breaks. The Sgs1(BLM) helicase appears to be a central regulator of most of the recombination events that occur during S. cerevisiae meiosis. During normal meiosis Sgs1(BLM) is responsible for directing recombination towards the alternate formation of either early non-crossovers or Holliday junction joint molecules, the latter being subsequently resolved as crossovers. In the plant Arabidopsis thaliana, homologs of the Sgs1(BLM) helicase act as major barriers to meiotic crossover formation. These helicases are thought to displace the invading strand allowing its annealing with the other 3’overhang end of the double-strand break, leading to non-crossover recombinant formation by a process called synthesis-dependent strand annealing (SDSA) (see Wikipedia article “Genetic recombination”). It is estimated that only about 5% of double-strand breaks are repaired by crossover recombination. Sequela-Arnaud et al. suggested that crossover numbers are restricted because of the long-term costs of crossover recombination, that is, the breaking up of favorable genetic combinations of alleles built up by past natural selection. RECQL4 helicase In humans, individuals with Rothmund–Thomson syndrome, and carrying the RECQL4 germline mutation, have several clinical features of accelerated aging. These features include atrophic skin and pigment changes, alopecia, osteopenia, cataracts and an increased incidence of cancer. RECQL4 mutant mice also show features of accelerated aging. RECQL4 has a crucial role in DNA end resection that is the initial step required for homologous recombination (HR)-dependent double-strand break repair. When RECQL4 is depleted, HR-mediated repair and 5’ end resection are severely reduced in vivo. RECQL4 also appears to be necessary for other forms of DNA repair including non-homologous end joining, nucleotide excision repair and base excision repair. The association of deficient RECQL4 mediated DNA repair with accelerated aging is consistent with the DNA damage theory of aging. See also Bloom syndrome References Further reading External links RecQ Helicases , introduction at UNC's Sekelsky Lab. BLM gene encodes a RecQ Helicase, description of the gene EC 3.6.1 Aging-related enzymes Senescence