NEET 2020 Botany Question 31 Solution

Human female gametogenesis exhibits two prolonged developmental arrests: 1. First Meiotic Arrest: Primary oocytes enter meiosis I during fetal gestation and remain arrested at the diplotene stage of prophase I until puberty. 2. Resumption at Puberty: Each menstrual cycle, the pre-ovulatory LH surge triggers completion of meiosis I, producing a large haploid secondary oocyte ($n$) and a small first polar body. 3. Second Meiotic Arrest (Metaphase II): The secondary oocyte promptly enters meiosis II, but arrest occurs at metaphase II mediated by cytostatic factor (CSF) and maturation-promoting factor (MPF). 4. Ovulation: The secondary oocyte is ovulated into the fallopian tube in this arrested metaphase II condition.

When the fertilizing spermatozoon binds to the oolemma and fuses with the egg plasma membrane, a wave of intracellular calcium ($Ca^{2+}$) release activates the anaphase-promoting complex/cyclosome (APC/C). This degrades cyclin B, breaking the metaphase II arrest. Meiosis II is completed, extruding the second polar body and forming the mature haploid female pronucleus.