2 Telomere DNA Structure

Abstract

TELOMERE DNA SEQUENCES
The molecular biology of telomeres began in 1978 with a report describing the telomere repeat sequence from the rDNA gene of the ciliated protozoan Tetrahymena thermophila (Blackburn and Gall 1978). Telomeric restriction fragments were distinguished by their “fuzzy” appearance on agarose gels. This fuzzy appearance, due to length heterogeneity in the population of telomeres from any one chromosome in this organism, has since become a hallmark of telomeric restriction fragments, although this characteristic is not always diagnostic for a telomeric fragment. For example, some hypotrichous ciliates and budding yeasts have telomeric restriction fragments that appear as sharp bands on agarose gels (Klobutcher et al. 1981; Pluta et al. 1982; McEachern and Blackburn 1994). In addition to the fuzzy appearance of a telomeric restriction fragment on agarose gels, a second test for telomeric location is susceptibility to shortening by the exonuclease BAL 31. Since this exonuclease digests DNA from chromosomal termini inward, the first region of the chromosome to be removed upon BAL 31 treatment is the telomere. The appearance of a fuzzy band, in association with susceptibility of candidate telomere sequences to BAL 31, has led to the identification, cloning, and characterization of telomere sequences from many organisms (Table 1).

In their pioneering work, Blackburn and Gall (1978) used direct chemical methods to determine the Tetrahymena telomere sequence. These studies defined the hexamer repeat sequence for Tetrahymena, CCCCAA/TTGGGG, and showed that telomere repeats were polarized such that the TTGGGG strand formed the 3′ end of the telomere...