1. DNA DAMAGE
Dr.Riddhi H Patel,
3rd year resident,
Biochemistry

2.  An alteration in the chemical structure
of DNA, such as a break in a strand of DNA,
a base missing from the backbone of DNA

3. Difference between DNA damage
and Mutation
DNA Damage DNA Mutation
• Physical abnormalities in the DNA • is a change in the base
sequence of the DNA
• Can be recognized by enzymes • cannot be recognized by
enzymes once the base change
is present in both DNA strands
• can be correctly repaired if redundant
information, such as the undamaged
sequence in the complementary DNA
strand or in a homologous chromosome,
is available for copying
• a mutation cannot be repaired

4. lTypes of Damage to DNA

5. Types of DNA Damage
• Single Base Alteration:a)Depurination
b)Deamination
c)Alkylation
d)Base analogue incorporation
e)Mismatch base
• Double base Alteration: a)Pymidine Dimer
b)Purine Dimer
• Chain break : a) Single Stranded Break
b) Double Stranded Break
• Cross linking : a)Between DNA to DNA
b)Between DNA to Protein

6. Single base alteration

7. A) Depurination
Causes: -Spontaneous
- Chemical induced like estrogen
- UV radiation
- Ionizing radiation
- Monofunctional alkylating agents
- Free radical

8. lDue to some chemical reaction of endogenous
metabolites
Hydrolytic Cleavage of beta-N glycosidic bond of
PURINE NUCLEOSIDES
Release of Nucleic base
Forms APURINIC SITE
Repaired effeciently by BER
If Not repaire causes Mutaion

10. •Depurination produced by chemical like estrogen

11. 2)Deamination
• Hydrolytic removal of an amine group from a molecule
• Causes:- spontaneous
l -chemically induced(Nitric Oxide induced)

12. Spontaneous Deamination

13.  deamination of 5-methylcytosine
forms thymine. This conversion of a DNA base
from cytosine (C) to thymine (T) can result in
a transition mutation

14. Nitric Oxide induced Deamination
• Auto-oxidation of nirtic acid or from Acidic nitrite
• Nitrosating agent like nitrous anhydrite (N2O3)
• react with amines, thiol and other necleophles
• Deamination of DNA

16. • Repair mainly done by Base excision reapir by
formation of AP site.
• If not repaired :
• Guanine and Adenine deamination leads to the
transition mutation

19. 3)ALKYLATION
• Alkyl group:CnH2n+1
-examples: methyl, ethyl, propyl
• Transfer of an alkyl group from one molecule to
another
• Alkylating Agents:
l -Monofunctional agents
-Bifunctional agents

20. •electrophilic compounds
•electrophiles encounter negative centers in DNA
•attack by alkylating agents are the N7 of guanine and the
N3 of adenine and add alkyl group
•Making bond between sugar and base more labile
•Forms apurinic site
•Repair: -Base Excision Repair
•Not repaired: -Add incorrect base-- mutation
•Example of alkylating agent: ethylmethane sulfonate
(EMS)
Monofunctional Agents

21. Alkylation done by EMS

22. Bifunctional Agents
• Bifunctional agents have two Alkyl groups
• Causes cross linking in DNA

25. Base analogue incorporation

27. • Cell makes mistake for recognizing the bases due to
poor proof reading function sometimes
• Causes change in base
• Example: G could look more like an A

28. • Repair by Mismatch repair
• Mutation to the protein involve in the MMR is
associated with Hereditary NonPolyposis
 Colorectal Cancer (HNPCC) also ka Lynch
syndrome
• HNPCC- causes increase risk of developing
colon cancer

29.  Single Base Alteration: a)Depurination
b)Deamination
c)Alkylation
d)Base analogue incorporation
e)Mismatch base
 Double base Alteration: a)Pymidine Dimer
b)Purine Dimer
 Chain break : a) Single Stranded Break
b) Double Stranded Break
 Cross linking : a)Between DNA to DNA
b)Between DNA to Protein

30. Double Base Alteration

31. • Causes : UV radiation
• May be direct effect of UV radiation

32. UV Radiation
lUVR : 3 types
• UVA:315nm-400nm :Poor efficiency for DNA Damage as
lnot absorbed by native DNA
l-able to generate singlet oxygen that can damage DNA via
indirect photosensitizing reactions
lUVB:280-315nm : Mainly absorbed by DNA : Main damage
occur by this
lUVC :<280nm : absorbed by oxygen and ozone in the Earth’s
atmosphere

