Translession DNA Synthesis Ds Break repair Homologous Recombination

2. site specific recombination
is illegitimate..
Transposase enzyme
recognized the self DNA
and cut it and also
recognized the target site.
Target site is not specific .

4. Transposons

6.  Reterovirus Like elements (RLE)- like reterovirus
 42 % human genome made up on
reterotransposons ..

7. Repair system
Direct Repair
1) Photoreactivation
Excision Repair
BER
NER
Mismatch
Transcriptional coupled
repair system
Double stranded
Break Repair
1.Homologous
recombination
2. Non-homologous
End joining Mech:
Single stranded DNA Repair
(Translession DNA Synthesis)
SOS
heavy damage 
translesion
polymerases
Any type of damage remove before cell division
Genetic information can be stored stably in DNA sequences only because a large set of DNA repair
enzymes continuously scan the DNA and replace any damaged nucleotides.

8. • Translession DNA Synthesis
• oxidizing agents , metabolites , radiation and reactive
chemicals DNA suffers heavy damage  translesion
polymerases
• When DNA damage is excessive, a special class of inaccurate
DNA polymerases, called translesion polymerases, is used to
bypass the damage, allowing the cell to survive but
sometimes creating permanent mutations at the sites of
damage.
• Y- Family DNA polymerase
• Lack of proofreading Activity
• Add nucleotide at lesion without base pairing
DNA Polymerase IV ---Din B Gene
DNA Polymerase V – UMU, UMUD Gene /------ Repressor Lex -A
SOS Repair / Translession DNA Synthesis /
Error prone Repair

9. DNA Polymerase IV ---Din B Gene
DNA Polymerase V – UMU, UMUD Gene /------ Repressor
Lex –A
In normal condition Lex –A Repressor protein bind to
Din B , UMU, UMUD Gene . And inhibit expression of
translesion polymerases.
Excessive DNA Damage or single strand DNA
Damage = High
Rec-A Protein Active and Interact with Lex-A and
Auto-Protease Activity of Lex-A is on.
Cleaved Lex-A – not bind to Din B , UMU, UMUD Gene
DNA Pol- IV DNA Pol-V
Absent Proofreading Activity
Add nucleotide at lesion without
base pairing

10. Double strand break repair
• Prokaryotic and lower eukaryotic – Homologous Recombination
• Higher eukaryotic – Non – Homologous End Joining (NHEJ)
Error prone Repair mechanism .

12. HOMOLOGOUS RECOMBINATION
• the mechanisms that allow the DNA sequences in cells to be
maintained from generation to generation with very little change.
• repair mechanism is essential for every proliferating cell
• homologous recombination (also known as general recombination)is an
exchange of DNA strands between a pair of homologous duplex DNA
• Work in meiosis in plants and animals.
• daughter DNA molecules are still held close together. homologous
recombination  flexible series of reactions

13. Prokaryotics
RecA first binds cooperatively to the invading single
strand, help to pairing of homologous strands.
Rec B,C,D – processing of ds DNA break.
5’3’ exonuclease
3’5’ Exonuclease
strand exchange protein complex
DNA Pol III and DNA Ligase
DNA synthesis and ligation
RUV-A,B – Branch Migration steps (Helicase Activity)
RUV-C – Resolution ( Holiday Junction)
 Both strand separate.

14. Homologous recombination in eukaryotic
• DNA Damage / Mutant sensor Kinase – ATM (Ataxia-Telangiectasia
Mutated kinase) .
• Sensory Kinase- chk-2 .
• Pairing of Homologous strands – BRCA-1, BRCA-2, WRN, Rad-5 .
• Processing of double strand Break – Rad-50,Rad 58,Rad 60 or MRX
complex in Yeast.
• Strand exchange protein Assembly – Rad 52, Rad 58.
• Resolution ( Holiday Junction)
 Both strand separate  Nuclease or resolvase

15. NHEJ
If these lesions were left unrepaired , they would
quickly lead to the breakdown of chromosomes
into smaller fragments and to loss of genes .
Nonhomologous end joining  loss of nucleotides
at the site of joining .
end-joining mechanism “quick and dirty” solution to
the repair of double-strand breaks  common in
mammalian somatic cells .
Age of 70.
Nonhomologous end joining  one broken
chromosome becomes covalently attached to
another. This can result in chromosomes with two
centromeres and chromosomes lacking
centromeres.

16. NHEJ
1. Recognition  Ku protein 70/80 
heterodimer binds to broken chromosome
ends.
2. End processing-> Additional proteins
(ARTEMIS,MRN) needed to hold the broken
ends together while they are processed and
eventually joined covalently.
3. Strand invasion , DNA synthesis and
Resolution
4. Ligation- Lig-IV