Telomeres are structures found at the ends of chromosomes in the cells of eukaryotes. Telomeres function by protecting chromosome ends from recombination, fusion to other chromosomes, or degradation by nucleases. They permit cells to distinguish between random DNA breaks and chromosome ends. They also play a significant role in determining the number of times that a normal cell can divide. Unicellular forms whose cells have no true nuclei (prokaryotes) possess circular chromosomes that, therefore, have no ends. Thus, prokaryotes can have no telomeres.

Structure

Telomeres are extensions of the linear, double-stranded DNA molecules of which chromosomes are composed, and are found at each end of both of the chromosomal strands. Thus, one chromosome will have four telomeric tips. In humans, the forty-six chromosomes are tipped with ninety-two telomeric ends.

In most eukaryotic forms, telomeres consist of several thousand repeats of the specific nucleotide sequence TTAGGG and occur in...