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PinkMonkey Online Study Guide-Biology
This division occurs simultaneously in both the nuclei formed as a result of first meiotic division. It is essentially similar to mitotic division and shows following four steps:
Prophase II : The nuclear membrane begins to disappear and chromosomes get shortened. Each chromosome showing two sister chromatids attached at a single undivided centromere.
Metaphase II : The spindle is formed usually at right angle to the plane of the spindle formed in Meiosis I. The chromosomes get arranged in an equatorial plane with centromeres attached to spindle fibers.
Anaphase II : The centromere of each chromosome now divides for the first time separating sister chromatids which travel to the opposite poles.
Telophase II : The set of chromatids at each pole organizes into a new daughter nucleus with the reappearance of nuclear membrane and nucleolus. Thus four daughter nuclei are formed each containing haploid number of chromosomes.
Cytokinesis follows and four daughter cells are formed (tetrads) at the end of Meiosis-II, each with the nucleus containing haploid number of chromosomes. Meiosis, therefore, is a profound modification of mitosis in which the chromosomes first pair and then divide to form four haploid cells.
Significance of Meiosis
(i) It compensates for the doubling of chromosomes at gametic fusion and thus helps to keep the number of chromosomes constant, characteristic of each species.
(ii) It is responsible for giving rise to large number of variations in the offsprings by crossing over and random distribution of maternal and paternal chromosomes.
(iii) The variations coupled with heredity form the raw
material for organic evolution. If it was not for meiosis, the evolution
of species would be suspended.
MEIOSIS - II
Figure 14.19 Cell division : meiosis II.
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