What are telomeres and why are they important for genome stability?

• A. Telomeres are enzymes that cut DNA ends.
• B. Telomeres are repetitive DNA at chromosome ends that protect against end-to-end fusions.
• C. Telomeres are repetitive DNA at chromosome ends that protect against end-to-end fusions and information loss; maintained by telomerase.
• D. Telomeres are regions within genes that code for proteins.

Answer: (C)

Telomeres are repetitive DNA sequences at the ends of linear chromosomes that act as protective caps. They prevent two major threats to genome stability: end-to-end fusions with other chromosomes, which would create unstable chromosome structures during cell division, and the loss of genetic information from crucial genes near chromosome ends due to the end-replication problem.

During DNA replication, polymerases can’t fully copy the extreme ends of chromosomes, so telomeres provide a buffer of noncoding repeats that can be shortened without harming essential genes. Telomeres are maintained by the enzyme telomerase, which extends the ends by adding repeats using an RNA template. This maintenance is crucial in cells that divide a lot (like germ cells and many stem cells) and is often hijacked in cancer. Protective protein complexes also help form structures that hide chromosome ends from DNA damage sensors.

So, telomeres are the repetitive DNA at chromosome ends that protect against end-to-end fusions and information loss, and they are maintained by telomerase, making them central to genome stability.