MAMMALIAN TELOMERES AND CHROMOSOME FRAGMENTATION

Human chromosomes terminate in variable length tandem arrays (2-20 kb in somatic cells) of the simple repeat (TTAGGG)n, orientated 5'-3' tow ard the chromosome end. The DNA adjacent to the terminal array consist s of a complex mixture of repetitive sequences, which vary in distribu tion over different chromosomes. The polymorphic variation in subtelom eric structure implies that these sequences do not have a significant role in telomere function. In addition, whilst (TTAGGG)n is present at the chromosome ends of all vertebrates studied the subtelomeric repea ts are conserved only within primates. As will be discussed below, evi dence in support of the idea that the (TTAGGG)n repeats alone are suff icient for telomere function has come from the molecular analysis of b oth spontaneously-arising and experimentally-induced chromosome fragme ntation events.