- Meiosis is the general name given to two successive nuclear divisions called meiosis I and meiosis II.
- Meiosis takes place in special diploid cells called meiocytes.
- Because of the two successive divisions, each meiocyte cell gives rise to four cells, 1 cell →2 cells →4 cells. The four cells are called products of meiosis.
- In animals and plants, the products of meiosis become the haploid gametes.
- In humans and other animals, meiosis takes place in the gonads, and the products of meiosis are the gametes—sperm (more properly, spermatozoa) and eggs (ova).
- In flowering plants, meiosis takes place in the anthers and ovaries, and the products of meiosis are meiospores, which eventually give rise to gametes.
- Before meiosis, an S phase duplicates each chromosome’s DNA to form sister chromatids, just as in mitosis.
Stages of Meiosis:-
Prophase I: As in mitosis, the sister chromatids become visible, closely adhered side by side. However, in contrast with mitosis, the sister chromatids (although fully replicated at the DNA level) show an apparently undivided centromere. The sister chromatid pairs at this stage are called dyads, from the Greek word for “two.”
Metaphase I: The homologous dyads now pair to form structures called bivalents. Thus, any one bivalent contains a total of four chromatids, sometimes referred to as a tetrad (Greek; four). This stage represents the most obvious difference from mitosis.
Pairing of the dyads to form a bivalent is accomplished by molecular assemblages called synaptonemal complexes along the middle of the tetrads.
Although the existence of synaptonemal complexes has been known for some time, the precise working of these structures is still a topic of research.
Nonsister chromatids of the tetrad engage in a breakage-and-reunion process called crossing over, The crossover is visible as two chromatids crossing each other to form a structure called a chiasma (plural, chiasmata).
For those species that have been carefully studied, a minimum of one crossover per tetrad is known to be a necessary prelude to subsequent orderly chromosome separation.
Anaphase I: Each of the two pairs of sister chromatids (dyads) is pulled to a different pole.
Telophase I: A nucleus forms at each pole.
Prophase II: The dyads reappear.
Metaphase II: The dyads move to the equatorial plane.
Anaphase II: Each of the sister chromatids of a dyad is pulled into a different daughter nucleus as the cells divide for a second time.
We see that the fundamental events of meiosis are DNA replication and sister chromatid adhesion, followed by homologous pairing, segregation, and then another segregation.
Hence, within a single cell, the number of copies of a chromosome of the same type goes from 2 → 4 → 2 → 1, and each product of meiosis must therefore contain one chromosome of each type, half the number of the original meiocyte.
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