Mitosis and Meiosis Quiz

See Generation of genetic diversity. Option D is correct. Spermatogenesis is the production of sperm in the seminiferous tubules of the testis (the coiled tubes where sperm form). It starts with spermatogonia, the stem cells of the testis, which divide by mitosis and produce primary spermatocytes. Each primary spermatocyte then undergoes meiosis I to give two secondary spermatocytes, and each of those undergoes meiosis II, so that one primary spermatocyte yields four haploid spermatids (cells with 23 chromosomes). Early spermatids still look like ordinary round cells, so a final step called spermiogenesis reduces the cytoplasm and begins forming the parts of a true sperm. The whole sequence from spermatogonium to formed sperm takes about 64 days, which is roughly two months, and a new cycle starts about every 16 days, so sperm are produced continuously from puberty onwards. This is a clear contrast with oogenesis: each primary spermatocyte gives four functional gametes, whereas each primary oocyte gives only one. A memory aid is to remember four sperm from one spermatocyte, one egg from one oocyte.

• Four gametes: one primary spermatocyte gives four haploid spermatids through meiosis I and II.
• Timing: about 64 days from spermatogonium to formed sperm, roughly two months.
• Site and onset: seminiferous tubules of the testis, continuous from puberty.