between parts of non-sister chromatids allows for genetic recombination. This results in the genetic variance in horses. B) Metaphase I: Tetrads line up along the midline of the cell and microtubules connect their centromeres with centrosomes at opposite poles. C) Anaphase I: each homologous chromosome pair separate to opposite poles of the cell. Centromeres do NOT split; therefore, the chromosome pairs remain attached. D) Telophase I & cytokinesis: cleavage furrow forms and pinches parent cells into 2 daughter cells, each with 1/2 of the parent chromosome. The net effect of meiosis I is that each cell contains a haploid number of chromosomes because it only contains 1 member of each pair of chromosomes that were present in the starting cell.
Meiosis II: immediately follows meiosis I and is comparable to the stages of mitosis.
A) Prophase II: chromatin condenses into chromosomes and the nucleus disintegrates.
B) Metaphase II: chromosomes line up at the midline and mitotic spindle is formed from the centrosomes.
C) Anaphase II: chromosomes separate to opposite poles and the centromeres split.
D) Telophase II & cytokinesis: cleavage furrow forms and divides cell into 2 daughter cells, each with ½ of the parent chromosome. In summary, meiosis I begins with 1 diploid (2n) cell and ends with 2 cells, each with the haploid (n) number of chromosomes. During meiosis II, each of the haploid cells divides and the net result is 4 haploid gametes that are genetically different from the original diploid starting cell.
Spermatogenesis
Spermatogenesis is the formation of sperm and begins with spermatogonia (stem cell which undergoes mitosis) which contains the diploid number of chromosomes. They enter the seminiferous tubules of the testes during their 5 th week of development. Spermatogonia undergo more developmental changes and differentiate into primary spermatocytes . Like spermatogonia, primary spermatocytes are diploid. Primary spermatocytes then replicate its DNA and undergo meiosis. Again, in meiosis I chromosome pairs line up at the metaphase plate and cross over. The meitotic spindle pulls 1 duplicated chromosome of each pair to an opposite pole of the dividing cell. The 2 cells formed by meiosis I are called secondary spermatocytes, each with having 32 chromosomes of haploid.
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