Define telomeres and the role of telomerase.

• A. Telomeres are repetitive sequences at chromosome ends that protect genes; telomerase extends telomeres by adding repeats
• B. Telomeres are coding regions that regulate transcription
• C. Telomerase degrades telomeres to shorten chromosomes
• D. Telomeres are the primary site of replication initiation

Answer: (A)

Telomeres are the protective caps at the ends of chromosomes, made of short, repetitive DNA sequences. Their main job is to shield essential genes from being lost during DNA replication and to prevent end-to-end chromosome fusions and DNA damage signaling. Each time a chromosome is copied, some terminal DNA is not fully replicated, so telomeres serve as a buffer. Telomerase is the enzyme that counteracts this shortening by adding repeats to the ends, using an RNA template within the enzyme complex. This activity helps maintain chromosome stability in cells where telomerase is active, such as germ cells and many stem cells, and is also a feature observed in many cancers.

The other descriptions don’t fit because telomeres are noncoding, repetitive regions—not coding sequences that regulate transcription. Telomerase does not degrade telomeres; it extends them. And telomeres are not the primary sites where DNA replication initiates; origins of replication are the starting points for replication, not telomeres.