Telomere Position Effect in Human Cells
Telomeres are tracts of repetitive DNA that cap the ends of linear chromosomes. Each time the chromosome is duplicated, a small amount of telomeric DNA is lost from the end due to factors inherent in the mechanism of DNA replication. The result is a net shortening of telomeres with each cell division, unless new repeats are synthesized through the action of the enzyme telomerase. Most human somatic cells lack telomerase activity and so continued cell division leads to telomere shortening. After a limited number of divisions (the "Hayflick limit"), it is believed that a few critically shortened telomeres trigger a state of growth arrest termed replicative senescence.