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Chapter 8: Bacterial Genetics Genetic changes in bacteria occur via: -mutations -gene transfer What are mutations? • Change in the base sequence of the DNA • Do they always change the genetic code? Causes of mutations in bacteria • Most are spontaneous • Errors made by DNA Polymerase • UV light exposure Types of Mutations • Base-pair mutation – Missense mutation – Nonsense mutation – Silent mutation Base-pair mutation: missense Results of base-pair mutations Types of Mutations • Frameshift – Changes the reading frame What can cause mutations? • Chemicals (nitrous acid) • Physical mutagens (uv light) • Biological mutagens (transposons) Barbara McClintock: “jumping genes” biological mutagen Nitrous acid as a chemical mutagen Nitrosoguanidine alters guanine bases Nucleoside analogs are mutagens Intercalating agents UV light as a mutagen Repair mechanisms • Wrong nucleotide inserted – Proofreading – Mismatch repair Repair: Mismatch Repair of UV damage • Two repair mechanisms – Light repair – Dark repair Why use bacteria to study mutations? • Only have one chromosome…one copy of each gene • Easy to grow Direct selection • Testing for traits that are easily identified – Colony color – Motility – Resistance to antibiotics Indirect selection • A way to look at traits that are not easily identified, at changes in metabolic pathways • Replica plating – A way to identify AUXOTROPHS from PROTOTROPHS Replica Plating: indirect selection Penicillin enrichment of mutants Testing chemicals for mutagenicity…Ames test