Gametogenesis and Fertilization

Figure 2-16 Human spermatogenesis in relation to the two meiotic divisions. The sequence of events begins at puberty and takes approximately 64 days to be completed. The chromosome number (46 or 23) and the sex chromosome constitution (X or Y) of each cell are shown. See Sources & Acknowledgments.

As discussed earlier, normal meiosis requires pairing of homologous chromosomes followed by recombination. The autosomes and the X chromosomes in females present no unusual difficulties in this regard; but what of the X and Y chromosomes during spermatogenesis? Although the X and Y chromosomes are different and are not homologues in a strict sense, they do have relatively short identical segments at the ends of their respective short arms (Xp and Yp) and long arms (Xq and Yq) (see Chapter 6). Pairing and crossing over occurs in both regions during meiosis I. These homologous segments are called pseudoautosomal to reflect their autosome-like pairing and recombination behavior, despite being on different sex chromosomes.



Figure 2-17 Human oogenesis and fertilization in relation to the two meiotic divisions. The primary oocytes are formed prenatally and remain suspended in prophase of meiosis I for years until the onset of puberty. An oocyte completes meiosis I as its follicle matures, resulting in a secondary oocyte and the first polar body. After ovulation, each oocyte continues to metaphase of meiosis II. Meiosis II is completed only if fertilization occurs, resulting in a fertilized mature ovum and the second polar body.

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Nov 27, 2016 | Posted by in GENERAL & FAMILY MEDICINE | Comments Off on Gametogenesis and Fertilization

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