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What keeps the other sperm from fertilizing the egg once 1 sperm has penetrated?

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Final answer:

The egg releases proteins to prevent multiple sperm from fertilizing it. This is achieved through the fast block and slow block mechanisms, which prevent the fusion of additional sperm. The slow block involves the release of inhibiting proteins and mucopolysaccharides that destroy sperm receptors and create a protective barrier around the zygote.

Step-by-step explanation:

When the first sperm fuses with the egg, the egg releases proteins to prevent other sperm from fusing with it. This prevents polyspermy, which is the penetration of the egg by more than one sperm. The egg deploys two mechanisms to ensure only one sperm fertilizes it. The fast block involves an instant change in sodium ion permeability, preventing additional sperm from fusing. The slow block is initiated by an influx of calcium ions and involves the release of inhibiting proteins and mucopolysaccharides that destroy the sperm receptors and coat the zygote in an impenetrable barrier.

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