Females are born with a set number of primary oocytes at birth. Oocytes are immature eggs, and oogenesis results in the formation of primary oocytes during the fetal period.
Meiosis I only occurs once a month in females. Primary oocytes enter meiosis I as diploid and are reduced to haploid secondary oocytes. Genetic diversification occurs here because of crossing over and independent assortment.
The products of meiosis I are a secondary oocyte and one polar body. The polar body is produced because the secondary oocyte takes a disproportionate amount of cytoplasm. The polar body, subsequently, does not undergo any additional divisions.
Secondary oocytes are frozen in metaphase II (metaphase in meiosis II) until fertilization occurs (if it does occur). If fertilization occurs, an ootid and another polar body are produced.
Ovulation is the phase of the menstrual cycle where a mature egg is released into the oviduct from the ovarian follicles. Afterwards, it is available for fertilization by sperm.
The secondary oocyte requires fertilization in order to finish meiosis II and proceed along the developmental pathway.
Meiosis II finishes upon fertilization by sperm. Once this occurs, the ootid that leaves meiosis II can further develop.
The ootid develops into a mature ovum after meiosis II, which is a haploid female gamete.
The nuclei of the ovum and the penetrating sperm fuse which results in zygote formation. The zygote is now diploid and is the starting cell of the child.
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