What is the end-replication problem in linear eukaryotic chromosomes and which enzyme mitigates it?

What is the end-replication problem in linear eukaryotic chromosomes and which enzyme mitigates it?

• A. Gaps in the DNA are repaired by mismatch repair.
• B. Telomerase seals nicks between fragments.
• C. Telomere shortening; telomerase extends telomeres in germline and some stem cells.
• D. Ligase extends telomeres.

Answer: (C)

In linear chromosomes, each round of DNA replication leaves the very ends hard to copy. DNA polymerases need a starting 3' end and primers, so the final segment at the end of the chromosome can’t be fully replicated. Over many cell divisions this would progressively trim away important genetic information, which is the end-replication problem. The genome avoids this loss conceptually with telomeres, which are repetitive, noncoding sequences at chromosome ends that absorb most of the shortening. The enzyme that counters this issue is telomerase. It carries its own RNA template and adds repeats to the 3′ end of telomeres, effectively lengthening them and preserving the ends of chromosomes. This activity is especially important in germline cells and some stem cells, where many divisions are needed and telomerase remains active. In most somatic cells, telomerase is not active, so telomeres shorten with each division, contributing to aging. So, the end-replication problem is about telomere shortening, and telomerase is the enzyme that extends telomeres in germline and some stem cells.

In linear chromosomes, each round of DNA replication leaves the very ends hard to copy. DNA polymerases need a starting 3' end and primers, so the final segment at the end of the chromosome can’t be fully replicated. Over many cell divisions this would progressively trim away important genetic information, which is the end-replication problem. The genome avoids this loss conceptually with telomeres, which are repetitive, noncoding sequences at chromosome ends that absorb most of the shortening.

The enzyme that counters this issue is telomerase. It carries its own RNA template and adds repeats to the 3′ end of telomeres, effectively lengthening them and preserving the ends of chromosomes. This activity is especially important in germline cells and some stem cells, where many divisions are needed and telomerase remains active. In most somatic cells, telomerase is not active, so telomeres shorten with each division, contributing to aging.

So, the end-replication problem is about telomere shortening, and telomerase is the enzyme that extends telomeres in germline and some stem cells.