Biology Reference
In-Depth Information
late S and G2 phases of the cell cycle.
29
Importantly, crossover events during
mitotic recombination could lead to loss of heterozygosity (LOH), which is a
common process during tumorigenesis.
29,33,34
A synthesis-dependent strand annealing (SDSA) model was proposed to
account for the low numbers of crossover recombination events (
Fig. 2
; Refs.
35-37
). In SDSA, once DNA synthesis occurs on the invading strand of a
D-loop, it is unwound and displaced such that it may anneal with the second
end to prime DNA synthesis on the latter (
Fig. 1
). This pathway leads exclu-
sively to noncrossover products. SDSA aside, dHJ-based recombination prod-
ucts have been observed between homologous chromosomes in yeast during
mitotic DSB repair, but as a minor pathway.
38
During NHEJ, the broken DSB ends are prevented from resection by the
Ku70-Ku80 heterodimer.
39
The strong affinity of Ku70-Ku80 for DSB ends
appears to recruit DNA ligase IV, which is capable of sealing the DSB (
Fig. 2
).
The Alt-NHEJ pathway appears to resect a 5- to 25-nt region where micro-
homology may be used prior to ligation of the ends (
Fig. 2
; Refs.
28,40
).
SSA occurs in regions flanked by direct repeat DNA sequences (
Fig. 2
; Ref.
41
). Hence, this pathway is seen in higher eukaryotes where direct repeated
sequences are prevalent.
41-44
In SSA, both 5
0
flanking the DSB are resected by
nucleases and the resulting 3
0
overhangs annealed by RPA and RAD52.
29
As
strand invasion and exchange are not involved, this process is independent of
RAD51. 3
0
Single-stranded overhangs are subsequently resected and ligated by
nucleases and ligases, respectively.
41
NHEJ, Alt-NHEJ, and SSA are mutation-
prone pathways due to the lack of fidelity and loss of genetic information during
the repair process.
A. Collapsed or Stalled Replication Forks
When a replication fork collapses or if a telomere becomes uncapped, a
single-ended DSB is formed.
45-47
This end may be processed to produce a 3
0
overhang that can invade a homologous region of the sister chromatid, the
homologous chromosome, or a homologous region of another chromosome to
initiate DNA synthesis. This process is called break-induced replication (BIR;
Fig. 3
). Use of any other template other than the sister chromatid leads to LOH
during BIR. During mitotic recombination, however, BIR is disfavored over
SDSA apparently because of its slower kinetics.
48
Stalled replication machinery or lesions on leading or lagging strands may
lead to the formation of DNA gaps or DSBs that often result in replication fork
collapse.
49,50
In vertebrate cells, replication fork collapse occurs in every cell
cycle.
51,52
Translesion synthesis (TLS), template switching, or HR can restore
replication. TLS is error-prone due to the low fidelity of the polymerase
employed.
53
However, template switching and HR are error-free. Many repli-
cation mutants with defective checkpoint activation are dependent on HR gene
