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Microbial genetics (Ch. 7) Part 3 -- Creating genetic diversity • • • • • • Types of mutations Mutagens Transduction Conjugation Transformation Transposons 1 Types of mutations (Fig. 7.22) • Mutations are changes in the sequence of the genome • Point mutations are changes in only one basepair (bp) • AT-> GC, transition • AT-> CG, TA, transversion 2 Mutagens (Fig. 7.22, 7.23) • UV light causes pyrimidine dimers • Chemical mutagens are analogs of normal bases or cause a chemical change that causes misreplication of DNA 3 Frameshift mutagens (Fig. 7.24) • Frequently mutagens like ethidium bromide intercalate into DNA and cause DNAP to insert or delete bases 4 Horizontal gene transfer among prokaryotes via transformation • Horizontal genetic transfers occurs within a population of the same generation (vs. vertical) • Typically occurs with only 1% of a population • Occurs cross-species and cross-genera, i.e., can pass genes to unrelated organisms • Transformation, transduction and conjugation all cause horizontal gene transfer • Many factors that contribute to pathogenesis may be transferred via these processes 5 Transformation (Fig. 7.30) • Recipient cells (competent) take up DNA from environment and incorporate pieces into the chromosome • Some pathogens (Haemophilus, Neisseria, Streptococcus, Bacillus, Staphylococcus) are naturally competent • Competent cells can be artificially produced by injuring 6 cell membranes (Biotechnology) Transduction (Fig. 7.31) • Carried out by bacteriophage that sometimes package the host’s DNA by mistake • Phage also can transduce genes when they insert themselves into the chromosome and later pop out with attached host DNA 7 Bacterial conjugation (Fig. 7.32) • Depends on specialized plasmids (F) that can transfer themselves from one cell into another (often unrelated genetically) via F pilus • Some transfer themselves to eukaryotic cells (Ti plasmid, R751) • Some R plasmids also transfer antibiotic resistance genes 8 9 Transposons (Fig. 7.35) • Jumping genes first discovered by Barbara McClintock in corn • Transposons are mobile genetic elements that carry a gene that carries out the transposition and may carry other jeans as well • Transposons can be carried by plasmids or phage 10 Transposon structure (Brock, Fig. 10.29) 11 Mechanism of transposition (Fig. 7.36) • Many of these sequences carry drug resistance genes and many drug resistant plasmids carry several transposons 12