Diplotene stage. |
| Anaphase. Homologues separate. Two levels of
recombination: completion of crossing over, and random reassortment of
maternal and paternal homologues. |
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| Nondisjunction: faulty separation of
bivalents. Gametes with abnormal chromosome numbers may survive, giving
rise to embryonic defects. |
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| Meiotic division II: similar to mitosis. At
anaphase chromatids separate to yield gametes with haploid chromosome
number. |
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1.10.1 Gametogenesis
| Oogenesis. Oocytes begin meiotic division I
but arrest at diplotene stage. Cell then enlarges and builds up stocks of
proteins and RNA. At puberty, cohorts of oocytes mature to metaphase of
division II; at fertilization, division II is completed. (see also 13.3.2) |
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| Spermatogenesis (see also 13.3.1) |
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