Getting to the end: Telomerase access in yeast and humans

In organisms with linear chromosomes, telomeres are essential to maintain genome integrity. However, inappropriate telomere addition, for example to double-stranded DNA breaks, might stabilize deleterious genetic changes. Therefore, telomere addition by telomerase is highly regulated, for example by mechanisms that determine the accessibility of telomeres to elongation by telomerase. These mechanisms, which have been studied mainly in budding yeast and human cell culture, can be subdivided into two classes: mechanisms that modulate the telomeric chromatin structure and those that sequester active telomerase from chromosome ends.