ABSTRACT
A ll species with linear chromosomes require telomeres, whose role is to stabilize chromosome ends and prevent undesirable recombination-mediated or DNA repair-mediated events involving these DNA ends. The telomeres of most higher eukaryotic species are composed of very long tracts of a short repeated DNA sequence that is G-rich on one strand. These tracts are variable in length, ranging from approximately 3 kb in Arabidopsis, 15 to 50 kb in some rodents, to 100 kb or longer in some plants such as garden peas and tobacco.15 Telomeric DNA interacts with histones and other chromatin proteins to form chromatin, which in turn forms a higher order looped structure called a t-loop.6 Under some circumstances, t-loops may be converted to or generate extrachromosomal t-circles; for example, t-circles are associated with the Alternative Lengthening of Telomeres (ALT) pathway, which maintains telomere length by a telomerase-independent recombination-dependent mechanism.7,8 Recent studies show that forma tion of t-circles in human ALT cells is dependent on several recombination proteins.9
