Page 79 - Genetics_From_Genes_to_Genomes_6th_FULL_Part2

238 Chapter 7 Anatomy and Function of a Gene: Dissection Through Mutation

base pairing distorts the double helix, resulting in abnor- Using sloppy DNA polymerases
mal bulges and hollows. But how does the system know One type of emergency repair in bacteria, called the SOS
whether to correct the pair to a G–C or to an A–T? system (after the Morse code distress signal), relies on
Bacteria solve this problem by placing a distinguish- error-prone (or sloppy) DNA polymerases. These sloppy
ing mark on the parental DNA strands at specific places: DNA polymerases are not available for normal DNA repli-
Everywhere the sequence GATC occurs, the enzyme ad- cation; they are produced only in the presence of DNA
enine methylase puts a methyl group on the A (Fig. 7.19a). damage. The damage-induced, error-prone DNA polymer-
Shortly after replication, the old template strand bears ases are attracted to replication forks that have become
the methyl mark, while the new daughter strand—which stalled at sites of unrepaired, damaged nucleotides. There
contains the wrong nucleotide—is as yet unmarked the enzymes add random nucleotides to the strand being
(Fig. 7.19b). A pair of proteins in E. coli, called MutL synthesized opposite the damaged bases.
and MutS, detect and bind to the mismatched nucleo- The SOS polymerase enzymes thus allow the cell with
tides. MutL and MutS direct another protein, MutH, to damaged DNA to divide into two daughter cells, but be-
nick the newly synthesized strand of DNA at a position cause at each position the sloppy polymerases restore the
across from the nearest methylated GATC; MutH can proper nucleotide only one-quarter of the time, the ge-
discriminate the newly synthesized strand because its nomes of these daughter cells carry new mutations. In bac-
GATC is not methylated (Fig. 7.19c). DNA exonucleases teria, the mutagenic effect of many mutagens either
then remove all the nucleotides between the nick and a depends on, or is enhanced by, the SOS system.
position just beyond the mismatch, leaving a gap on the
new, unmethylated strand (Fig. 7.19d). DNA polymerase
can now resynthesize the information using the old, Sloppy repair of double-strand breaks
methylated strand as a template, and DNA ligase then Another kind of emergency repair system, microhomology-
seals up the repaired strand. With the completion of rep- mediated end-joining (MMEJ), deals with dangerous
lication and repair, enzymes mark the new strand with double-strand breaks that have not been corrected by
methyl groups so that its parental origin will be evident homologous recombination or NHEJ. The mechanism of
in the next round of replication (Fig. 7.19e). MMEJ is similar to that of NHEJ (previously shown in
Eukaryotic cells also have a mismatch correction sys-
tem, but we do not yet know how this system distinguishes Fig. 7.18), except in MMEJ the broken DNA ends are cut
back on either side of the break (resected) by enzymes. The
templates from newly replicated strands. Unlike prokary- resection exposes small single-stranded regions of comple-
otes, GATCs in eukaryotes are not tagged with methyl mentary DNA sequence (microhomology) on either side of
groups, and eukaryotes do not seem to have a protein closely the break that help in bringing the ends together.
related to MutH. One potentially interesting clue is that the Because nucleotides are removed at the sites of the
MutS and MutL proteins in eukaryotes associate with DNA double-stranded breaks during resection, MMEJ results in
replication factors; perhaps these interactions might help deletions of tens to hundreds of base pairs in the rejoined
MutS and MutL identify the strand to be repaired.
DNA. These deletions are longer than the small deletions
of a few base pairs that sometimes result from NHEJ.
Error-Prone Repair Systems Serve
as Last Resorts
Mutations in Genes Encoding DNA Repair
The repair systems just described are very accurate in Proteins Impact Human Health
repairing DNA damage because they can either replace
damaged nucleotides with a complementary copy of the DNA repair mechanisms appear in some form in virtually
undamaged strand or ligate breaks back together. However, all species. For example, humans have six proteins whose
cells sometimes become exposed to levels or types of amino acids are about 25% identical with those of the
mutagens that they cannot handle with these high-fidelity E. coli mismatch repair protein MutS. DNA repair systems
repair systems. Strong doses of UV light, for example, are thus very old and must have evolved soon after life
might make more thymine dimers than the cell can mend. emerged ~3.5 billion years ago. Some scientists think DNA
Any unrepaired damage has severe consequences for cell repair became essential when plants started to deposit oxy-
division; in particular, the DNA polymerases normally gen into the atmosphere, because oxygen helps form free
used in replication will stall at such lesions, so the cells radicals that can damage DNA.
cannot proliferate. These cells can initiate emergency The many known human hereditary diseases associ-
responses that may allow them to overcome these problems ated with the defective repair of DNA damage reveal how
and thus survive and divide, but their ability to proceed in crucial these mechanisms are for survival. In one example,
such circumstances comes at the expense of introducing the cells of patients with xeroderma pigmentosum lack the
new mutations into the genome. ability to conduct nucleotide excision repair; these people