What is the role of telomeres and telomerase in chromosome stability?

• A. Telomeres encode essential genes at chromosome ends; telomerase repairs them.
• B. Telomeres protect chromosome ends from degradation; telomerase extends telomeres in germ cells and some stem cells to maintain stability, while most somatic cells lack active telomerase.
• C. Telomeres are centromere-like regions; telomerase binds to them to promote recombination.
• D. Telomeres cause chromosomes to fuse; telomerase terminates replication.

Answer: (B)

The concept being tested is how telomere ends and telomerase activity keep chromosomes stable during replication. Telomeres are short, repetitive, noncoding DNA sequences at chromosome ends that form a protective cap, helping to prevent degradation and the mistaken repair of natural chromosome ends as breaks. Each time a cell divides, a small amount of DNA is lost from the end, a consequence of the replication machinery not fully copying chromosome termini. Telomerase counteracts this loss by using its RNA template to add back telomeric repeats, extending the ends. This activity is strong in germ cells and some stem cells, where long-term division is needed, but is low or absent in most somatic cells, so their telomeres shorten with age. Shortening limits how many times a cell can divide, helping maintain genome stability and preventing dangerous end-to-end fusions. The other descriptions misstate the role: telomeres do not encode essential genes, they are not centromere-like regions that promote recombination, and telomeres do not cause fusion or terminate replication; rather, the telomerase activity helps maintain ends and protect stability.