What is telomerase and why is it needed?

• A. Telomerase extends telomeres to counteract the end-replication problem in linear chromosomes.
• B. Telomerase unwinds the DNA double helix.
• C. Telomerase removes RNA primers.
• D. Telomerase ligates Okazaki fragments.

Answer: (D)

Telomerase adds telomere repeats to the ends of linear chromosomes to counteract the end-replication problem. When DNA is copied, the lagging strand can’t fully replicate the extreme end, so chromosomes shorten with each division. Telomerase uses an RNA template within the enzyme to extend the 3′ end with repetitive telomere sequences (such as TTAGGG in humans). Once the end is lengthened, conventional DNA synthesis can complete the gap, helping preserve chromosome integrity in cells that divide a lot, like germ cells and stem cells. In most body cells, telomerase activity is low, so telomeres gradually shorten and cell aging or senescence can occur; in many cancers, telomerase is reactivated to enable more divisions.

Unwinding the DNA double helix is the job of helicase, not telomerase. Removing RNA primers is done by nucleases and polymerase activities, not telomerase. Ligation of Okazaki fragments is performed by DNA ligase, not telomerase.