Under which stage of meiosis does chromosome number reduce by half?
MEIOSIS I Meiosis is the process by which replicated chromosomes undergo two nuclear divisions to produce four haploid cells, also called meiocytes (sperms and eggs). Diploid (2n) organisms rely on meiosis to produce meiocytes, which have half the ploidy of the parents, for sexual reproduction. Halving the ploidy in meiocytes is essential for restoring the genetic content of the zygote to that of the parents. Meiosis uses similar mechanisms as those employed during mitosis to accomplish the separation and redistribution of chromosomes. However, several features, namely, the pairing and genetic recombination between homologous chromosomes, are unique to meiosis. The steps leading up to meiosis are similar to those of mitosis – the centrioles and chromosomes are replicated. The amount of DNA in the cell has doubled, and the ploidy of the cell remains the same as before, at 2n. In meiosis I, the phases are analogous to mitosis: prophase I, metaphase I, anaphase I, and telophase I (below figure). Meiosis I proceeds directly to meiosis II without going through interphase.
Meiosis I is unique in that genetic diversity is generated through crossing over and random positioning of homologous chromosomes (bivalent chromosomes). In addition, in meiosis I, the chromosomal number is reduced from diploid (2n) to haploid (n) during this process. (See figure below, where meiosis I begins with a diploid (2n = 4) cell and ends with two haploid (n = 2) cells.) In humans (2n= 46), who have 23 pairs of chromosomes, the number of chromosomes is reduced by half at the end of meiosis I (n= 23).
The steps leading up to meiosis are similar to those of mitosis – the centrioles and chromosomes are replicated. The amount of DNA in the cell has doubled, and the ploidy of the cell remains the same as before, at 2n. In meiosis I, the phases are analogous to mitosis: prophase I, metaphase I, anaphase I, and telophase I (below figure). Meiosis I proceeds directly to meiosis II without going through interphase.
Meiosis I is unique in that genetic diversity is generated through crossing over and random positioning of homologous chromosomes (bivalent chromosomes). In addition, in meiosis I, the chromosomal number is reduced from diploid (2n) to haploid (n) during this process. (See figure below, where meiosis I begins with a diploid (2n = 4) cell and ends with two haploid (n = 2) cells.) In humans (2n= 46), who have 23 pairs of chromosomes, the number of chromosomes is reduced by half at the end of meiosis I (n= 23).
