Telomerase

Telomeres are special, essential DNA sequences at both ends of each chromosome. Each time chromosomes replicate a small amount of the DNA at both ends is lost, by an uncertain mechanism. Because human telomeres shorten at a much faster rate than many lower organisms, we speculate that this telomere shortening probably has a beneficial effect for humans, namely mortality. The telomere hypothesis of aging postulates that as the telomeres naturally shorten during the lifetime of an individual, a signal or set of signals is given to the cells to cause the cells to cease growing (senesce). At birth, human telomeres are about 10,000 base pairs long, but by 100 years of age this has been reduced to about 5,000 base pairs.

Telomerase is actually an enzyme (a catalytic protein) that is able to arrest or reverse this shortening process. Normally, telomerase is only used to increase the length of telomeres during the formation of sperm and perhaps eggs, thus ensuring that our offspring inherit long "young" telomeres to propagate the species.