Figure 2.

The DNA-end replication problem in eukaryotes and the rate of telomere shortening. (A) In many eukaryotes, including budding yeast and humans, telomeres end with a 3′ single-stranded overhang. (B) Most telomeric DNA is replicated by replication forks that arise unidirectionally from subtelomeric elements (Miller et al., 2006; Makovets, 2009; Sfeir et al., 2009). Thus, most TG-rich strands, containing the 3′ end, are replicated by the lagging synthesis machinery (green), and most CA-rich strands, containing the 5′ end, are replicated by the leading synthesis machinery (red). (C) The presence of the overhang implies an asymmetry between the two template strands and is expected to result in an asymmetry in the length of the semi-conservative DNA replication products. On one hand, the lagging strand telomere is expected to conserve the longest strand containing the 3′ end (but this has not been addressed experimentally). The 5′ newly synthesized lagging strand starts with an RNA primer of a few ribonucleotides. The positioning of this last Okazaki fragment and subsequent removal of the RNA primer is expected to determine the length of the overhang of this strand. In mammals, the positioning of the first RNA primer is random and its removal is delayed (Chow et al., 2012). The enzyme(s) involved in the RNA primer maturation are currently unknown. On another hand, the leading strand synthesis (red) presumably stops prematurely due to a lack of template that corresponds to the length of the overhang. Thus, this telomere is shorter than the parental telomere and the lagging telomere. (D) 5′–3′ Resection of the CA-rich strands was shown to occur in both leading and lagging telomeres in mammals (Wu et al., 2012). In S. cerevisiae, this resection is probably limited to the leading telomere, as represented (Faure et al., 2010). (E) C-strand fill-in compensates for this resection. This synthesis likely requires the polymerase alpha/primase with the synthesis of an RNA primer that must be processed, similarly to the lagging telomere. Thus, the generation of the overhangs is different between leading and lagging telomeres and these processes are expected to modulate telomere-shortening rate and subsequently the onset of senescence.