Telomeres in the majority of model systems have the general DNA structure of a simple sequence repeat at the end of the chromosome that is adjacent to a middle-repetitive family of repeats. Experiments with chromosome truncations in yeast and the discovery of viable chromosome truncations in humans indicate that the simple sequence repeats are all that are required to maintain a stable chromosomal telomere in mitosis and meiosis (Gottschling et al., 1990; Wilkie et al., 1990). In contrast, broken DNA ends lacking these repeats are highly recombinagenic, as are yeast telomeres that have lost their simple sequence repeats (Hackett et al., 2001). In humans, the 3' end of the chromosome bears the repeat sequence TTAGGG while S. cerevisiae has the more divergent repeat (TG2-3)(TG)1-6, abbreviated as TG1-3, and S. pombe has a more divergent sequence with a core repeat of GnTTACA that can be interrupted by additional bases (Hiraoka and Chikashige, 2004; Smogorzewska and De Lange, 2004). The current model for telomere end structure in humans and yeast is that the 3' end extends past the 5' end by several telomere repeats (Makarov et al., 1997; McElligott and Wellinger, 1997; Wellinger et al., 1993).
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