DNA repair is essential to prevent damage from impairing organism survival and potentially causing cancer. The main types of DNA repair are mismatch repair, direct repair, base-excision repair, and nucleotide-excision repair. Mismatch repair in E. coli involves the MutS, MutH, and MutL proteins detecting mismatches that are then repaired. Direct repair directly changes altered bases back to their original structures using enzymes like MGMT. Base-excision repair removes small base lesions using DNA glycosylases and polymerases. Nucleotide-excision repair removes bulky UV-induced DNA adducts by cleaving and removing the damaged strand.

1. DNA REPAIR
PRESENTED BY
SHIDA DHAR
PRIYA KAMAT

2. DANGER OF DNA DAMAGE
 Structural damage: prevent replication, transcription and
translation
 Harmful mutation: impair survival of organism
 Mutation in tumour suppressor genes for examples
 If unrepaired:
 Senecence
 Apoptosis
 Aberrant cell division- cancer

3. TYPES OF DNA REPAIR
1. Mismatch repair
2. Direct repair
3. Base-excision repair
4. Nucleotide- excision repair.

4. MISMATCH REPAIR IN E.COLI
 E. coli has identified a number of genes that, when
mutationally inactivated, cause hypermutable strains.
 The gene products are, therefore, called the "Mut"
proteins, and are the major active components of the
mismatch repair system.
 Three of these proteins are essential in detecting the
mismatch and directing repair machinery to it:
 MutS
 MutH
 MutL

5.  This can be detected and repaired but it is important that
the repair enzyme can distinguish the new strand from the
old.
 This is possible in E.coli because there is an
enzyme(DAM methylase) which methylates the A
sequence GATC.
 This methylation does not occur immediately after
synthesis and until it does the two strands are
distinguishable.

6.  Proteins Mut S binds to the mismatch bases and forms a
complex with Mut H and Mut L and the mismatch is
brought close to a methylated GACT sequence and the
new strands is identified
 Exonucleases remove nucleotides on the new strand
between the GATC sequence and the mismatch
 DNA polymerase then replaces the nucleotides
,correcting the mismatch, and DNA ligase seals the nick
in the sugar-phosphate backbone.

8. DIRECT REPAIR
 Direct repair mechanisms do not replace altered nucleotides but instead
change them back to original structures
 Some normal methylated bases in DNA becomes alkylated by
carcinogens or by interaction with the normal methyl carriers in cells.
 Alkylation on positions that affect base pairing is highly mutagenic.eg. O⁶
methyl-guanine
 Cells have glycosylases that recognize methylated bases and trigger
base excision repair.
 O⁶- methyl guanine methyl transferase(MGMT) recognizes O⁶- methyl guanine
and transfers the methyl group from the guanine to a cysteine on the protein
itself
 Methylated MGMT protein is inactive, becomes ubquitinated, and is then
degraded.

10. BASE-EXCISION REPAIR(BER)
 In base-excision repair, is a cellular mechanism that repairs damaged
DNA throughout the cell cycle.
 It is responsible primarily for removing small, non-helix-distorting base
lesions from the genome.
 A modified base is first excised & then the entire nucleotide is
replaced.

11. BER CONTD..
 Proteins involved in BER are:
1. DNA glycosylases
2. AP endonucleases
3. End processing enzymes
4. DNA polymerases
5. Flap endonuclease
6. DNA ligase

12. BER CONTD…
 BER has 2 subpathways:
1. Short patch:- replaces the lesion with single nucleotide.
2. Long patch:- replaces the lesion with approximately 2 to 10
nucleotides.

13. BER PATHWAY
1 Each DNA glycosylase recognizes &
removes a specific type of damaged base producing an
apurinic or apyrimidinic site(AP site).
2 AP endonuclease cleaves the phosphodiester bond
on the 5’ side of the AP site.
3 Other enzymes remove the deoxyribose sugar.
4 DNA polymerase adds new nucleotides to the exposed
3’-OH group.
5 The nick in the phosphodiester backbone is sealed by
DNA ligase.

16. NUCLEOTIDE EXCISION REPAIR(NER)
 Nucleotide excision repair (NER) is a particularly important excision
mechanism that removes DNA damage induced by ultraviolet
light (UV).
 UV DNA damage results in bulky DNA adducts- these adducts are
mostly thymine dimers and 6,4-photoproducts.

17. NER PATHWAY
1 Damage to the DNA distorts the configuration of the
molecule.
2 An enzyme complex recognizes the distortion
resulting from damage.
3 The DNA is separated & single-strand-binding proteins
stabilize the single strands.
4 An enzyme cleaves the strand on both sides of the
damage.

18. NER PATHWAY CONTD..
5 Part of the damaged strand is removed
6 Gap is filled in by DNA polymerase & sealed by DNA
ligase.