Telomeres limit cell divisions through progressive shortening

Steps:

• Recall telomeres are protective repetitive DNA sequences at chromosome ends that shorten during replication due to the end-replication problem.
• Evaluate each option against telomere structure and function: protection from degradation, role in limiting divisions via Hayflick limit.
• Eliminate options misstating structure (not genes, not solely unpaired) or function (not about introns/exons).
• Identify the option correctly linking telomeres to finite cell divisions.

Why A is correct:

• Telomeres act as a buffer, allowing DNA replication and cell division until they shorten critically, enforcing a fixed limit (Hayflick limit) before senescence.

Why the others are wrong:

• B: Telomeres are non-coding repetitive sequences, not genes, and cap both chromosome ends (5' and 3').
• C: Telomeres are mostly double-stranded with a short 3' overhang; they are not defined as unpaired regions.
• D: Telomeres protect chromosome ends from loss or fusion, unrelated to preserving introns/exons within genes.

Final answer: A