Which statement best describes the end-replication problem and the role of telomeres?

Which statement best describes the end-replication problem and the role of telomeres?

• A. The need for RNA primers.
• B. The problem of supercoiling during replication.
• C. The requirement to ligate Okazaki fragments.
• D. The end-replication problem in linear chromosomes.

Answer: (D)

The end-replication problem arises because DNA polymerases can only synthesize DNA in the 5' to 3' direction and require a primer. On linear chromosomes, the final RNA primer on the lagging strand leaves a gap after its removal that cannot be filled, so the chromosome would shorten with each round of replication. Telomeres—short, repetitive noncoding sequences at chromosome ends—act as a protective buffer so essential genes aren’t lost during shortening. In germ cells and many stem cells (and some cancer cells), telomerase adds telomeric repeats back onto ends, helping maintain length. In most somatic cells, telomerase activity is low, so telomeres gradually shorten with divisions, which can contribute to aging and limiting cell lifespan. This captures why the end-replication problem matters and how telomeres mitigate it.

The end-replication problem arises because DNA polymerases can only synthesize DNA in the 5' to 3' direction and require a primer. On linear chromosomes, the final RNA primer on the lagging strand leaves a gap after its removal that cannot be filled, so the chromosome would shorten with each round of replication. Telomeres—short, repetitive noncoding sequences at chromosome ends—act as a protective buffer so essential genes aren’t lost during shortening. In germ cells and many stem cells (and some cancer cells), telomerase adds telomeric repeats back onto ends, helping maintain length. In most somatic cells, telomerase activity is low, so telomeres gradually shorten with divisions, which can contribute to aging and limiting cell lifespan. This captures why the end-replication problem matters and how telomeres mitigate it.