Cell Cycle and Cell Division
Meiosis, significance of meiosis
- Meiosis in human occurs in Testis and ovary to produce sperm and egg cells.
- It also happens in plants in pollen sac and ovule to form pollen grains (microsporogenesis) and embryo sac (megasporogenesis).
- Meiosis is also termed as reduction division as it reduces the chromosome number into half to the meiotic products.
- Meiotic products are always four, where all the four cells possess different genetic composition.
- Meiosis involves 2 sets of division – meiosis I → (reduction division) and meiosis II → (Equatorial division) similar to mitosis.
- The time gap between meiosis I and II is called interkinesis.
- Meiosis I and II, in males happen only after puberty, whereas in females meiosis I is locked at Diplotene sub stage of prophase I till they reach puberty.
- Meiosis I and II are further divided in prophase, metaphase, Anaphase and Telophase.
- Prophase of meiosis I is sub-divided as follows.
- Leptotene, where chromosomes show bouquet shaped arrangement i.e., all their telomeric ends join with the nuclear membrane.
- Zygotene phase proceeds by pairing homologous chromosomes ie Tetrad/bivalents formation, with the help of synthesis of the synaptonemal complex.
- Synaptonemal complex is a tripartite structure with a central recombination nodule that posses enzyme such as Endo/Exonuclease, unwindase, recombinase and ligase. The complex looks like a zipper fashion.
- Crossing over happens in the pachytene phase (longest cycle). The genetic exchange of non-sister chromatids of a homologous chromosome is crossing over.
- During diplotene, a temporary structure (X-Shaped structure) formed by homologous chromosomes called as chiasmata is seen.
- Terminalization of chiasmata is noted in diakinesis with the disappearing of nuclear membrane and formation of spindle apparatus.
- Further steps are similar to mitosis.
- In meiosis, one-time replication and 2 times division is observed.
- Crossing over is the main event for the formation of new species and evolution.
- 2 Cells are formed at the end of meiosis I, which enters meiosis II to further divide to finally form 4 cells, which are genetically dissimilar.
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