Which one of the following assures prevention of polyspermy in an egg?
(1) Activation of PI3 kinase
(2)Cortical reaction
(3) Acrosome reaction
(4) Cross-linking between ZP1 and ZP3
Prevention of Polyspermy: How Does the Egg Ensure Successful Fertilization?
Polyspermy is the condition in which an egg is fertilized by more than one sperm, leading to an abnormal number of chromosomes and often resulting in the death of the embryo. To prevent this, the egg has evolved several mechanisms that ensure only one sperm enters and fertilizes the egg. In this article, we’ll explore the primary mechanisms that prevent polyspermy, with a focus on the cortical reaction, and answer a common question:
Which one of the following assures the prevention of polyspermy in an egg?
Options:
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Activation of PI3 kinase
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Cortical reaction
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Acrosome reaction
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Cross-linking between ZP1 and ZP3
✅ Correct Answer: (2) Cortical Reaction
What is Polyspermy?
Before diving into how the egg prevents polyspermy, let’s first understand what it is:
Polyspermy occurs when multiple sperm enter an egg during fertilization. This results in an abnormal number of chromosomes, which can lead to genetic abnormalities, embryo death, or developmental issues. In most animals, the egg has mechanisms to ensure that only one sperm fertilizes it.
How Does the Egg Prevent Polyspermy?
There are several mechanisms that ensure only one sperm fertilizes an egg, and the cortical reaction is one of the most critical processes. Let’s break down the options.
1. Cortical Reaction: The Key to Polyspermy Prevention
The cortical reaction is a rapid and critical process that occurs immediately after sperm binding to the egg’s surface. It involves the release of enzymes from the cortical granules inside the egg. These enzymes modify the zona pellucida (ZP), a protective glycoprotein layer surrounding the egg, making it impermeable to additional sperm. This ensures that once one sperm has fertilized the egg, no other sperm can enter.
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How it works:
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When the sperm fuses with the egg’s plasma membrane, the cortical granules under the egg’s membrane are triggered.
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These granules release enzymes that alter the zona pellucida, causing it to harden and change its structure.
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This reaction prevents other sperm from binding and entering the egg, effectively blocking polyspermy.
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This is the primary mechanism that ensures only one sperm fertilizes the egg and prevents multiple sperm from entering.
2. Acrosome Reaction
The acrosome reaction occurs when the sperm encounters the egg. It involves the release of enzymes from the sperm’s acrosome (a cap-like structure). These enzymes help the sperm penetrate the zona pellucida and reach the egg membrane for fertilization. While important for sperm entry, the acrosome reaction does not directly prevent polyspermy once the sperm has already entered the egg.
3. Activation of PI3 Kinase
PI3 kinase activation is a signaling pathway that plays a role in several cellular processes, including cell growth and survival. While it is involved in egg activation during fertilization, it does not directly participate in the prevention of polyspermy. Its role is more related to metabolic changes and egg activation after fertilization.
4. Cross-linking between ZP1 and ZP3
ZP1 and ZP3 are proteins in the zona pellucida, the egg’s protective layer. ZP3 is responsible for sperm binding, while ZP1 helps to maintain the structure of the zona. Although the zona pellucida plays a key role in fertilization, it is the cortical reaction that modifies the zona to prevent polyspermy. Cross-linking between ZP1 and ZP3 contributes to zona stability, but it does not directly block additional sperm after fertilization.
Why is the Cortical Reaction So Important?
The cortical reaction is the primary safeguard against polyspermy. Here’s why:
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Immediate Action: The cortical reaction occurs quickly after sperm-egg fusion, ensuring that no additional sperm can enter the egg.
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Permanent Change: The changes in the zona pellucida are permanent, making the egg impenetrable to other sperm after fertilization.
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Egg Protection: This reaction protects the developing embryo from genetic abnormalities caused by an incorrect number of chromosomes.
Other Mechanisms for Polyspermy Prevention
While the cortical reaction is the main mechanism, some organisms also use electrical changes in the egg membrane to prevent polyspermy. These electrical changes occur immediately after sperm entry and create a temporary block to further sperm binding.
Conclusion
The prevention of polyspermy is essential for successful fertilization, and the cortical reaction is the key mechanism that ensures only one sperm fertilizes the egg. After sperm binding, the cortical reaction leads to the modification of the zona pellucida, making it impermeable to additional sperm.
So, the correct answer to the question “Which one of the following assures the prevention of polyspermy in an egg?” is:
(2) Cortical reaction
Understanding the cortical reaction and its role in egg fertilization highlights the complexity of cellular processes that ensure successful reproduction and genetic stability
