74
Aging and Sex, DNA Repair in
BASC
ATM
BRCA1
Cell cycle
arrest
p53
p53
BLM
WRN
p73
E2F1
Apoptosis
BLM
Rad51
BLM
Recruit second
chromosome
Strand invasion
and form D-loop
to make Holliday
junctions
Nick crossovers
to resolve
Holliday junctions
DNA synthesis
across gaps
and ligation
p66Shc
Completion of
homologous recombinational repair (HRR)
BASC complex senses and binds
to the double-strand break
Fig. 3
Homologous recombinational repair (HRR).
strand invasion from the ends of the
double-strand break plus formation of a
‘‘D-loop’’ in the invaded chromosome by
strand migration facilitated by the heli-
case action of BLM, (3) nicking of the
crossover strands to resolve the Holliday
junctions, carried out by BLM and WRN
helicases in association with p53 (which
has a 3
0
to 5
0
exonuclease function and
a strand transfer function that may act at
this step), and (4) DNA synthesis across
single-strand gaps and a Fnal ligation step.
An interstrand cross-link will be converted
to a double-strand break, in part by action
of ERCC1 (Table 3), and thus can also be
repaired by this pathway.
ModiFed forms of HRR have been sug-
gested for replicating somatic cells. In
these models, the ‘‘D-loop’’ (the strand
of DNA from the intact chromosome
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