What is a biological clock?

• A. It regulates the number of cell divisions; immature cells always complete 50 divisions no matter how long it had been frozen or what stage cell division was suspended
• B. It tracks the age of the organism
• C. It controls the rate of DNA replication only
• D. It signals cells to stop dividing after a fixed lifespan

Answer: (A)

Biological clocks in cells reflect a limited number of times a normal cell can divide, governed by telomere shortening and cell cycle controls. This finite replication potential, often called the Hayflick limit, means cells typically stop dividing after a certain number of divisions—around fifty for many somatic cells. The clock is intrinsic to the cell and doesn’t reset if the cell is paused or frozen; when division resumes, the remaining number of possible divisions is still based on that built-in limit. This aligns with the idea that the division count, not the organism’s calendar age or a fixed time lifespan, regulates how long a cell can keep proliferating. The other ideas—tracking age, controlling only the rate of DNA replication, or stopping after a fixed time—don’t capture that the key feature is a finite number of divisions before senescence, rather than a clock tied to calendar time or a simple rate limit. Some cells, like germ or certain stem cells, can bypass this limit with telomerase, which is an important exception to keep in mind.