Turning telomeres off and on

We envision multiple steps in telomere maintenance, based largely on geneti c data from budding yeast. First, the telomere must unfold or open itself s uch that the free end is accessible to the appropriate enzymatic machinery. Second, telomerase must be recruited, together with the DNA replication ma chinery that synthesizes the C-rich strand. The processivity of telomerase is regulated both by a length-sensing feedback mechanism and by second-stra nd synthesis. Finally, the telosome refolds into a protective end structure . If telomerase is nonfunctional, recombination may occur once telomeres ar e open. Multiple pathways regulate these different steps, producing a highl y dynamic chromosomal cap.