DNA damage and repair pathways

1. DNA Repair
Mechanisms
ZAINAB ZAIDI
17-BTM-18
GI2842
Department Of Botany
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2. INTRODUCTION
What is meant by DNA damage?
Any change in the chemical structure of DNA which prevents the
replication mechanism to function properly or alters the genetic code.
What causes DNA damage?
Exogenous sources- ionizing radiation (IR), ultraviolet (UV) radiation,
and various chemicals agents.
Endogenous sources- hydrolysis, oxidation, alkylation, and mismatch of
DNA bases.
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3. Types of DNA damage
Oxidation- some chemical compounds include reactive oxygen
species such as hydroxyl group. Hydroxyl radical reacts with DNA
bases and alters their structure.
Guanine is converted to 8-Oxoguanine which pairs with adenine.
Deamination- Adenine is deaminated to hypoxanthine which pairs
with cytosine.
Alkylation-
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4. Dimerisation- UV component of sunlight is a DNA damaging agent. It
covalently links adjacent pyrimidine residues along a DNA strand. Such dimer
cannot fit into a double helix and so replication and gene expression is
blocked.
Cross-links- It can be intra strand or inter strand. A thymine dimer is an
example of intra strand cross-link.
Inter strand cross links can also be introduced by various agents such as
psoralens. Inter strand cross links disrupt replication because they prevent
strand separation.
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5. Strand breaks- High energy electromagnetic radiation such as x-rays can
damage DNA by producing high concentrations of reactive species in
solution. X-ray exposure can induce single stranded and double stranded
breaks in DNA.
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6. DNA repair mechanisms
DNA
repair
Direct reversal of
DNA damage
Excision repair
Base excision
Nucleotide
excision
Mismatch
repair
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7. Direct Reversal of DNA Damage
• Damage induces by UV light causes formation of pyrimidine dimers.
• Cyclobutane ring is formed by saturation of the double bonds between
carbon 5 and 6.
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8. • Methylation of O6 position of guanine forms O6-
methylguanine.
• O6-methylguanine pairs with thymine instead of cytosine.
• Repair takes place by the help of an enzyme called O6-
methylguanine methyltransferase.
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9. Excision Repair
• Excision repair is a more general means of repair
mechanisms in both prokaryotic and eukaryotic cells.
• The damaged DNA is removed, either as free bases or
as nucleotides.
• It is of three types- base-excision, nucleotide excision,
and mismatch repair.
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10. Base-excision Repair
• Used to repair many minor damage like
alkylation and deamination.
• The repair of uracil containing DNA is a
good example.
• DNA glycosylases also recognize and
remove other abnormal bases, including
hypoxanthine, pyrimidine dimers,
alkylated purines(other than O6-
alkylguanine).
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11. Nucleotide Excision Repair
• The damaged bases are removed as a part of oligonucleotides.
• The uvr system of excision in E.coli is best-studied example.
• It involves the removal of relatively short, usually 12
nucleotides in length.
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12. Mismatch Repair
• The newly synthesized DNA in prokaryotes are recognized
by methylated adenine while in eukaryotes by the presence
of single-strand breaks.
• Mismatch repair in E.coli involves three Mut proteins-
MutH, MutL, MutS.
• MutH and MutS recognizes the mismatch sequence at the
GATC sites.
• MutH endonuclease cleaves the unmethylated strand on 5’
side of G in the GATC sequence.
• MutL and MutS act together with an exonuclease and a
helicase to excise the DNA between the strand break and
mismatch, with the resulting gap being filled by DNA
polymerase and ligase.
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13. Recombinational Repair
• It relies on replacement of the damaged DNA
by recombination with an undamaged
molecule.
• This mechanism is used to repair damage
encountered during DNA replication, where
the presence of thymine dimers or other
lesions that cannot be copied by the normal
replicative DNA polymerases block the
progress of a replication fork.
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14. Sometimes the damaged site cannot be bypassed then the
replication fork stalls and reverses a short distances, so
that a duplex is formed between the daughter strands.
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15. Double-strand break repair
• Ionizing radiation, oxidizing agents and replication errors
may cause double-strand break in DNA.
• The cellular breaks can be repaired by Non-Homologous
End Joining (NHEJ) and Homology-Directed Repair (HDR)
pathway.
• NHEJ is the process by which the cell can repair strand
breaks without the need for a homologous template.
• HDR performs error-free DNA repair using homologous
DNA as a template.
• NHEJ is common in the GO ad G1 phases whereas HDR
predominates in the S and G2 phases.
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16. What is the need of DNA repair?
Damaged DNA that is not properly repaired can lead to genomic
instability, apoptosis, or senescence, which can greatly affect the
organism's development and ageing process.
Loss of genomic integrity predisposes the organism to
immunodeficiency, neurological disorders, and cancer.
Various diseases like Ataxia-telangiectasia, Bloom's and Werner’s
syndrome, Hereditary Non-Polyposis Colon Cancer, and breast and
ovarian cancer syndrome are caused by faulty DNA repair mechanisms.
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17. Thank you.
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