How does telomerase extend chromosome ends without a conventional template at the edge?

• A. Telomerase degrades telomeres to reset length.
• B. Telomerase uses a DNA template to extend ends.
• C. Telomerase carries its own RNA template to extend 3' telomere overhang, after which DNA polymerases fill in and ligate.
• D. Telomerase recruits telomerase inhibitors to stop elongation.

Answer: (C)

Telomeres protect chromosome ends, and replication leaves a gap at that edge. Telomerase solves this by carrying its own RNA template inside the enzyme. It binds the 3' end of the telomere and uses this RNA template to reverse-transcribe and extend the 3' overhang, adding telomeric repeats (in humans, TTAGGG). This extension creates a longer 3' overhang that can then serve as a primer for the usual DNA replication machinery to synthesize the complementary strand. After that, DNA polymerases fill in the remaining gap on the opposite strand and ligase seals the nicks, restoring a proper telomere. This sequence of events is exactly what option describing RNA-templated extension by telomerase, followed by filling in and ligating, conveys.