Problem 1 - Campbell Biology

Question

Knowledge/Comprehension 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

The cortical reaction directly causes (A) the formation of a fertilization envelope.

Step 1 - Understand the Cortical Reaction

The cortical reaction in sea urchin eggs involves the release of cortical granules following the penetration of the egg by a sperm. These granules release enzymes that modify the egg's exterior.

Step 2 - Identify The Purpose of The Cortical Reaction

The primary function of the cortical reaction is to prevent additional sperm from fertilizing the egg, a process known as polyspermy.

Step 3 - Evaluate The Options

Option A: The formation of a fertilization envelope is indeed a result of the cortical reaction. Option B: Production of a fast block to polyspermy is related but not a direct function. Option C: The generation of an electrical impulse is involved in the fast block to polyspermy. Option D: The fusion of egg and sperm nuclei happens after the cortical reaction but is not a direct function of it.

Step 4 - Select The Best Answer

Given that the cortical reaction directly prevents polyspermy by forming a fertilization envelope, the correct answer is (A) the formation of a fertilization envelope.

Key Concepts

fertilization envelope

When a sea urchin egg is fertilized by a sperm, immediate changes occur to ensure no other sperm can penetrate the egg. A crucial part of this defense mechanism is the creation of the fertilization envelope. This envelope forms as a direct result of the cortical reaction, where cortical granules release enzymes that alter the egg's membrane and surrounding layers.
This alteration makes the outer layer of the egg hard and impenetrable, thus establishing the fertilization envelope.
The formation of this envelope is crucial as it acts as a barrier to ensure that only one sperm fertilizes the egg, thereby maintaining the correct number of chromosomes for the developing embryo.

prevention of polyspermy

Preventing multiple sperms from fertilizing the same egg is vital for healthy development. This problem is known as polyspermy, and the sea urchin egg has evolved several mechanisms to stop it.
The cortical reaction is paramount among these defenses. Upon entry of the first sperm, the egg releases cortical granules that prompt the formation of the fertilization envelope.
This reaction creates a physical and chemical barrier around the egg to block additional sperm. By doing so, the process ensures that the egg undergoes proper chromosomal separation and development. Without these mechanisms, multiple sperms could fertilize the egg, leading to genetic complications.

cortical granules

Cortical granules are specialized vesicles located just beneath the plasma membrane of the egg. They play a critical role during fertilization by undergoing exocytosis shortly after sperm entry.
The granules release their contents, which include enzymes and other chemicals, into the space between the plasma membrane and the vitelline layer. These enzymes modify the egg's outer layers, making it impermeable to other sperms.

• The enzymes alter the properties of the vitelline layer.
• The released substances cause the hardening of this layer, forming the fertilization envelope.
• This process ensures that only one sperm can fertilize the egg.

The strategic release and action of cortical granules are key to successful fertilization and prevention of polyspermy.