Download Repair of Damaged DNA

Repair of Damaged DNA
• DNA is the only cellular macromolecule that can be
repaired
• DNA damage includes:
base modifications
nucleotide deletions or insertions
cross-linking of DNA strands
breakage of phosphodiester backbone
•
Specific repair enzymes scan DNA to detect any
alterations
•
Repair protects individual cells and subsequent
generations
• Mechanisms:
1. direct repair - does not require breaking the
phosphodiester backbone of DNA
2. Mismatch repair – replaces incorrect bases
shortly after replication
3. Base-excision repair- cuts out damaged bases
4. Nucleotide-excision repair – removes largerscale distortions by excision
Methyl-Directed Mismatch Repair
-Most DNA is methylated, particularly at adenosine
-Newly synthesized DNA is not methylated for
perhaps several minutes
-Mismatch repair enzymes recognize incorrect bases
close to GATC’s
-Cuts them out and replaces
Base-Excision Repair
• DNA can be damaged by alkylation, methylation,
deamination, loss of heterocyclic bases
(depurination or depyrimidization)
• Glycosylases recognize and remove base (leaves an
AP site – abasic site)
• Sugar and phosphate not removed yet
• AP endonucleases cut backbone
• Segment is removed and replaced
• Details are specific to type of lesion and type of AP
endonuclease
• Hydrolytic deamination
of cytosine to uracil
• Uracil in place of cytosine
causes incorporation of an
incorrect base during
replication
• DNA glycosylases hydrolyze
base-sugar N-glycosidic bonds
• Deaminated bases are then
removed and replaced
Nucleotide excision repair
• Enzyme complex cuts backbone in two places
– Prokaryotes 12-13 bases apart
– Eukaryotes 27-29 bases apart
• E. coli uses the ABC excinuclease (catalyzes 2
specific cuts on same strand)
• Helicase helps remove damaged section
• Section replaced
Repair after Photodimerization:
Direct Repair
• Double-helical DNA is very sensitive to damage by
ultaviolet (UV) light
• Dimerization of adjacent pyrimidines in a DNA
strand is common (e.g. thymines)
• Replication cannot proceed in the presence of
pyrimidine dimers (distort the template strand)
• Thymine dimers are repaired in all organisms
• DNA photolyase is primary repair protein
• Repair of thymine dimers
by DNA photolyase
Homologous Recombination
•
Recombination - exchange or transfer of pieces of
DNA from one chromosome to another or within a
chromosome
• Three types
1. Homologous - exchange between sections of
DNA with closely related sequences
2. Site-specific
3. Transposition - occurs between unrelated
sequences (e.g. Transposons; jumping genes )
Homologous Recombination
Three purposes:
1. Recombinational DNA repair
2. DNA organization during meiosis (eukaryotes)
3. Genetic diversity (exchanging alleles)
Two Chromatids showing
crossover points
The Holliday Model of
Recombination
Site-specific Recombination
Purposes: 1.
2.
3.
Requires:
gene expression regulation
embryonic programmed rearrangement
viral & plasmid replication processes
1. Recombinase enzyme
2. 20-200 bp recombination site
3. auxiliary proteins
Can occur between 2 different DNA strands or 2
points within the same strand
1. Recombinase recognizes
recombination sites.
2. Backbone bond are
broken.
3. Transient bonds to
recombinase are made.
4. Holliday intermediates
are formed.
5. Recombination is
completed by cleavage and
ligation.
Exchange is reciprocal and precise.
Recombination sites are preserved.
Outcome depend on location and orientation (5'→3')
of site
1. On same DNA molecule – inversion or deletion
2. On different DNA’s - recombination
3. On circular DNA’s - Insertion
Tranposition
New segment moved to a
target site.
Either directly moved
or
a copy is moved (replicative
transposition).