Chromosome Substructure Investigation: Telomeres

Telomeres are the specialized structures that define chromosome ends and have been the subject of several recent reviews (1 –5 ). Telomeres allow a linear replication unit to be maintained as a linear molecule and overcome the end replication problem. The telomere must also distinguish a bona fide chromosome end from ends derived from an interstitial chromosome break, since broken chromosome ends are unstable. Most telomeres isolated to date are composed of tandem repetitive sequence arrays that are generally rich in T and G residues, for example, the vertebrate repeat is (TTAGGG)_n (6 ) and that of many plants appears to be (TTTAGGG)_n (7 ). Telomeres are synthesized by telomerase, identified in ciliates and mammalian cells, which adds on additional terminal repeats to a preexisting telomere (reviewed in 2 –5 ). It is these repetitive sequences that form a functional telomere.