Problem 1 - Campbell Biology

Question

The cortical reaction of sea urchin eggs functions directly in (A) the formation of a fertilization envelope. (B) the production of a fast block to polyspermy. (C) the generation of an electrical impulse by the egg. (D) the fusion of egg and sperm nuclei.

Step-by-Step Solution

Answer

(A) The formation of a fertilization envelope.

Step 1: Understanding the Cortical Reaction

The cortical reaction is a process that occurs in the egg cells of many animals, including sea urchins. It involves the exocytosis of cortical granules from the egg's cortex. The main function of this reaction is to prevent polyspermy, which is the fertilization of an egg by multiple sperm.

Step 2: Analyzing the Answer Choices

Review each of the given choices to see which one directly relates to the function of the cortical reaction.(A) Formation of a fertilization envelope.(B) Production of a fast block to polyspermy.(C) Generation of an electrical impulse by the egg.(D) Fusion of egg and sperm nuclei.

Step 3: Evaluating Each Choice

Now, evaluate each choice:(A) The cortical reaction primarily results in the formation of a fertilization envelope that blocks additional sperm from entering.(B) While the cortical reaction aids in blocking polyspermy, a 'fast block' is achieved through an electrical change soon after sperm entry, not via the cortical reaction.(C) Electrical impulses are involved in the 'fast block' but are not a direct result of the cortical reaction itself.(D) Fusion of egg and sperm nuclei occurs after the cortical reaction but is not its direct function.

Step 4: Conclusion

Given the evaluation, the authentic function of the cortical reaction in the context provided is the formation of a fertilization envelope, which directly prevents polyspermy by establishing a physical barrier.

Key Concepts

Fertilization Envelope

Once a sea urchin egg is fertilized by a sperm, a crucial process called the cortical reaction occurs. This reaction primarily leads to the formation of a fertilization envelope. Think of this envelope as a protective barrier that forms around the egg.
This physical barrier is essential because it keeps out additional sperm, ensuring that only one sperm fertilizes the egg.
Without this envelope, multiple sperm could enter the egg, leading to developmental problems. This barrier is vital for successful fertilization and subsequent development of the embryo.

Polyspermy Prevention

Preventing polyspermy, or the fertilization of an egg by multiple sperm, is critical for the healthy development of an organism. The cortical reaction directly helps to achieve this by forming the fertilization envelope. When multiple sperm penetrate an egg, the resulting zygote may have an abnormal number of chromosomes, leading to inviable or defective embryos.
There are two main blocks to polyspermy:

The fast block, which is an electrical change immediately after the first sperm enters the egg, preventing other sperm from fusing with the egg's membrane.

The slow block, which involves the cortical reaction and the formation of the fertilization envelope.

The slow block creates a long-term barrier, ensuring that no additional sperm can fertilize the egg after the initial sperm has entered.

Cortical Granules Exocytosis

The cortical reaction in sea urchin eggs involves the exocytosis of cortical granules found inside the egg.
When the first sperm penetrates the egg, these cortical granules move to the egg's surface and release their contents into the space outside the egg membrane.
The substances released by these granules cause changes in the outer layers of the egg, leading to the hardening and swelling of the fertilization envelope.
This process ensures that no other sperm can penetrate the egg, establishing a foolproof mechanism against polyspermy. Exocytosis of the cortical granules is a well-coordinated and rapid response crucial for the integrity and future development of the fertilized egg.