The document discusses DNA repair, emphasizing its critical role in maintaining genetic stability. It elaborates on the types of DNA damage, sources, causes, consequences, and various repair mechanisms, including direct repair, excision repair, mismatch repair, and recombinational repair. Ultimately, these processes are essential to prevent genome instability, cancer risk, and other diseases.

1. UNIVERSITY OF RAJASTHAN
SUBMITTED TO:
STANI MEMORIAL PG COLLEGE, JAIPUR
DNA REPAIR
SUBMITTED BY :
DR. RAKHI SHARMA
HOD ZOOLOGY
SUNIDHI
M.SC ZOOLOGY(PRE.)
2017-18

2. INTRODUCTION:
Genetic variations are important for evolution but the survival of the
individual demands genetic stability. Maintaining genetic stability
requires an extremely accurate mechanism for replicating DNA and
mechanism for repairing the many accidental lesions that occur
continually in DNA.
Most such changes are temporary because they are immediately
corrected by a set of processes that are collectively called as DNA
repair.

3. CONTENTS:
• Double helical structure of DNA
• DNA damage
• Sources of DNA damage
• Causes of DNA damage
• Consequence of DNA damage
• DNA repair
• Direct repair
• Excision repair
• Mismatch repair
• Recombinational repair
• References

4. DOUBLE HELICAL STRUCTURE OF DNA:

5. DNA DAMAGE:
DNA damage is the alteration in the chemical structure of DNA,
such as a break in a strand of DNA, a base missing from the
backbone of DNA, a chemically changed base.
Damage to DNA that occurs naturally can result from metabolic or
hydrolytic processes.

6. SOURCES OF DNA DAMAGE:
• Endogenous damage:
It includes damage from within the cell.
It also includes replication errors.
Example: Attack by reactive oxygen species produced from normal metabolic byproducts.
• Exogenous damage:
It includes damage caused by external agents.
Examples: 1.UV, X rays and gamma rays.
2.Plants of comfrey species.
3.Viruses.

7. CAUSES OF DNA DAMAGE:
• UV Rays:
Random photons of ultraviolet light induce aberrant bonding between neighbouring
pyrimidines(thymine and cytosine) bases on the same strand of DNA. This will prevent the
replication.

8. CONTD…
• Alkylating agents:
Methyl and ethyl group added to DNA bases alters the structure of DNA.
• Deamination:
An amino group of cytosine is removed and the base becomes uracil.
An amino group of adenine or guanine is removed and the base becomes hyoxanthine.

9. CONTD…
• Depurination:
The base is simply ripped out of the DNA molecule leaving a gap(like a missing tooth).

10. CONSEQUENCES OF DNA DAMAGE:
• Leads to genome instability.
• Increased cancer risk.
• Accelerated ageing.
• Neurodegenerative diseases.

11. • DNA repair refers to the number of processes by which a cell
identifies and corrects damage to the DNA molecules that encode
its genome.
• Depending on the type of damage inflicted on DNA‘s double
helical structure, a variety of repair strategies have evolved to
restore lost information.
DNA REPAIR :

12. TYPES OF DNA REPAIR MECHANISMS:
1. Direct reversal
2. Excision repair
a) Base excision repair
b)Nucleotide excision repair
3. Mismatch repair
4. Recombinational repair

13. DIRECT REPAIR:
• This system act directly on damaged nucleotides and convert each
one back to its original structure.But only a few damaged
nucleotides can be repaired directly.
• Pyrimidine dimers are repaired by a light-dependent direct system
called photoreactivation.

14. DIRECT REPAIR:

15. EXCISION REPAIR:
• It involves excision of a segment of the polynucleotide containing a
damage site, followed by resynthesis of the correct nucleotide
sequence by a DNA polymerase.
• Excision → Resynthesis → Ligation.
• Excision repair is of two types:
a)Base excision repair
b) Nucleotide excision repair

16. BASE EXCISION REPAIR:
• BER repairs damage to a single base caused by oxidation, alkylation,
hydrolysis, or deamination.
• The damaged base is removed by a DNA glycosylase, resynthesized
by a DNA polymerase, and a DNA ligase performs the final nick-
sealing step.

17. BASE EXCISION REPAIR:

18. NUCLEOTIDE EXCISION REPAIR:
• Nucleotide excision repair
recognizes bulky helix-
distorting lesions such as
pyrimidine dimers and 6,4
photoproducts and treats
them.

19. MISMATCH REPAIR:
• Mismatch repair corrects errors of DNA replication and
recombination that results in mispaired but undamaged
nucleotides.

20. RECOMBINATIONAL REPAIR:
• Recombinational repair requires the presence of an identical or
nearly identical squence to be used as a template for repair of the
break in the DNA strand.
• The enzymatic machinery responsible is nearly identical to the
machinery responsible for chromosomal crossover during meiosis.

21. REPLICATIONAL REPAIR :

22. REFERENCES:
• Albert, B., Bray, DD., Lewis, J., Rafif M., Roberts, K, Walson, J.D., Molecular Biology
of the Cell. Garland Publishing Company, Inc., New York.
• Benjamin, Lewin, Gene VIII, Oxford University Press, U.K.
• http://www.ncbi.nlm.nih.gov/pubmedhealth/PMH0002369.
• http://www.suit101.com/content/defects-in-dna-repair-and-resulting-diseases-
a137961