Problem 8 - Campbell Biology

Question

The drug cytochalasin B blocks the function of actin. Which of the following aspects of the animal cell cycle would be most disrupted by cytochalasin B? \begin{equation}\begin{array}{l}{\text { (A) spindle formation }} \\ {\text { (B) spindle attachment to kinetochores }} \\ {\text { (C) cell elongation during anaphase }} \\ {\text { (D) cleavage furrow formation and cytokinesis }}\end{array}\end{equation}

Step-by-Step Solution

Answer

Option (D) cleavage furrow formation and cytokinesis

Step 1 - Understand the function of actin

Actin is a protein that is part of the cytoskeleton in eukaryotic cells. It plays a crucial role in various cellular processes, including maintaining cell shape, enabling cell motility, and facilitating cell division.

Step 2 - Be aware of what cytochalasin B does

Cytochalasin B is a drug known to inhibit the polymerization of actin. This means it stops actin from forming its usual structures and performing its regular functions.

Step 3 - Review the stages of the animal cell cycle

The animal cell cycle contains several stages including interphase (where the cell prepares for division), mitosis (where the nucleus divides), and cytokinesis (where the cell itself divides).

Step 4 - Identify processes that involve actin in the cell cycle

During the cell cycle, actin is particularly important during cytokinesis, where it helps form the cleavage furrow that ultimately divides the cell into two daughter cells.

Step 5 - Evaluate each option in the context of actin's role

Now, let's analyze each option: (A) Spindle formation is primarily related to microtubules, not actin. (B) Spindle attachment to kinetochores is also related to microtubules. (C) Cell elongation during anaphase is mainly driven by microtubules. (D) Cleavage furrow formation and cytokinesis directly require actin for the formation of the cleavage furrow.

Step 6 - Choose the correct answer based on the analysis

Given that actin is essential for cleavage furrow formation during cytokinesis, the process most disrupted by cytochalasin B would be option (D).

Key Concepts

actin function

Actin is a fundamental protein found in the cytoskeleton of eukaryotic cells. It has various roles, including maintaining cell shape, enabling cell movement, and facilitating cell division. The cytoskeleton, supported by actin structures, provides the cell with mechanical support.
Every time cells divide, particularly during cytokinesis, actin is crucial. This protein is part of contractile rings that form around the inside of the cell membrane, allowing the cell to pinch in two.
Moreover, actin filaments are dynamic—they can grow and shrink. This ability is vital for their functions, including cell motility and response to external signals.

cytochalasin B

Cytochalasin B is known for its ability to disrupt actin function. It achieves this by inhibiting the polymerization of actin filaments. In simpler terms, cytochalasin B prevents actin from forming the structures it needs to function properly.
This interference can have significant consequences, especially during crucial cellular processes. For instance, if actin filaments cannot form or function correctly, cells may struggle to divide properly.
Specifically, in the context of cell division, cytochalasin B's inhibition of actin polymerization directly affects cytokinesis—blocking the cell from successfully forming two distinct cells.

cytokinesis

Cytokinesis is the final stage of the cell cycle. This is where the cell physically divides into two daughter cells. Following mitosis, where the nuclear contents are divided, cytokinesis involves the separation of the cytoplasm and other organelles.
Actin plays an integral role in this process. It forms what is known as the contractile ring—comprised mainly of actin filaments and myosin. This ring contracts, leading to the pinching of the cell membrane and the eventual split of the cell.
If actin function is disrupted by agents like cytochalasin B, the contractile ring cannot form, preventing cytokinesis from occurring efficiently.

cleavage furrow formation

The cleavage furrow is a critical structure during cytokinesis. It is the indentation that begins to appear on the cell's surface as the contractile ring tightens.
As the actin-myosin ring contracts, the membrane is drawn inward to create this furrow, which deepens until the cell splits into two new daughter cells. This entire process is heavily reliant on the dynamic polymerization and depolymerization of actin.
When cytochalasin B inhibits actin function, the cleavage furrow cannot form properly. This failure hinders the cell from completing division, demonstrating cytochalasin B's impact on the cell cycle.