INTRODUCTION ■ DNA DAMAGE ■ TYPES OF DNA REPAIR: ►BASE EXCISION REPAIR(BER) ►NUCLEOTIDE EXCISION REPAIR(NER) ►MISMATCH REPAIR ►HOMOLOGOUS RECOMBINATION REPAIR(HRR) ►NON-HOMOLOGOUS END JOINING(NHEJ) ■ CONCLUSION ■ REFERENCES

1. “DNA REPAIR”
By
KAUSHAL KUMAR SAHU
Assistant Professor (Ad Hoc)
Department of Biotechnology
Govt. Digvijay Autonomous P. G. College
Raj-Nandgaon ( C. G. )

2. CONTENTS
■ INTRODUCTION
■ DNA DAMAGE
■ TYPES OF DNA REPAIR:
►BASE EXCISION REPAIR(BER)
►NUCLEOTIDE EXCISION REPAIR(NER)
►MISMATCH REPAIR
►HOMOLOGOUS RECOMBINATION REPAIR(HRR)
►NON-HOMOLOGOUS END JOINING(NHEJ)
■ CONCLUSION
■ REFERENCES

3. DNA DAMAGE
• DNA damage is an alteration in the chemical
structure of DNA,such as a break in a strand of
DNA,a base missing from backbone of DNA
or a chemically modified base.
• DNA damage naturally or via environmental
factors.

4. WHAT IS DNA REPAIR?
• It is a mechanism for repairing many
accidental lesions that DNA continually
suffers.
• Most such spontaneous changes in DNA are
temporary because they are immediately
corrected by a set of processes that are
collectively called DNA repair.

8. DIRECT REPAIR
• Damaged bases can be repaired directly
by specific enzymes.
• Photolyase can repair thymine dimers
It splits the dimers restoring the DNA to
its original condition.
•
1.DIRECT REPAIR

10. • It removes a damaged base.
• BER involves the function of variety of enzymes known as
DNA N-glycosylases.
• This type of enzyme can recognise an abnormal base and
cleave the bond between it and the sugar in the DNA
backbone,creating an apurinic or apyrimidinic site.
• Living organism produce multiple type of DNA N-glcosylase
which can eliminate bases such as Uracil,3-Methyladenine,7-
methylguanine,and pyrimidine dimers.
☼ (BER is important for the repair of oxidative DNA damages)
2. BASE EXCISION REPAIR(BER)

12. NUCLEOTIDE EXCISION REPAIR(NER)
• NER can repair many different types of large DNA
damages (bulky lesions) including thymine
dimers,chemically modified bases,missing bases.
• In NER,several nucleotides in the damaged strand are
removed from the DNA,and the intact strand is used as
a template for resynthesis of a normal complementary
strand.
• NER is found in both eukaryotes and prokaryotes,
although its molecular mechanism is better understood
in prokaryotes.
3.NUCLEOTIDE EXCISION REPAIR(NER)

13. NER IN E.coli
• In E. coli, the NER system requires four key proteins.
■ These are designated as UrvA, UrvB, UvrC and UvrD.
■ Named as such because they are involved n Ultraviolet
light repair of pyrimidine dimers.
■ They are also important in repairing chemically damaged
DNA.
■ UvrA, B, C, and D recognises and remove a short segment of
damaged DNA.
■ DNA polymerase and ligase finish the repair job.

15. MISMATCH REPAIR
• It recognise and correct a base pair mismatch.
• The structure of DNA double helix obeys the AT/GC
rule of base pairing.
• During DNA replication an incorrect nucleotide
may be added to the growing strand by mistake this
produces a mismatch between parental and newly
synthesised strand.
4.BASE MISMATCH REPAIR

17. DOUBLE STRANDED
BREAK REPAIR

18. 6.HOMOLOGOUS
RECOMBINATION REPAIR(HRR)
• HRR also called homology directed repair, occurs when homologous dna
strand,usually from a sister chromatid,are used to repair a ds break in the
other sister chromatid.
• First, the DSB is processed by the short digestion of DNA strands at the
break site.
• This processing event is followed by the exchange of DNA strands between
the broken and unbroken sister chromatids.
• The unbroken strands are then used as templates to synthesize DNA in the
region where the break occurred. Finally, the crisscrossed strands are
resolved, which means they are broken and then rejoined in a way that
produces separate chromatids.

20. NON-HOMOLOGOUS END JOINING(NHEJ)
• During NHEJ,the two broken ends of DNA are simply pieced
back together.
• This mechanism requires the participation of several proteins that
play key roles in the process.
• First, the DSB is recognized by end-binding proteins. These
proteins then recognize additional proteins that form a cross-
bridge that prevents the two ends from drifting apart.
• Next, additional proteins are recruited to the region that may
process the ends of the broken chromosome by digesting
particular DNA strands.
• Finally, any gaps are filled in via DNA polymerase,
and the DNA ends are ligated together.

22. CONCLUSION

23. REFERENCES