Which option describes the telomere concept in replication?

• A. Telomeres code for essential genes
• B. Telomeres are unaffected by replication
• C. Telomeres are the origin of replication
• D. Telomeres shorten with each replication

Answer: (D)

The idea being tested is what happens to telomeres during cell division. On linear chromosomes, ends carry telomeres, which are repetitive, noncoding sequences that protect essential genes from erosion. DNA polymerase can only synthesize DNA in a 5' to 3' direction and cannot fully copy the very ends of chromosomes—the end-replication problem. Because of this, without a special solution, each round of replication trims the telomeres a little shorter. Over many divisions, telomeres shorten, contributing to a limit on cell divisions and to aging processes. Some cells express the enzyme telomerase, which can extend telomeres and counteract shortening, but in most somatic cells telomeres do shorten with each replication. This is why the correct statement is that telomeres shorten with each replication.