Only custom-written papers / Professional writers / Always on-time delivery
Gametogenesis begins in the seminiferous tubules of the testis, in human males. Meiosis begins with a diploid cell, spermatogonium, which divides through mitosis. The spermatogonia accumulate cytoplasm and replicate DNA. They are then called primary spermatocytes. In meiosis I, the primary spermatocytes’ chromosomes undergo synapsis, in which homologous pairs of chromosomes find each other, during Prophase I. Crossing over also takes place during Prophase I.
In Metaphase I, independent assortment happens as chromosome pairs line up side by side randomly at the equator of the cell and attach to the spindle fibers. During Anaphase I the chromosome pairs are pulled apart from each other, and following Telophase I two haploid cells form. The cells are now known as secondary spermatocytes. During Meiosis II, the chromosome pairs line up at the equator of the cell (Metaphase II), and are pulled apart during Anaphase II, separating into two daughter cells following Telophase II. They resulting cells are spermatids. The spermatids develop a flagellum and then they are mature spermatozoa (sperm).
Human female Gametogenesis occurs in the ovaries. A female embryo has about 2 million primary oocytes, and about 400,000 remain at puberty. After being arrested in Prophase I since birth, a female’s oocytes complete meiosis I one or a few at a time each month with ovulation. After ovulation, the secondary oocyte is arrested in Metaphase I. If fertilization occurs, then meiosis will be completed and the zygote will develop. Meiosis in females yields one oocyte for every primary oocyte and the other 3 resulting haploid cells are called polar bodies, and are absorbed by the female’s body.
Males produce millions of sperm about every 74 days, but females have only about 400,000 primary oocytes. Excluding crossing over, the chances of producing the same zygote from the same two parents is 223 multiplied by 223.