A. DNA damage caused by ultraviolet light: 1. cyclobutane-type pyrimidine dimer is the major photoproduct formed 2. a second product, the 6-4 photoproduct, is formed in about 10 % of UV induced pyrimidine dimmers 3. the cyclobutane type dimer can be reversed by a process called photoreactivation (a) this is carried out by an enzyme called DNA photolyase (photoreactivating enzyme) (b) importance of the photolyase enzyme in humans is questionable 4. DNA photoproducts can also be repaired by excision repair
B. Spontaneous deamination of cytosine: 1. deamination of cytosine is common and results in the conversion of cytosine to uracil 2. can be repaired by excision repair process (i) uracil-DNA glycosylase hydrolyzes N-glycosidic bond to remove uracil base (ii) AP endonuclease removes deoxyribose-phosphate (iii) gap is extended by exonuclease (iv) gap is filled by DNA polymerase I and nick is sealed by DNA ligase
C. Depurination: Pathway is similar to that above except that excision repair begins with AP endonuclease
D. DNA damaged by alkylating agents: 1. Some simple alkylating agents 2. examples of products of alkylating agents 3. many of these products can be repaired by excision repair that is initiated by specific glycosylases 4. some damage resulting from methylation can be reversed by methyltransferases (a) O6-methylguanine-DNA methyltransferase
E. Mismatch repair: 1. mismatches can occur when DNA polymerase inserts the wrong nucleotide during replication 2. mismatch repair is “coupled” to replication
F. Recombinational repair: 1. occurs during DNA replication 2. major steps: (a) DNA polymerase skips over damaged DNA leaving a gap opposite the lesion (b) the undamaged parental strand recombines into the gap (this is facilitated by recA protein in E. coli) (c) the new gap in the parental strand is filled by DNA polymerase and ligase
G. Genetic Defects in DNA repair and human disease: 1. Xeroderma pigmentosum is an inherited disease that is characterized by severe photosensitivity and a very high incidence of skin cancers. It is due to defective excision repair. 2. Bloom’s syndrome. 3. Cockayne’s syndrome 4. Fanconi’s anemia 5. Ataxia telangiectasia
B. Spontaneous deamination of cytosine: 1. deamination of cytosine is common and results in the conversion of cytosine to uracil 2. can be repaired by excision repair process (i) uracil-DNA glycosylase hydrolyzes N-glycosidic bond to remove uracil base (ii) AP endonuclease removes deoxyribose-phosphate (iii) gap is extended by exonuclease (iv) gap is filled by DNA polymerase I and nick is sealed by DNA ligase
C. Depurination: Pathway is similar to that above except that excision repair begins with AP endonuclease
D. DNA damaged by alkylating agents: 1. Some simple alkylating agents 2. examples of products of alkylating agents 3. many of these products can be repaired by excision repair that is initiated by specific glycosylases 4. some damage resulting from methylation can be reversed by methyltransferases (a) O6-methylguanine-DNA methyltransferase
E. Mismatch repair: 1. mismatches can occur when DNA polymerase inserts the wrong nucleotide during replication 2. mismatch repair is “coupled” to replication
F. Recombinational repair: 1. occurs during DNA replication 2. major steps: (a) DNA polymerase skips over damaged DNA leaving a gap opposite the lesion (b) the undamaged parental strand recombines into the gap (this is facilitated by recA protein in E. coli) (c) the new gap in the parental strand is filled by DNA polymerase and ligase
G. Genetic Defects in DNA repair and human disease: 1. Xeroderma pigmentosum is an inherited disease that is characterized by severe photosensitivity and a very high incidence of skin cancers. It is due to defective excision repair. 2. Bloom’s syndrome. 3. Cockayne’s syndrome 4. Fanconi’s anemia 5. Ataxia telangiectasia
