USA Biology Olympiad (USABO) Practice Exam

Terminally differentiated cells are most often found in which phase of the cell cycle?

1. M phase

2. G1 phase

3. S phase

4. G0 phase

The correct answer is: G0 phase

Terminally differentiated cells are typically found in the G0 phase of the cell cycle. This phase is characterized by a state of quiescence where the cells are no longer actively dividing. Terminal differentiation refers to a process wherein a cell attains a specialized function and, as a result, often leaves the cell cycle entirely, entering G0. Cells in G0 may remain metabolically active but do not replicate their DNA or undergo mitosis. This is essential for maintaining the specific functions of specialized cells, such as neurons and muscle cells, which are crucial for the overall operation of tissues and organs. In contrast, cells in the M phase are actively dividing, while cells in the G1 phase are preparing for DNA synthesis, and those in the S phase are engaged in the actual replication of DNA. Hence, terminally differentiated cells, having completed their function and no longer requiring division, reside in the G0 phase.