DNA repair mechanism - Brief description, causes and cure

DNA repair mechanism

1. DNA Repair Mechanisms Dr.Balaji.K . Professor , Dept of Biochemistry

2. DNA repair Despite proof reading and mismatch repair during replication – some mismatched bases persist DNA can be damaged by mutagens produced in cells or from environment Environmental mutagens – radiation , chemicals  mutation If damage is not repaired  permanent mutation  deleterious effects  cancer

3. Repair mechanisms Recognition of distorted region of the DNA Removal / excision of the damaged region Filling the gap left by excision of the damaged DNA by DNA polymerase Sealing the nick – ligase

4. Types of DNA repair systems System Type of damage Mismatch repair Mismatches , copying errors Base excision repair Spontaneous , chemical , radiation damage – to a single base. Pyrimidine dimers , alkylated bases Nucleotide excision repair DNA lesions that cause structural changes , eg : pyrimidine dimers – may be due to chemical, radiation or spontaneous Direct repair Pyrimidine dimers , O6- methylguanine

5. Mismatch repair * Discrimination between new strand and template strand ? Old strands tagged with methyl group * Enzyme involved in methylation ? Dam methylase * Action of Dam methylase ? Methylates DNA at the N6 position of all adenines that occur within 5’- GATC sequences * Methylation of New strands ? New strands are not immediately methylated . There is a short lag during which the template strand is methylated and the new strand is notb yet methylated

6. Mismatch repair Mut S , Mut H , and Mut L – recognise the mutation and nick the strand The difference in methylation allows repair enzymes to identify the strand with mismatch nucleotide Replication mismatches in the vicinity of a GATC sequence are then repaired according to information in the methylated parent strand If both strands are methylated at GATC sequence – little repair occurs If neither strands are methylated , repair occurs but does not favour either strand

7. 9 protein components Dam methylase Mut H Mut L Mut S DNA helicase SSB Exonuclease I DNA polymerase III DNA ligase Mismatch repair enzyme complex

8. Bases of DNA can be altered : spontaneously / action of deamination or alkylating compounds . Bases can be lost spontaneously Ex: * Uracil from spontaneous deamination of cytosine * Hypoxanthine arising from adenine deamination * Alkylated bases - 3 methyl adenine , 7 methyl guanine Base excision repair : Removal or replacement of modified bases Base excision repair

9. Base excision repair DNA glycosylases recognize particularly common DNA lesions DNA glycosylase : recognises the lesion and remove the affected base by cleaving the N- glycosyl bond AP sites or basic sites : action of DNA glycosylase creates apurinic ( devoid of A or G) or apyrimidic (devoid of C or T) site in the DNA. AP endonuclease nick the backbone *AP : Apurinic / Apyrimidinic DNA polymerase fills in the gap DNA ligase seals the nick

10. Exposure of a cell to ultraviolet light can result in covalent joining of 2 adjacent pyrimidines ( usually Thymines ) producing a dimer Nucleotide excision repair mechanism helps in lesions involving pyrimidine dimers , cytobutane and other types of base adducts Nucleotide excision repair

11. Several types of damage are repaired without removing a base or nucleotide Ex: photochemical cleavage of pyrimidine dimers DNA photolyase contains : N5, N10 – methyl THF and flavin adenine dinucleotide cofactors Direct repair pathway

12. System Type of damage Enzymes / proteins involved Mismatch repair Mismatches , copying errors Dam methylase , Mut H , Mut L , Mut S , DNA helicase , DNA polymerase III , Exonuclease I , DNA ligase Base excision repair Spontaneous , chemical , radiation damage – to a single base. Pyrimidine dimers , alkylated bases DNA glycosylases AP endonucleases DNA polymerase I DNA ligase Nucleotide excision repair DNA lesions that cause structural changes , eg : pyrimidine dimers – may be due to chemical, radiation or spontaneous ABC excinuclease DNA polymerase I DNA ligase Direct repair Pyrimidine dimers , O6- methylguanine DNA photolyases O6 methylguanine methyl transferase