telomeres

Meaning: Telomeres are repeated, noncoding DNA sequences at the ends of linear eukaryotic chromosomes, bound by protective proteins. They keep the ends from being treated as DNA breaks and act as a buffer so coding DNA is not lost. DNA polymerase extends only 5′→3′ and needs a primer, so once the last RNA primer on the lagging strand is removed, the gap at the 5′ end cannot be filled and the telomere shortens with each division (the end-replication problem). Telomerase carries its own RNA template and works as a reverse transcriptase, adding repeats to the 3′ end to offset this. It is active in germ cells, stem cells and most cancer cells, but low or absent in many somatic cells.

On the MCAT: A passage on aging, senescence or cancer. Predict that cells lacking telomerase lose telomere length each division until they stop dividing or undergo apoptosis, while cells with active telomerase can keep dividing much longer. You may also be asked to identify telomerase as an RNA-containing enzyme that makes DNA from an RNA template.

Common confusion: Telomeres are not centromeres. A centromere is the region where sister chromatids join and spindle fibers attach through kinetochores; telomeres are at the tips. Also, the shortening is not a random copying error. It follows from how lagging-strand synthesis works.