33. 1)Pyrimidine dimer

34. Pyrimidine dimer

36. Purine Dimer

37. UV induced DNA lesions
lPyrimidine dimer
lPurine dimer
lDepurination
lSingle and Double strand break

38. • Repair mainly done by Nucleotide Excixion repair
• If not reapir- during replication it may incorrectly insert
base
- may skip that dimer
• NER defect: Xeroderma Pigmentosa
-XP protein encoding gene defect
-Accumulation of these mutation induced by UV light
-Causes various skin cancer: Basal Cell Carcinoma
Squamous Cell Carcinoma
Melanoma

39. • Single Base Alteration:a)Depurination
 b)Deamination
 c)Alkylation
 d)Base analogue incorporation
 e)Mismatch base
• Double base Alteration: a)Pymidine Dimer
 b)Purine Dimer
• Chain break : a) Single Stranded Break
 b) Double Stranded Break
• Cross linking : a)Between DNA to DNA
 b)Between DNA to Protein

40. Chain Break

41.  Causes :- Mainly by irradiation
- Free radical damage
 Repair : By Homologus end joining

42. Double strand break:
Causes : - Direct by irradiaction
- unrepaired UV-induced DNA lesions
- the repair of single strand breaks
passing through base excision repair
- Free radical injury
- Radiomimetic compound

43. Mechanism of DSB

44.  Repair by Homologus End joining Or Non
Homologus End Joining

45. •Oxidative damage mainly occurs by formation of Free
redical or Radiomimetic compunds
•Free radical damage to the DNA:
-70% damage by OH-
-Radiolysis of H2O produce peroxides
-formation of OH. requires metal near to the DNA
-Fe+2 mainly forms OH. formation by fenton reaction
Fe2+ + H2O2 → Fe3+ + OH· + OH−
Free radical damage to DNA

46. • OH. Abstracts H from deoxyribose carbon
• Deoxyribosyl radical formation
• Radical react with molecular O2- forms peroxyl
radical
• Break the DNA strand and release of Bases(AP site)
• Repair by BER
• When OH. Abstrcts 1-deoxyribose-- forms
 2- deoxyribonolacton which is resistant to repair
 enzyme --- Mutation

47.  Depurination
 Single strand break
 Double strand break

48. Radiomimetic compounds
•radiomimetic compounds are enediyens
•Undergose cyclization
•Forms Para-benzene diredicals
•Highly reactive- Abstract hydrogen from any possible
hydrogen donor
•In DNA- it abstracts H from the deoxyribose sugar
backbone, predominantly at the C-1’,C-4’ and C-5’
positions
radical formation at the reacted carbon

49. • carbon radical reacts with molecular oxygen
• Para-benzyne is able to position itself in such a way
that it can abstract proximal hydrogens from both
strands of DNA
• produces a double-strand break in the DNA
• Mostly this will not be repaired
• Cell apoptosis

50.  Direct Damage:
 ionizing radiation directly interact with
target structure to cause ionization
 initiating the chain of events to lead to
biological changes
 direct action of radiation done by High
Linear energy transfer like alpha partical
and neutron

51.  Indirect Damage : done by free radical
formation

53.  Clustered DNA lesion : which represent two
or more lesions formed within one or two
helical turns of DNA
-less readily repaired than individual
lesions
-cytotoxic, mutagenic and carcinogenic
effects

54. • Single Base Alteration:a)Depurination
 b)Deamination
 c)Alkylation
 d)Base analogue incorporation
 e)Mismatch base
• Double base Alteration: a)Pymidine Dimer
 b)Purine Dimer
• Chain break : a) Single Stranded Break
 b) Double Stranded Break
• Cross linking : a)Between DNA to DNA
 b)Between DNA to Protein

55. Cross linking

56. • Causes : - UV radiation
-ionizing radiation like X rays and gamma
rays
- Free radical like H2O2 and OH.
• Mechanism: By irradiation – formation of H2O2
• Protein peroxidation occur
• Fornation of Protein hydroperoxide
• Causes cross linking with DNA