BIOL 039 Lecture Notes - Lecture 20: Ap Site, Escherichia Coli, Helicase
Document Summary
Base excision repair pathways remove abnormal or chemically modified bases. Nucleotide excision repair pathways remove larger defects. Such as thymidine dimers and bases with bulky side-groups. Both excision pathways occur by similar mechanisms in bacteria and eukaryotes. Base excision repair is a multi-step process. Dna glycosylase recognizes and removes modified purine bases, leaving an apurinic or apyrimidinic site (ap site) Ap endonuclease removes the remainder of the nucleotide. Finally, dna polymerase and dna ligase replace the gap with the appropriate nucleotide. Nucleotide-excision is also called the uv-repair system. In e. coli, the enzymes responsible are encoded by the genes uvr-a, uvr-b, uvr-c, and uvr-d. Uv repair removes approximately 12 nts surrounding dna damage. New dna is synthesized to replace the removed nucleotides. Two molecules of uvr-a protein and one uvr-b. Bind the dna strand opposite the site of the thymine dimer.
