What typically happens to telomeres as cells divide?

As cells divide, telomeres typically undergo shortening. Telomeres are repetitive DNA sequences located at the ends of chromosomes, and they serve to protect the genetic information contained within the chromosome from degradation during cell division. Each time a cell divides, a small portion of the telomere is lost due to the inability of DNA polymerase to fully replicate the ends of linear chromosomes. This shortening occurs because the mechanisms that replicate DNA cannot completely duplicate the very end of the chromosome, a phenomenon known as the "end replication problem."

Over time, with successive cell divisions, the telomeres shorten to a critical length. Once they reach this critical point, it can trigger cellular senescence, apoptosis, or other mechanisms that limit the cell's ability to divide further. This shortening process is a natural part of the cellular aging process and is one of the factors that contribute to the overall aging of tissues and organisms.

In contrast, elongation or replication of telomeres does not typically occur in somatic cells, and they do not remain unchanged due to the limitations posed by DNA replication mechanisms. Some specialized cells, such as germ cells and certain stem cells, possess telomerase, an enzyme that can extend telomeres, but this is not a common