chapter 24
DNA Replication, Repair, and Mutagenesis
Excision and
gap filling
Breakage of N-glycosidic bond of
n cisio n
T dimer and cleavage 3' lo uncut T
Displacement by polymerase
FIGURE 24-15
Two modes of excision repair, (a)
Escherichia coli
mechanism. Two incision steps are followed by gap-filling and
displacement by polymerase I. (b)
Micrococcus luteus
mechanism. A pyrimidine dimer glycosylase breaks an
N-glycosidic bond and makes a single incision. Pol I displaces the strand, which is removed by an exonucleolytic event.
In both mechanisms, the final step is ligation.
E. coli,
pol I polymerizes from this end and displaces
the dimer-containing strand. After the dimer has been dis-
placed, a second cut is made, the displaced strand falls
away, and ligase forms the final phosphodiester bond. Re-
pair of dimers in mammalian cells is more complicated
and poorly understood, and requires a larger collection of
Recombination Repair
Recombination repair is a mechanism for generating a
functional DNA molecule from two damaged molecules.
It is an essential repair process for dividing cells because
a replication fork may arrive at a damaged site, such as
a thymine dimer, before the excision repair system has
eliminated damage.
When pol III reaches a thymine dimer, an adenine is
added to the growing strand. However, the distortion of the
helix caused by the dimer weakens the hydrogen bond and
activates the polymerase editing function, and the adenine
is removed. The cycle begins again—an adenine is added
and then removed—the net result of which is that the repli-
cation fork fails to advance. A cell in which DNA synthesis
is permanently stalled cannot complete a round of repli-
cation and does not divide. However, in a way that is not
understood, after a pause of ~5 seconds per dimer, chain
growth begins again beyond the thymine dimer block. The
result of this process is that the daughter strands have large
gaps, one for each unexcised thymine dimer. Viable daugh-
ter cells cannot be produced by continued replication alone
because the strands having the thymine dimer will con-
tinue to turn out defective daughter strands and the first
set of daughter strands would be fragmented when the
growing fork enters a gap. However, by a recombination
mechanism called
sister-strand exchange
proper double-
stranded molecules can be made.
The essence of sister-strand exchange is that a single-
stranded segment free of any defects is excised from an
undamaged strand on the homologous DNA segment at
the replication fork and somehow inserted into the gap
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