DNA repair mechanism, topic of Genetics and Genomics

1. By
Dr. Sonia Goel
SGT University,
Gurugram

2.  DNA repair is a process by which the cell repairs
mutations that occur in the DNA.
 DNA carries all the necessary information for the
growth and function of the cell, and it is therefore
important that n nay inadvertent change is set right
before it can become detrimental to the cell
 The cell’s DNA repair mechanism continuously scans
the genome to remove or mend such mutations.

3.  Direct Repair (photoreactivation)
 Excision Repair
a) base excision repair
b) Nucleotide excision repair
 Mismatch repair
 Recombinational repair
 SOS - repair

4.  Direct repair acts directly on damaged nucleoetides,
converting each one back to its original structure . But only
few types of damaged nucleotides can be repaired directly.
 Example- UV radiations-mediated damages are repaired by
the PHOTOREACTIVATION.

5.  The UV damages caused in cells are repaired after
exposure of cells in visible light (300-500nm).
 Enzyme ‘DNA photolyase’ (lacks in mutant cell)
activated when exposed to visible light.
 This enzyme absorbs energy, binds to cyclobutane ring
to defective sites of DNA
 Promotes cleavage of bond bw T-T dimer and cause
reverse to original monomeric nucleotides.

7. Another example of direct repair:
 removal of alkyl group from O⁶-methylguanine.

8.  More elaborate system in which the damaged
nucleotides is not repaired , but removed from the
DNA.
 In this system, undamaged strand serves as a template
for the reincorporation of correct nucleotide by the
DNA polymerase.
 Types: 1) base excision repair
2) nucleotide excision repair

9.  Removal of damaged nucleotides base, excision of
short piece of polynucleotide and resynthesis with
DNA polymerase.
ENZYMES INVOLVED:
1. DNA glycocyclase: cleaves N- glycosyl bond to remove
damaged base. (ex.-uracil DNA glycocylcase)
2. AP Endonuclease: cleaves the phosphdieseter bond at
5’ .
3. DNA polymerase: fills the gap
4. DNA ligase: joins the ends.

10. STEPS:
1. UDG recognizes Uracil.
2. UDG cleaves N- glycosyl
bond
3. UDG flip out Uracil.
4. AP Endonuclease cleaves
phosphodiester bckbone
5. DNA polymerase refills
the gap.
6. DNA ligase seals the nick.

11.  Versatile process that can remove many forms of DNA
damage by nuclease cleavage on either side of the
damaged bases, removal of the damaged
oligonuclotide, and resynthesis of a patch using the
undamaged strand as the template.
 NER occurs in almost all organisms examined.
 Common feature: modified nucleotides cause a
significant distortion in the DNA helix.
 Disease due to defective NER:
1. Xeroderma Pigmentosum
2. Cockayne syndrome

12. Uvr system of NER in E. Coli:

13.  Base pair mismatch occur during DNA replication are fixed by
this repair.
 In E.Coli. ,3 protiens involved : ‘Mut S, Mut L, Mut H’
 Mut S: detects mismatch site & binds to it ; form complex with
Mut L.
 Formed complex recruits Mut H to GATC sequence at
hemimethylated site. This complex intraxct with Mut H by DNA
looping mechanism.
 Mut H: has site specific endonuclease activity that is inactive until
complex encounters a hemimethylated GATC sequence.
 Unmethylated strand is unwound and degraded in 3’ to 5’
direction.
 This segment is replace with new DNA with combined actions of
DNA helicase, SSB, exonuclease, DNA polymerase and ligase.

14. Mismatch
repair
mechanism
in E.Coli

15.  The process of filling gap in one strand of dsDNA by
retrieving a homologous single strand from another
dsDNA.
 Occurs after replication (post translation method).
 In E.Coli, when DNA is replicated, dimer prevents the
damaged sites from acting as template. Replication is
forced to bypass it.
 This results in gap in newly synthesised strand, while
other parental strand forms normal complementary
strand.
 Gap opposite the damaged site in one double strand is
filled by taking the homologous ssDNA from normal
duplex.

16. Recombination
al repair in
E.Coli

17.  SOS (save our soul) is a global response to DNA
damage in which the cell cycle is arrested and the
DNA repair mutagenesis are induced.
 Sos repair reconstruct the chemical structure of DNA
but the hereditary information is lost.
 Also called inducible repair.
 It involves more than 40 gene which encodes protein
responsible for protection and replication of DNA as
well as repair and mutation.

18. Mechanism of SOS RESONSE