What problem at chromosome ends arises during replication, and which enzyme counteracts it?

• A. Telomere shortening causes replication fork stalling
• B. Okazaki fragment processing involves ligase
• C. Replisome disassembly problem; helicase unwinding
• D. End replication problem; telomerase extends telomeres using its RNA template

Answer: (D)

The issue being tested is the end replication problem: linear chromosomes tend to lose sequence from their ends during DNA replication because DNA polymerases can’t fully replicate the very end of the lagging strand. Telomeres, the protective caps at chromosome ends, help manage this by providing a buffer of repetitive, noncoding DNA. The enzyme that counteracts this shortening is telomerase, a reverse transcriptase that carries its own RNA template. Telomerase extends the 3’ end of the telomere by adding telomeric repeats, using its RNA as a template. This extra sequence provides the necessary primer/overhang for subsequent fill-in synthesis, helping preserve chromosome length. In most somatic cells telomerase activity is low, so telomeres shorten with each division, but in germ cells, stem cells, and many cancer cells telomerase remains active to maintain telomere length.