ABSTRACT
Telomeres Human telomeres are noncoding DNA sequences at the end of chromosomes, which are composed of (TTAGGG)n hexanucleotide repeats. During each cell division, telomeric DNA (30-100 bp) is lost because of the so-called end-replication problem. Telomeres maintain chromosomal integrity and prevent replication of defective genes (1,2). As all chromosomes begin life with a limited amount of telomeric DNA, there is a finite number of cell divisions that a cell can undergo before it reaches an irreducible lower limit of telomeres (1-3). Without this minimal amount of telomeric DNA, the cell is no longer capable of supporting chromosomal replication or cell division. When normal cells reach a critical telomere length, they exit the cell cycle, enter M2 (mortality stage 2) crisis, and undergo senescence. This mechanism is thought to be the clock that determines human lifespan (3,4).
