A comparison of sperm production and egg production in humans

(a) Beginning at puberty, some germ cells enter meiosis as primary spermatocytes. After meiosis I, the secondary spermatocytes contain 23 chromosomes composed of sister chromatids joined by a common centromere. After the second division (meiosis II), the haploid spermatids undergo developmental changes and become mature sperm. (b) In oogenesis, cells enter meiosis as primary oocytes during embryonic development and arrest in meiosis I. After puberty, usually one oocyte per menstrual cycle completes meiosis I just before ovulation. Formation of the haploid secondary oocyte is accompanied by unequal cytokinesis to produce a polar body, which is nonfunctional. Meiosis is completed only if the secondary oocyte is fertilized, when penetration of the sperm stimulates meiosis II and the second division.