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.