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 organisms ranging from slime molds to humans. The entire length of repeated telomere sequences is known as the terminal restriction fragment (TRF). Sequences different from TTAGGG are found in more primitive eukaryotic forms, such as the ciliated protozoan Tetrahymena, in which Elizabeth Blackburn first characterized the repeated telomere sequence.
The polymerases that copy the chromosomes of DNA strands are unable to copy completely to the end. This became known as the "end-replication problem" when it was first recognized in the late 1960s. The TRF acts like a buffer that protects the information-containing genes, so that the loss of some telomeric nucleotide sequences at each round of DNA replication does not result in the loss of genetic information. The telomeres themselves end in large duplex loops, called T-loops.