DNA damage and repair mechanism

1. JIWAJI UNIVERSITY,GWALIOR
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TOPIC-DNA damage and repair mechanism
NAME –MANISHA SINGHAI
M.Sc. 3rd SEM

2. Contents
 INTRODUCTION
 SOURCES OF DNA DAMAGE
 TYPES OF DNA DAMAGE
 DNA REPAIR
 TYPES OF DNA REPAIR
 REFERENCE
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3. Introduction
 DNA damage is an alteration in the chemical structure of DNA,such as break in a strand
of DNA,a base missing from the backbone of DNA.
 DNA damage can occur naturally or via environmental factors.
 A DNA damage causes changes in the structure of genetic material and prevents the
replication mechanism from functioning and performing properly.
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4. Sources of DNA damage
 Sources of DNA damage are of two types-
1. Endogenous damage
2. Exogenous damage
 Endogenous damages-1.by reactive oxygen speices.
2.by natural metabolic byproducts.
3.hydrolysis of nucleotide bases.
4.replication error
 Exogenous damage- 1.ionizing radiation.
2. Ultraviolet radiation
3. Chemical agents
4. virus
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5. Types of DNA damage
 Single base alteration-1. depurination
2. deamination
3. alkylation
 Double base alteration- 1.pyrimidine dimer
2. purine dimer
 Chain breaks –1.single strand break
2. double strand break
 Cross linkage-1. between DNA-DNA molecule
2. DNA-protein molecule
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6. SINGLE BASE ALTERATION
1.Depurination-hydrolytic cleavage of N glycosidic bond of purine nucleotide ( A and
G)
 Loss of purine bases from the DNA.
 It forms apurinic site.
2.Deamination- removal of an amino grp from dna bases.
 Cytosine gets deaminated to form uracil while adenine form hypoxanthine and
guanine form xanthine.
 If left unrepaired then uracil would form a base pair with adenine during
replication which give rise to mutation.
3.Alkylation- transfer of alkyl grp to bases.
 Two types of alkylating agents are used-
1.Monofunctional agents- ethyl methane sulphonate.
2.Bifunctional agents- ethylene ammonium sulphate.
 It have 2 alkyl grps causes crosslinking in DNA. 6

7. DOUBLE BASE ALTERATION
 It consist of two types-purine dimer and pyrimidine dimer.
1.Pyrimidine dimer-(thymine and cytosine) DNA absorbs uv radiation in the range of (280-
315)nm.
2.Purine dimer- (adenine and guanine) after the exposure of uv radiation the adjacent purine
base forms dimer.
CHAIN BREAK
 Break in phosphodiester bond.
 It is of two type- single strand break and double strand break.
1.Single strand break- causes by radiation and free radical(peroxidase).
2.Double strand break- causes by radiation,repair of single strand break,free radical injury.
 CROSS LINKING
 Cross linking is caused by uv radiation,ionizing radiation like x rays, gamma rays,free radicals
like Peroxide.
 Cross linking is between DNA-DNA molecule and DNA- protein molecule. 7

8. DNA repair
 It refers to the process by which a cell identifies and correct the damage of DNA
molecule that encodes its genome.
TYPES OF DNA REPAIR -PHOTOREACTIVATION
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9. 1.photoreactivation
 Uv rays shows lethal effect when the normal DNA comes into the contact of uv rays
then it causes damage.
 In nucleotide the adjacent pyramidine bases forms the covalent bond and it forms t-t
dimer
 visible light is used for the repair.
 Then photolyase enzyme attached to the damaged part it cleaves the dimer which
formed between two same pyramidine.
 Then the hydrogen bond reformation takes place and the damaged DNA is repair.
 Then photolyase enzyme will be separated from the DNA.
 Photolyase enzyme is present in bacterial cell and mammal placenta.
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10. Base excision repair
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11. 2.Base excision repair
 Deamination of cytosine takes place and uracil comes in place of cytosine.
 Uracil glycosylase enzyme is active and it is used to remove the uracil from DNA
backbone but it has to hydrolyse the glycosidic bond from which uracil and guanine
joined.
 So that uracil will removed and the place of uracil will remain empty.
 Ap endonuclease means apurine site or apyramidine site.
 Ap endonuclease cuts the whole strand of the damage site so that all the nucleotide
present in the damaged DNA will be removed.
 DNA polymerase enzyme is used for the synthesize of nucleotide base pair and DNA
ligase enzyme is used to sealed the newly synthesized strand.
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12. Nucleotide excision repair
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13. 3.Nucleotide excision repair
 Exonuclease enzyme is made of 3 sub units- Uvra,Uvrb,Uvrc.
 Uvra recognize the damaged part and Uvrb attached to damage part of DNA.
 Then Uvra enzyme will released and Uvrc is added.
 The complex formed is Uvrb+Uvrc=Uvrbc dimer.
 The Uvrbc dimer cleave the damaged part of DNA or the phosphodiester bond also break.
 Phosphodiester bond break 8 phosphate bond from 5’ end and 5 phosphate bond from 3’ end
from damaged DNA strand.
 Pyramidine dimer formation takes place in the strand of DNA between two adjacent bases.
 Exonuclease enzyme cleave the phosphpodiester bond so that we separate the damaged part of
DNA.
 DNA polymerase enzyme synthesize new strand and DNA ligase enzyme sealed the new strand.
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14. Mismatch repair
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15. 4.Mismatch repair
 During replication one is parental strand and another one is newly synthesize strand.
 In parental strand the methylation of adenine takes place and it acts as a marker.
 The damage occurs in newly synthsize strand and it shows the activity for repair.
 Different enzyme system involved in this repair-Mut L, Mut S, Mut H this all are known as
mut gene.
 Mismatch occurs when guanine in parental strand attached to adenine whereas guanine
attached to cytosine.
 Now the Mut L and Mut S recognize and attached with the damaged part of DNA.
 Mut H cut the damaged part. Helicase and Endonuclease enzyme separate the damaged
part.
 Now the gap is created, SSB(single strand binding protein) it binds the single strand and
maintained its structure.
 DNA polymerase enzyme synthesize the complimentary strand where the DNA ligase
enzyme sealed the complimentary strand.
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16. Reference
 AUTHOR;- U.SATYANARAYANA; 2016PUBLISHED; THIRD EDITION;
BIOTECHNOLOGY; PAGE NO-34-37.
 AUTHOR; ALEXANDER MCLENNAN, PHIL TURNER; 2015 PUBLISHED; FOURTH
EDITION; MOLECULAR BIOLOGY; PAGE NO-86-89.
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17. Thank you
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