1. Group 3
Hadia azhar
Amna hafeez
Um-e-Farwa
Muqadas shehzad

2. “ DNA Damage and
Repair ”

3. Introduction
 DNA in the living cell is subjected to many
 chemical alterations.
 The genetic information encoded in the DNA
 has to remain uncorrupted
 Any chemical changes must be corrected.
 A failure to repair DNA produces a mutation.

4. Agents that Damage DNA
1. Radiations:
 Highly reactive oxygen radicals produced during
normal cellular respiration as well as by other
biochemical pathways
 Ionizing radiation such as gamma rays and x-rays
 Ultraviolet rays, especially the UV-C rays (~260nm)
that are absorbed strongly by DNA but also the
longer-wavelength UV-B that penetrates the ozone
shield

5. Agents that Damage DNA
2. Chemicals in the environment:
 Aromatic hydrocarbons, including some found in
cigarette smoke
 Plant and microbial products, e.g. the Aflatoxin
produced in moldy peanuts
 Chemicals used in chemotherapy, especially
chemotherapy of cancers.

6. DNA Repair
i) Base excision repair (BER)
ii) Nucleotide excision repair (NER),
iii) Mismatch repair (MMR)
iv) Strand break repair.

7. o In these reactions a nucleotide segment
containing base damage, double-helix
distortion or mispaired bases is replaced by the
normal nucleotide sequence in a new DNA
polymerase synthesis process.
o All of these pathways have been characterized
in both bacterial and eukaryotic organisms.

8. 1. Base Excision Repair (BER)
i. Removal of abnormal bases:
 In BER, abnormal bases are recognized by
specific glycosylases that hydrolytically cleave
them from the deoxyribose -phosphate
backbone of the strand.
This leaves an apyrimidinic site or apurinic site,
both referred to as AP sites.

9. ii. Recognition and repair of an AP site:
 The AP sites are substrates for AP
endonucleases.
These enzymes recognize that a base is
missing and initiate the process of excision and
gap filling by making an endonucleolytic cut to
the 5’-side of the AP site.
 A deoxyribose phosphate lyase removes the
single, base- free, sugar phosphate residue.
 DNA polymerase and DNA ligase complete the
repair process.

10. • The enzyme uracil DNA
glycosylase removes the
uracil created by
spontaneous deamination
of cytosine in the DNA.
• An endonuclease cuts the
backbone near the defect
• An endonuclease
removes a few bases
• The defect is filled in by
the action of a DNA
polymerase and
• The strand is rejoined by
a ligase.
Base Excision Repair
(BER)

11. 2. Nucleotide excision repair (NER)
This mechanism is used to replace regions of
damaged DNA up to 30 bases in length.
Exposing of cell to UV light can result in the
covalent joining of two adjacent pyrimidines,
producing a dimer.
These dimers prevent DNA pol from replicating
the DNA strand.

12. i. Recognition and excision of dimers:
 A UV-specific endonuclease recognizes the
dimer and cleaves the damaged strand on both
5’-side and 3’-side of the dimer.
 A short oligonucleotide containing dimer is
released, leaving a gap in the DNA strand.
 This gap is filled in using a DNA pol and DNA
ligase.

13. Nucleotide
Excision
Repair (NER)

14. 3. Mismatch repair (MMR)
i. Identification of the mismatched
strand:
Mismatch repair corrects errors made when
DNA is copied.
 Specific proteins scan the newly synthesized
DNA, using adenine methylation within a
GATC sequence as the point of reference
The template strand is methylated, and the
newly synthesized strand is not.

15. ii. Repair of damaged DNA:
If a mismatch is found, a GATC endonuclease
cuts the strand bearing the mutation and is
removed by exonuclease.
 The gap is then filled by DNA pol and DNA
ligase.

17. 4. Repairing Strand Breaks
 Ionizing radiation and certain chemicals can produce
both single-strand breaks (SSBs) and double-strand
breaks (DSBs) in the DNA backbone.
i. Single-Strand Breaks (SSBs):
Breaks in a single strand of the DNA molecule are
repaired using the same enzyme systems that are used in
Base-Excision Repair (BER).

18. ii. Double-Strand Break (DSBs):
There are two systems by which the cell
attempts to repair a complete break in a DNA
molecule:
a) Non-homologous end joining (NHEJ)
b) Homologous Recombination (HR)

19. a) Non-homologous end joining
(NHEJ):
 In this system, a group of proteins mediates
the recognition, processing, and ligation of the
ends of two DNA fragments. However some
DNA is lost during the process.
Consequently, this mechanism of repair is
error prone and mutagenic.
 Defects in this system are associated with a
predisposition to cancer and
immunodeficiency syndromes.

20. b) Homologous Recombination:
This repair system uses the enzymes that
normally perform genetic recombination
between homologous chromosomes during
meiosis.
HR is much less error prone than NHEJ
because any DNA that was lost is replaced
using homologous DNA as a template.

21. “Diseases associated with
defective DNA repair
system”

22. 1) Xeroderma pigmentosum:
 Xeroderma pigmentosum (XP) is an autosomal
recessive genetic disease.
 It includes marked sensitivity to sunlight (ultraviolet)
with subsequent formation of multiple skin cancers
and premature death.
 The inherited defect seems to involve the repair of
damaged DNA, particularly thymine dimers.
 Cells cultured from patients with xeroderma
pigmentosum exhibit low activity for the nucleotide
excision-repair process.

23. Xeroderma pigmentosum

24. 2) Ataxia telangiectasia (A-T):
 Ataxia-telangiectasia (A-T) is an autosomal recessive,
complex, multisystem disorder characterized by
1. progressive neurologic impairment,
2. cerebellar ataxia,
3. immunodeficiency
4. impaired organ maturation,

25. Ataxia telangiectasia

26. 3)Bloom syndrome:
 Head is disproportionately small,
 less amount of subcutaneous fat tissue throughout
infancy and childhood,
 A redness of the cheeks and nose that
characteristically makes its appearance in infancy after
sun exposure.
 Chronic obstructive lung disease, Diabetes mellitus
and malignancies of varied types are some of the
common complications of Bloom syndrome

27. Bloom syndrome

28. 4) Cockayne's syndrome:
 Cockayne syndrome is a rare autosomal recessive
congenital disorder characterized by:
1. growth failure,
2. impaired development of the nervous system,
3. abnormal sensitivity to sunlight (photosensitivity), and
premature aging.
4. Hearing loss and eye abnormalities are other
common features, but problems with any or all of the
internal organs are possible.
5. It is associated with a group of disorders called
leukodystrophies.

29. Cockayne's syndrome

30. 5) Trichothiodystrophy:
 Brittle hair, rough skin and extreme photosensitivity
are the characteristic features.
 The trichothiodystrophies (TTD) are named primarily
for the hair sulphur deficiency which is their most
specific feature and which leads to brittleness of the
hair.
 There is defect in DNA excision repair system along
with other defects.

31. Trichothiodystrophy