Questions: Telomeres and the End-Replication Problem

Question 1

A somatic cell from an elderly individual is found to have critically short telomeres and subsequently enters permanent cell cycle arrest. What is the most direct explanation for this sequence of events?

• A. Telomere shortening caused mutations to accumulate in coding genes at chromosome ends, triggering DNA damage
• B. Progressive shortening over ~50–70 cell divisions eroded telomeres to a critical length, at which point the shelterin complex could no longer protect the chromosome end, exposing it to DNA damage checkpoints that halt the cell cycle
• C. Telomerase, which normally maintains telomere length in all somatic cells, became inactivated in old age, causing sudden shortening
• D. The end-replication problem caused loss of essential genes adjacent to the telomere, disrupting cell cycle regulation

Answer: (B)

Somatic cells lack active telomerase; they accumulate shortening passively over cell divisions. When telomeres fall below a critical length, the shelterin complex that caps the chromosome end can no longer maintain its protective structure. The exposed chromosome end resembles a double-strand break and activates p53/Rb-mediated damage checkpoints — inducing senescence. Option C is wrong because telomerase is not normally active in somatic cells to begin with. Option D is wrong because telomeres are non-coding TTAGGG repeats specifically designed to be sacrificed before coding sequence is at risk.

Question 2

Why does telomerase reactivation contribute to cancer cell immortality?

• A. Telomerase repairs accumulated DNA mutations in cancer cells, restoring genomic stability
• B. Telomerase replenishes telomere repeats, allowing cancer cells to bypass replicative senescence and divide indefinitely
• C. Telomerase enhances DNA polymerase activity on the leading strand, speeding up replication in rapidly dividing cells
• D. Telomerase shortens telomeres in cancer cells, which paradoxically increases the rate of cell division

Answer: (B)

The mitotic clock — progressive telomere shortening — normally limits somatic cells to ~50–70 divisions before senescence. By reactivating telomerase to add TTAGGG repeats, cancer cells reset this clock after each division, bypassing the natural limit on proliferation. This is one of the hallmarks of malignancy. Telomerase does not repair coding-gene mutations or enhance leading-strand synthesis — it is specialized to extend the G-rich 3' overhang of chromosome ends.

Question 3

Telomere shortening in somatic cells functions as a 'mitotic clock' that limits the number of times a cell can divide, providing a natural brake on uncontrolled proliferation.

• T. True
• F. False

Answer: True

Each round of DNA replication shortens telomeres by 50–200 bp due to the end-replication problem. After approximately 50–70 divisions in human somatic cells, telomeres reach a critical minimum length. This triggers the DNA damage response (via p53 and Rb pathways), inducing senescence — permanent cell cycle arrest. This mechanism is evolutionarily thought to protect against cancer by limiting the replicative lifespan of potentially mutated cells. Telomerase reactivation in cancer cells circumvents precisely this protective limit.

Question 4

The end-replication problem is caused by DNA polymerase making errors specifically at chromosome ends, and telomeres protect the genome by preventing these errors.

• T. True
• F. False

Answer: False

The end-replication problem is not a polymerase error — it is a structural inevitability of semiconservative replication on linear DNA. The lagging strand requires an RNA primer to initiate each Okazaki fragment; when the final primer at the chromosome tip is removed, there is no upstream primer for DNA polymerase to extend from, leaving a single-stranded gap. This shortening occurs correctly and inevitably, not through error. Telomeres do not prevent the shortening — they provide a buffer of non-essential repetitive sequence so that coding genes are not lost when shortening occurs.

Question 5

Why do germ cells and stem cells maintain active telomerase, while most somatic cells do not? What are the biological consequences of this difference?

Model answer: Germ cells must ensure that offspring begin life with full-length telomeres rather than inheriting the accumulated shortening of a parent's lifetime. Stem cells need to divide extensively throughout an organism's life, so telomerase activity prevents premature senescence that would deplete stem cell pools. Somatic cells lack telomerase activity, meaning their division is limited by the mitotic clock — providing a tumor-suppressive effect by restricting the proliferative lifespan of potentially mutated cells. The trade-off is that tissue renewal capacity declines with age as somatic stem cell pools shorten. Cancer cells exploit telomerase reactivation to bypass this limit and achieve indefinite proliferation.

This difference reflects a fundamental tension between cancer suppression and tissue maintenance. Restricting telomerase in somatic cells limits runaway proliferation; activating it in germ and stem cells ensures long-term tissue function. The ~85% prevalence of telomerase reactivation in cancers reflects how essential this bypass is for achieving the hallmark of unlimited replicative potential.