DNA damage and repair

slides on DNA damage

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 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. 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. All of these pathways have been characterized in both bacterial and eukaryotic organisms.

8. Base Excision Repair (BER) 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. 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. 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 E xcision Repair (NER)

14. 3. Mismatch repair (MMR) 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. 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.

16. 4. Repairing S trand Breaks Ionizing radiation and certain chemicals can produce both single-strand breaks (SSBs) and double-strand breaks (DSBs) in the DNA backbone. 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).

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

18. 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. D efects in this system are associated with a predisposition to cancer and immunodeficiency syndromes.

19. 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.

20. “Diseases associated with defective DNA repair system”

21. 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.

22. Xeroderma pigmentosum

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

24. Ataxia telangiectasia

25. 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

26. Bloom syndrome

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

28. Cockayne's syndrome

29. 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 .

30. Trichothiodystrophy

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