DNA repair mechanism

DNA repair mechanism, topic of Genetics and Genomics

1. DNA REPAIR MECHANISM By Dr. Sonia Goel SGT University, Gurugram

2. INTRODUCTION 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. TYPES Direct Repair ( photoreactivation ) Excision Repair base excision repair Nucleotide excision repair Mismatch repair Recombinational repair SOS - repair

4. DIRECT REPAIR 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. Photo reactivation 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 b\w T-T dimer and cause reverse to original monomeric nucleotides.

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

7. EXCISION REPAIR 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

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

9. STEPS: UDG recognizes U racil . UDG cleaves N- glycosyl bond UDG flip out U racil . AP E ndonuclease cleaves phosphodiester bckbone DNA polymerase refills the gap. DNA ligase seals the nick.

10. NUCLEOTIDE EXCISION REPAIR 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: Xeroderma Pigmentosum Cockayne syndrome

11. Uvr system of NER in E. Coli:

12. MISMATCH REPAIR 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 .

13. Mismatch repair mechanism in E.Coli

14. RECOMBINATIONAL REPAIR 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.

15. Recombinational repair in E.Coli

16. SOS-REPAIR 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.

17. Mechanism of SOS RESONSE

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