?Iifi/N
Eukaryotic
Cetts
Have
Conserved Repair
Systems
o
The
yeast
R/D mutations, identified
by
radiation sensitjve
phenotypes,
are in
genes
that code for repair
systems.
.
Xeroderma
pigmentosum (XP)
is a human
disease caused by mutatjons in any one of
severaI
repair
genes.
.
A complex of
proteins
inctuding XP
prod-
ucts and the transcription factor TFnr
pro-
vides a human excision-repair mechanism.
r
Transcriptionatly
active
genes
are
prefer-
entiatty
repaired.
A Common System Repairs Double-Strand
Breaks
o
The NHEJ
pathway
can ligate blunt ends of
dupLex
DNA.
o
Mutations in the NHEJ
pathway
cause
human diseases.
Summary
@
Introduction
Any event
that
introduces
a deviation from the
usual double-helical structure
of
DNA is
a threat
to the
genetic
constitution of the cell. Injury to
r!**lt[
f*.i
Repairgenescan
be ctassified into
pathwaysthat
use
different mechanisms
to
reverse
or bypass
damage to
DNA.
DNA is minimized by systems
that
recognize
and correct the damage.
The repair
systems are
as complex
as the replication apparatus itself,
which indicates their importance for the
sur-
vival of
the cell. When a repair
system reverses
a change to DNA, there
is no
consequence. A
mutation may result, though,
when
it fails
to do
so.
The measured rate
of
mutation
reflects a bal-
ance between the number of damaging
events
occurring
in DNA and the number
that have
been corrected
(or
miscorrected).
Repair systems often can recognize a range
of distortions in DNA as signals for
action, and
a cell
is likely
to
have
several systems able to
deal with DNA damage. The importance
of DNA
repair in eukaryotes is indicated
by the
identi-
fication of
>130
repair
genes
in
the human
genome.
We may divide the repair
systems into
several
general
types, as summarized in
F:ili-8ftil
g*.1
.
Some enzymes directly reverse
specific
sorts of damage to DNA.
.
There are
pathways
for
base excision
repair, nucleotide excision
repair, and
mismatch repair, all
of which function
by
removing
and replacing material.
.
There are systems
that function
by
using
recombination
to retrieve
an undam-
aged copy that is then
used to replace a
damaged duplex sequence.
.
The nonhomologous
end-joining
path-
way rejoins
broken double-stranded
ends.
.
Several different DNA
polymerases
can
resynthesize
stretches of replacement
DNA.
Direct
repair is rare and involves
the rever-
sal or simple removal
of the damage.
Photore-
activation of
pyrimidine
dimers, in
which the
offending covalent
bonds are reversed
by a
light-dependent
enzyme. is a
good
example.
s00 CHAPTER 20
Repair
Systems