Abstract: The strand of telomeric DNA that runs 5' to 3' towards a chromosome end is typically G-rich. Due to the properties of conventional DNA polymerases, telomerase-generated tails of this G-rich strand are expected on only one end of individual linear DNA molecules. In Saccharomyces, TG.sub.1-3 tails are detected on chromosome and linear plasmid telomeres late in S-phase. Moreover, the telomeres of linear plasmids can interact when the TG.sub.1-3 tails are present. Molecules were generated in vitro that mimic the structures predicted for telomere replication intermediates. These in vitro generated molecules formed telomere--telomere interactions similar to those on molecules isolated from yeast but only if both ends that interacted had a TG.sub.1-3 tail. Moreover, TG.sub.1-3 tails were generated in vivo in cells lacking telomerase. These data suggest a new step in telomere maintenance cell cycle regulated degradation of the C.sub.

Abstract: Assays for measuring fragility or breakage of chromosomes at trinucleotide repeats in Saccharomyces cerevisiae are provided. These assays can also be used to measure trinucleotide repeat expansion and to screen drugs, proteins and cellular conditions which increase or decrease this expansion.

Abstract: Method for affecting viability of a eucaryotic cell by contacting the cell with a modulator of the activity of a PIF-1-type helicase in the cell. Such contacting specifically increases or decreases the specific activity of the helicase in the cell.