Fertilization to birth — A/L Biology

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1 question with a worked solution

1. What is the primary cellular mechanism that establishes the slow block to polyspermy immediately following the fusion of a sperm with the human egg membrane?
   1. A) Rapid depolarization of the ovum plasma membrane by sodium influx
   2. B) Secretion of human placental lactogen by trophoblast cells
   3. C) Immediate breakdown of the acrosomal vesicle by hyaluronidase
   4. D) Cortical reaction resulting in the hardening of the zona pellucida
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   Correct answer: D) Cortical reaction resulting in the hardening of the zona pellucida

   Upon sperm-egg membrane fusion, an intracellular calcium wave triggers exocytosis of cortical granules (cortical reaction). The released enzymes modify sperm receptors and harden the zona pellucida into a fertilization envelope, establishing the permanent slow block to polyspermy.

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   Step 1: Differentiate blocks to polyspermy: Fast block = transient membrane depolarization (Na⁺ influx, mainly in marine invertebrates); Slow block = cortical reaction. Step 2: Cortical reaction enzymes enzymatically harden the zona pellucida. Answer: Cortical reaction resulting in the hardening of the zona pellucida.
   Modelled on: the A/L Biology examination pattern

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Where does Fertilization to birth fit in the syllabus?

Year 2 · Term 1