What happens at the ends of linear chromosomes during replication?

What happens at the ends of linear chromosomes during replication?

• A. The 5' ends are completely preserved with no shortening
• B. The ends become circular
• C. The ends shorten over successive replication because they cannot be fully replicated
• D. The ends lengthen with every cycle

Answer: (C)

The end-replication problem is what’s being tested here. DNA polymerases can only synthesize DNA in the 5′ to 3′ direction and require a primer to start synthesis. On the lagging strand, after the final RNA primer is removed, there’s no upstream primer to extend from to fill that last gap. This leaves a small portion at the end unreplicated, so with each round of replication the chromosome becomes shorter. Telomeres—the repetitive, noncoding end sequences—protect essential genes by absorbing this loss, and in cells with active telomerase (like germ cells, some stem cells, and certain cancer cells) the enzyme can extend the ends by adding repeats, helping maintain length. In most ordinary somatic cells, telomerase is inactive, so the ends progressively shorten over time. The ends do not become circular, and they do not lengthen in typical somatic cells, which is why the statement about progressive shortening best describes what happens.

The end-replication problem is what’s being tested here. DNA polymerases can only synthesize DNA in the 5′ to 3′ direction and require a primer to start synthesis. On the lagging strand, after the final RNA primer is removed, there’s no upstream primer to extend from to fill that last gap. This leaves a small portion at the end unreplicated, so with each round of replication the chromosome becomes shorter. Telomeres—the repetitive, noncoding end sequences—protect essential genes by absorbing this loss, and in cells with active telomerase (like germ cells, some stem cells, and certain cancer cells) the enzyme can extend the ends by adding repeats, helping maintain length. In most ordinary somatic cells, telomerase is inactive, so the ends progressively shorten over time. The ends do not become circular, and they do not lengthen in typical somatic cells, which is why the statement about progressive shortening best describes what happens.