Problem 4 - Campbell Biology

Question

The decline of MPF activity at the end of mitosis is due to \begin{equation}\begin{array}{l}{\text { (A) the destruction of the protein kinase Cdk. }} \\ {\text { (B) decreased synthesis of Cdk. }} \\ {\text { (C) the degradation of cyclin. }} \\ {\text { (D) the accumulation of cyclin. }}\end{array}\end{equation}

Step-by-Step Solution

Answer

(C) the degradation of cyclin

Step 1: Identify MPF Components

MPF (Maturation Promoting Factor) is composed of two main components: Cyclin and Cyclin-dependent kinase (Cdk).

Step 2: Understand MPF Activity

MPF activity is regulated by the concentration of cyclin. When cyclin levels rise, they bind to Cdk, forming active MPF. Conversely, a decline in cyclin leads to a decrease in MPF activity.

Step 3: Link Cyclin Degradation to MPF Activity

At the end of mitosis, cyclin is ubiquitinated and subsequently degraded by the proteasome. This degradation of cyclin causes a decline in MPF activity.

Step 4: Determine the Correct Answer

Given that MPF activity declines due to the degradation of cyclin, the correct answer is (C) the degradation of cyclin.

Key Concepts

cyclin degradation

Cyclin degradation is a vital process in cell cycle regulation. Cyclins are proteins that control the progression of cells through the cell cycle by activating Cyclin-dependent kinases (Cdks). These cyclins undergo a tightly regulated process of synthesis and degradation to ensure cells move smoothly from one phase of the cell cycle to the next.
At the end of mitosis, cyclin is tagged for destruction by a sequence called ubiquitination. This tag directs cyclin to the proteasome, a protein complex responsible for breaking down unneeded or damaged proteins. As cyclin is degraded, its concentration drops, leading to the inactivation of Cdk. This ensures that the cell exits mitosis properly and does not immediately re-enter the cycle.
For MPF (Maturation Promoting Factor), the degradation of cyclin is crucial because it directly impacts MPF activity. When cyclin's levels fall, MPF activity declines, marking the end of mitosis. Hence, cyclin degradation is a key step in regulating the proper progression of the cell cycle.

maturation promoting factor

Maturation Promoting Factor, commonly known as MPF, is essential for driving a cell from the G2 phase into the M phase of the cell cycle. MPF is a complex made up of Cyclin and Cdk (Cyclin-dependent kinase).
When cyclins bind to Cdk, MPF becomes active. This activation triggers a range of processes that lead to mitosis. For example, MPF helps initiate the breakdown of the nuclear envelope, chromosome condensation, and the formation of the mitotic spindle.
MPF activity is tightly regulated by the levels of cyclin in the cell. When cyclins are present, they bind and activate Cdks, forming active MPF and driving the cell into mitosis. The decline in MPF activity at the end of mitosis, due to cyclin degradation, ensures that the cell can exit mitosis and reset for the next cycle. Without this regulation, cells would not be able to progress through the cell cycle correctly, leading to potential issues in cell division and growth.

cell cycle regulation

Cell cycle regulation is the series of tightly controlled events that ensure a cell replicates and divides accurately. The cell cycle is divided into four main phases: G1 (growth), S (DNA synthesis), G2 (growth and preparation for mitosis), and M (mitosis).
Various checkpoints exist within the cell cycle to check for errors before the cell proceeds to the next phase. These checkpoints are primarily regulated by Cyclins and Cdks. Each phase and checkpoint ensure that cells only divide when they are ready and that any DNA damage is repaired.
If the cell detects errors, such as DNA damage, the cycle can be halted to allow for repair or, if the damage is excessive, to trigger apoptosis (programmed cell death). The regulated synthesis and degradation of cyclins ensure that the cell cycle proceeds in a controlled manner. A key example is the role of MPF, whose activity is governed by cyclin levels. This careful orchestration prevents uncontrolled cell division, which could lead to conditions such as cancer.

mitosis

Mitosis is the process by which a cell divides its genetic material and cytoplasm into two daughter cells. It is crucial for growth, development, and tissue repair in multicellular organisms. Mitosis is divided into several stages: prophase, metaphase, anaphase, and telophase.
During prophase, the chromatin condenses into visible chromosomes. Each chromosome has two sister chromatids joined at the centromere. The mitotic spindle forms, and the nuclear envelope breaks down.
In metaphase, chromosomes align at the cell's equator, connected to spindle fibers. During anaphase, the chromatids are pulled apart to opposite poles of the cell. Finally, in telophase, the chromosomes de-condense and nuclear envelopes re-form around each set of chromosomes. Following telophase, cytokinesis occurs, dividing the cytoplasm and resulting in two distinct daughter cells.
The regulation of mitosis is critical for maintaining genomic stability. Proper control mechanisms, such as the decline in MPF activity at the end of mitosis due to cyclin degradation, ensure that cells divide accurately and efficiently. Solid understanding of these stages and regulations helps highlight the complexity and importance of cell cycle control.