DNA repair mechanism

DNA repair mechanism

1. DNA Repair Mechanism

2. DNA repair Damaged DNA must be repaired If the damage is passed on to subsequent generations. It must take place in the germ cells - the gametes - eggs and sperm If damage is to somatic cells (all other cells of the body bar germ cells) then just that one individual is affected.

3. How does the Damage occur? Consequences of DNA replication errors Chemical agents acting on the DNA UV light imparting energy into DNA molecule Spontaneous changes to the DNA

4. Why repair DNA? DNA pol does a great job, but not good enough Introduces errors in about 1 in 10^7 nucleotides added, which it does not correct Other mechanisms exist to correct many of the errors left by the replication system Most mistakes and damage corrected (99.999999999% -leaving just a few - only 1 in 10^9 errors are left) Mutations are permanent changes left in the DNA

5. Sickle Cell Disease This is a very good illustration of the devastating effects of even tiny changes to the DNA Red Blood Cells Hemoglobin - Has a large protein component 2 beta globin chains A single base change -substitution causes the disease

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7. DNA Repair Mechanisms Living organisms contain many enzymes that scan their DNA for damage and initiate repair processes when damage is detected.

8. DNA Repair Mechanisms in E. coli Light-dependent repair (photo-reactivation). Excision repair. Mismatch repair. Post-replication repair. Error-prone repair system (SOS response).

9. UV Light-Dependent Repair: Photolyase Cleaves Thymine Dimers. --No endonuclease --No Poly --No ligase photoreactivation

10. Excision Repair (steps) A DNA repair endonuclease or endonuclease-containing complex recognizes, binds to, and excise the damaged base or bases. A DNA Polymerase fills in the gap, using the undamaged complementary strand of DNA as a template. DNA ligase seals the break left by DNA polymerase.

11. Types of Excision Repair Base excision repair pathways remove abnormal or chemically modified bases. Nucleotide excision repair pathways remove larger defects, such as thymine dimers.

12. Base Excision Repair AP:apyrimidinic site ( apurinic / apyrimidinic site ), also known as an abasic site

13. ( ) ( apurinic / apyrimidinic site ), also known as an abasic site

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15. Nucleotide Excision Repair

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17. Mismatch Repair in E. coli Mismatching or mispairing of G and T (DNA polymerase/exonuclease proofreading activity) The A in GATC sequences is methylated subsequent to DNA replication. In newly replicated DNA, the parental strand is methylated , but the new strand is not. This difference allows the mismatch repair system to distinguish the new strand from the old strand. The mismatched nucleotide is excised from the new strand and replaced with the correct nucleotide, using the methylated parental strand as a template.

18. Mismatch Repair in E. coli MutS recognizes mismatches and binds to them to initiate the repair process. MutH and MutL join the complex. MutH cleaves the unmethylated strand at hemimethylated GATC sequences on either side of the mismatch. Excision requires MutS , MutL , MutU (activates DNA helicase II), and an exonuclease . DNA polymerase III fills in the gap, and DNA ligase seals the nick. Me Me Me Me

19. Post-replication Repair in E. coli A thymine dimer in the template strand blocks replication (DNA Polymerase III does not recognize thymidine dimer) DNA Polymerase III restarts DNA synthesis past the dimer, leaving a gap in the nascent strand. RecA binds to the single strand of DNA at the gap and mediates base pairing with the homologous segment of the sister double helix to fill the gap. DNA polymerase fills the gap in the sister double helix, and DNA ligase seals the nick.

20. The SOS Response in E. coli If DNA is heavily damaged by mutagenic agents, the SOS response, which involves many DNA recombination, DNA repair, and DNA replication proteins, is activated. DNA dependent DNA Polymerase V replicates DNA in damaged regions, but sequences in damaged regions cannot be replicated accurately . This error-prone system eliminates gaps but increases the frequency of replication errors (Pol II, IV and V are low-fidelity polymerases)

21. Induction of the SOS Response In the absence of DNA damage, LexA binds to DNA regions that regulate transcription of SOS response genes and keeps their expression levels low. When extensive DNA damage occurs, RecA binds to single-stranded regions of DNA in damaged regions. This activates RecA , which stimulates LexA to inactivate itself. When LexA is inactivated, the SOS response genes are expressed.

22. Inherited Human Diseases with Defects in DNA Repair Several inherited human disorders result from defects in DNA repair pathways.

23. Diseases associated with defective DNA repair system                                 Ataxia telangiectasia Bloom syndrome Cockayne's syndrome Progeria (Hutchinson-Gilford Progeria syndrome) Rothmund-Thomson syndrome Trichothiodystrophy Werner syndrome Xeroderma pigmentosum Hereditary non polyposis colon cancer.  26

24. Xeroderma Pigmentosum (XP) Individuals with XP are sensitive to sunlight (UV light). The cells of individuals with XP are deficient in the repair of UV-induced damage to DNA. Individuals with XP may develop skin cancer or neurological abnormalities.

25. Progeria   Progeria  (Hutchinson-Gilford Progeria Syndrome) is an extremely rare genetic disorder that causes the affected individual to undergo advanced aging at an early age. The symptoms closely resemble aging and include wrinkles, hair loss, and delayed growth. Affected individuals have normal development up to 18 months and suddenly stop gaining weight and display stunted height. As the individual ages, Progeria becomes more severe with an average life expectancy of 12 years.  28

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