USA Biology Olympiad (USABO) Practice Exam

What is the consequence of a mutation in cyclin D within the cell cycle regulatory system?

1. Cell cycle arrest

2. Loss of cyclin-dependent kinase activity

3. Increased cell proliferation

4. Reduced apoptosis

The correct answer is: Increased cell proliferation

A mutation in cyclin D can lead to increased cell proliferation. Cyclin D is an essential protein that helps regulate the cell cycle by partnering with cyclin-dependent kinases (CDKs), particularly CDK4 and CDK6, to drive the cell from the G1 phase into the S phase, where DNA replication occurs. When cyclin D is mutated, it may become overactive or deregulated, leading to unrestrained activation of CDK4/6. This situation can push the cell cycle forward without the usual checks and balances. As a result, the normal regulatory processes that prevent uncontrolled cell division may be bypassed, contributing to increased cell proliferation. In a healthy cell, cyclin D levels are tightly regulated to ensure proper progression through the cell cycle. If there is a mutation, this regulation may fail, resulting in cells dividing more frequently than they should, which is a hallmark of cancerous growth. Thus, the consequence of a mutation in cyclin D is significant in terms of cell cycle progression and can lead to pathologies such as cancer due to enhanced proliferation.