Figure 1.

Gametogenesis in yeast and female mice. (A) Outline of the lifecycle of the budding yeast S. cerevisiae, which can propagate vegetatively through mitotic division in both its haploid and diploid form. Haploid yeast of opposite mating types (a and alpha) mate and undergo conjugation to generate a zygote, upon which nuclei fuse (karyogamy). Starvation of diploid cells triggers meiosis, culminating in the production of four haploid spores. Note that the lifestyle of S. pombe is similar, except that meiosis and sporulation occur directly after karyogamy so that diploid cells are short-lived. (B) Oocyte differentiation in female mice. Gametes are generated from diploid germ cells, in the female embryo before birth for oocytes, and in the adult male for spermatocytes. After hormonal stimulation in adult females, some oocytes that are arrested in prophase I undergo a growth phase. During the menstrual cycle usually one (humans) or several (mouse) oocytes are induced to undergo meiosis I and enter meiosis II, where they will remain arrested to await fertilization with a male gamete that has already finished meiosis I and II and has progressed into G1 phase. Oocytes exit meiosis II after fertilization, and the male and female pronucleus fuse to form the diploid zygote, the first cell of the embryo.