As we have seen, DNA complementarity is an important
resource that is exploited by many error-free correction
systems. Such error-free repair is characterized by two
stages: (1) removal of damaged and nearby DNA from one
strand of the double helix and (2) use of the other strand
as a template for the DNA synthesis needed to fill the
single-strand gap. However, what would happen if both
strands of the double helix were damaged in such a way
that complementarity could not be exploited? One way
this might happen is if both strands of the double helix
were to break at sites that were close together. A mutation
like this is called a double-strand break. If left unrepaired,
double-strand breaks can cause a variety of chromosomal
aberrations resulting in cell death or a precancerous state.
Interestingly, the ability of double-strand breaks to initiate chromosomal instability is an integral feature of some
normal cellular processes that require DNA rearrangements. One example is the generation of the diversity of
antibodies in the cells of the mammalian immune system.
Another is meiotic recombination, which uses doublestrand breaks to generate genetic diversity. As will be seen
in the remainder of this chapter, the cell uses many of the
same proteins and pathways to repair double-strand breaks
and to carryout meiotic recombination. For this reason,
we begin by focusing on the molecular mechanisms that
repair double-strand breaks before turning our attention
to the mechanism of meiotic recombination.
Double-strand breaks can arise spontaneously (for
example, in response to reactive oxygen species), or
they can be induced by ionizing radiation. Two distinct
mechanisms are used to repair these potentially lethal
lesions: nonhomologous end joining and homologous
recombination.
NONHOMOLOGOUS END-JOINING As mentioned earlier,
DNA repair is important to prevent precancerous mutations from occurring in the nondividing cells of multicellular organisms. However, when a double-strand break
occurs in cells that have stopped dividing, error-free repair
is not possible because neither of the two usual sources
of undamaged DNA is available as a template for new
DNA synthesis. That is, complementarity cannot be exploited because both strands of the DNA helix are damaged and, in the absence of replication, there is no sister
chromatid. However, as was the case for the error-prone
translesion synthesis (including the SOS system in E. coli),
the consequences of imperfect repair may be less harmful
to the cell than leaving the lesion unrepaired. In this case,
it is better to put the free ends back together so they cannot initiate chromosomal rearrangements, even if this
means that some sequence may be lost. Putting the ends
back together is accomplished by a mechanism called
nonhomologous end-joining, which involves the three steps
shown in Figure 14-32. These steps include the binding of
the broken ends by 3 proteins (KU70, KU80, and a large
DNA-dependent protein kinase) followed by the trimming of the ends so that they can be ligated together. In
mammals, several of the proteins in this pathway also
participate in the end-joining reactions associated with
the programmed rearrangements of antibody genes.
