DNA can become damaged through external environmental factors like radiation or internally through natural chemical reactions. If left unrepaired, damaged DNA can lead to cancer or genetic disorders. The body has multiple DNA repair mechanisms including base excision repair, nucleotide excision repair, and double-strand break repair. These mechanisms recognize and remove damaged or incorrect DNA bases. Enzymes then excise the damage and DNA polymerases fill in the correct DNA sequence before ligases seal the DNA backbone. Without effective DNA repair, mutations can accumulate and cause cell harm.

1. Department of Biochemistry, Nobel College, Nepal
Friday, April 8,
2016
Rajesh Chaudhary
1
Students

2. Why DNA needs to be repaired?
RNA and proteins can be replaced by the information encoded
in the DNA molecule, but DNA itself can’t be replaced by a new
DNA when gets damaged.
Unrepaired DNA leads to cancer production.
Thus, needs repair.
Friday, April 8,
2016
Rajesh Chaudhary
2
MUTATION: A permanent change in the nucleotide base of the DNA is
called “mutation”.

3. Why DNA needs to be repair?
Friday, April 8,
2016
Rajesh Chaudhary
3
Xeroderma pigmentosum
Sickle cell anemia
Consequences of unrepaired DNA

4. Sickle cell anemia
Friday, April 8,
2016
Rajesh Chaudhary
4

5. How DNA gets damaged?
1. By the external environmental factors.
Example: Ionizing radiations, ultra violet rays of sun,
chemical agents, due to toxicity produced in the body.
2. Gets damaged spontaneously.
Friday, April 8,
2016
Rajesh Chaudhary
5
Chemistry of DNA damage is diverse and complex.
Rate of DNA damage: 1 million individual molecular lesion per cell/day.

6. Error made during DNA replication
Friday, April 8,
2016
Rajesh Chaudhary
6

7. Chemical reactions known to create
serious DNA damage
Friday, April 8,
2016
Rajesh Chaudhary
7
Both depurination and deamination doesn’t breaks phosphodiester bond.
During the time it takes to read this sentence 1012 purine bases will be lost from DNA due to
depurination reaction.

8. Deamination and Depurination
Friday, April 8,
2016
Rajesh Chaudhary
8

9. Deamination reaction
Friday, April 8,
2016
Rajesh Chaudhary
9
Cytosine Uracil
In DNA spontaneously deamination is corrected by removal of uracil by uracil-DNA glycosylase,
generating abasic (AP) site. The resulting AP is recognized by AP endonuclease enzyme that breaks
phosphodiester bond in the DNA, permitting the repair of the resulting lesion by replacement with
another cytosine.

10. What are the changes seen in damaged
DNA?
Friday, April 8,
2016
Rajesh Chaudhary
10
Formation of Thymidine dimer, Double stranded break
Xeroderma pigmentosum is the result of thymidine dimer
formation because of sunlight exposure

11. So, how to repair damaged DNA?
Through excision and repair mechanism.
Excision repair is divided into two major categories:
1. Nucleotide excision and repair
2. Base excision and repair
Friday, April 8,
2016
Rajesh Chaudhary
11

12. Basic DNA repair mechanism
1st. (Excision): Damage is cut by one of the
series of nucleases each specialized for a type
of DNA damage.
2nd. (Re-synthesis): Original DNA sequence is
restored by a repair DNA polymerase.
3rd. (Ligation): DNA ligase seals the nick left in
the sugar-phosphate backbone of repaired
strand.
 (NOTE: Ligation requires ATP as energy source.)
Friday, April 8,
2016
Rajesh Chaudhary
12

13. Nucleotide excision repair
Friday, April 8,
2016
13 Methyl-directed mismatch repair
1. Identification of mismatch strand.
2. Repair of damaged DNA

14. Methylation and Mismatch repair
Friday, April 8,
2016
Rajesh Chaudhary
14
1 2 3

15. “Mutation in the protein involved
in mismatch repair is associated
with hereditary nonpolyposis
colorectal cancer (HNCC).”
Friday, April 8,
2016
Rajesh Chaudhary
15
Lippincott’s Illustrated Review, 5th. Edition

16. Repair of damage caused by
ultraviolet (UV) light
1. Recognition and excision
of dimers by UV-specific
endonuclease
2. UV radiation and cancer
Friday, April 8,
2016
Rajesh Chaudhary
16
Example of E.Coli DNA

17. Base excision repair
 Bases of DNA can be altered, either spontaneously, as in the case
with cytosine, which slowly undergoes deamination to form uracil, or
by the action of delaminating or alkylating compounds.
 Approximately 10,000 purine bases are lost this way per cell per
day.
 1. Removal of abnormal bases.
 2. Recognition and repair of an AP site.
Friday, April 8,
2016
Rajesh Chaudhary
17
Correction of base alterations

18. Correction of base alteration by base
excision repair
Friday, April 8,
2016
Rajesh Chaudhary
18
Damage in DNA is recognized by the glycosylase enzyme.

19. Repair of double stranded break
Double stranded breaks are caused by the high energy
radiation or oxidative free radicals.
Such breaks occur naturally during gene rearrangements.
It can’t be corrected by any of the above mentioned
strategies.
But, can be repaired either by:
1. Non-homologous end-joining repair.
2. Homologous recombination repair.
Friday, April 8,
2016
Rajesh Chaudhary
19

20. Double stranded DNA repair break20

21. Mutation
Mutation is a sudden and permanent alteration of
nucleotide sequence of the genome of an organism, virus
or extrachromosomal DNA or genetic elements.
Friday, April 8,
2016
Rajesh Chaudhary
21

22. Chemical modifications of nucleotide if left
unrepaired leads to mutation
Friday, April 8,
2016
Rajesh Chaudhary
22

23. Mutation
Somatic mutation: Occurs in non-reproductive cells and
won’t be passed on to offspring and do not matter for
evolution.
Germ line mutation: occurs in reproductive cells like eggs
and sperm and are called germline mutation and it matters
for evolution.
Friday, April 8,
2016
Rajesh Chaudhary
23

24. Mutation on the control gene
Friday, April 8,
2016
Rajesh Chaudhary
24

25. Types of Mutation
1. Substitution
2. Insertion
3. Deletion
4. Frameshift
Friday, April 8,
2016
Rajesh Chaudhary
25
How about “Silent mutation”?

26. Mismatch repair eliminates replication error
and restores DNA sequence
Friday, April 8,
2016
Rajesh Chaudhary
26
DNA mismatch repair corrects 99% of
the replication errors increasing overall
accuracy to one mistake in 109 nucleotide
copied.

27. Failure to repair damaged DNA have severe
consequences
Friday, April 8,
2016
Rajesh Chaudhary
27

28. Importance of mutations
Creates the diversity in a population, thus some
group of population with a particular mutation
might be immune to some disease.
Example: Immune to malaria
Friday, April 8,
2016
Rajesh Chaudhary
28

29. Summary of DNA repair
Friday, April 8,
2016
Rajesh Chaudhary
29

30. Friday, April 8,
2016
Rajesh Chaudhary
30