Chemistry Reference
In-Depth Information
neither nucleotide is damaged or modifi ed, it is not obvious which strand carries the
correct genetic information and which carries the error; thus, mismatch repair cannot
be accomplished by a mechanism such as BER or NER, which simply excise the
damaged base, or a short DNA fragment containing the damage, respectively (see
above). Unlike BER and NER, postreplicative MMR has to be targeted exclusively
to the newly synthesized strand, which carries, by defi nition, the erroneous genetic
information.
21
However, as the parent and daughter strands cannot be distinguished
at the site of the mispair, this has to happen elsewhere. The most important features
of the current models of the molecular mechanism of MMR are the following:
22
The
MMR process (Figure 6.3) is initiated by the binding of the hMutS
(hMSH2/
hMSH6 heterodimer), which undergoes an ATP-driven conformational change and
recruits the hMLHl/hPMS2 heterodimer (also in an ATP-dependent manner). This
ternary complex can translocate in either direction along the DNA. Importantly, the
binding of a mismatch or modifi ed DNA base(s) by the hMSH2/hMSH6 heterodimer,
or by the hMSH2/hMSH6/hMLHl/hPMS2 complex, does not bring about DNA
incision at this site, nor does it lead to damage excision. When it encounters a strand
discontinuity that may be bound by a proliferating cell nuclear antigen, loading of
an exonuclease initiates degradation of the nicked strand towards the mismatch.
Notably, the exonucleolytic degradation of DNA takes place only if a pre-existing
strand discontinuity is present in the vicinity. If the exonuclease dissociates before
it reaches the mismatch, the single-stranded gap is stabilized by RPA. Loading of
the second hMutS
α
/hMLHl/hPMS2 complex at the mismatch stimulates a second
round of exonucleolytic degradation. This process is repeated until the mismatch is
removed. Also importantly, the modifi ed DNA bases are not removed during the
DNA degradation step unless they are present in the strand containing the discon-
tinuity. The RPA - stabilized single - stranded gap is fi lled in by the replicative polymer-
ase and the remaining nick can be sealed by DNA ligase.
23,24
α
6.2.3 Repair of Double - Strand Breaks
Double-strand breaks, in which the phosphodiester backbones of both strands in
the double helix are interrupted, are particularly hazardous to the cell because they
can lead to genome rearrangements. Two mechanisms exist to repair double-strand
breaks: nonhomologous end joining (NHEJ) and homologous recombination repair
(HHR) (also known as template-assisted repair).
Nonhomologous End Joining
DNA NHEJ is a predominant pathway of DNA double-strand break repair in mam-
malian cells, and defects in the pathway cause radiosensitivity at the cellular and
whole-organism levels. NHEJ is referred to as 'nonhomologous' because the break
ends are directly ligated without the need for a homologous template, in contrast
to homologous recombination (see below), which requires a homologous sequence
to guide repair. NHEJ typically utilizes the short homologous DNA sequences in
the single-stranded overhangs that are often present at the ends of double-strand
breaks and are used to promote restorative repair. When these overhangs are com-
