Test review 3 - Review Test number 3 Bacterial Genetics...

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Review Test number 3 Bacterial Genetics • Explain phenotype and genotype. Genotype - The specific set of genes present in a cell or virus Phenotype - Expressed, measurable characteristics. • Describe the effects of point mutations . The most frequent type of mutation and it happens at the - DNA level: change in a single nucleotide base pair - Transition – pyrimidine for a pyrimidine or a purine for a purine - Transversion – purine for a pyrimidine or vice versa - At the Protein level: Change translates in the amino acid sequence - Silent – No change in amino acid sequence – no phenotypic affect - Neutral - Change to equivalent amino acid - no phenotypic effect - Missense: Changes amino acid sequence: Phenotypic effect ranges from slight to strong depending on sequence. - Nonsense: Introduces stop codon producing truncated protein Phenotypic effect is usually strong. - Frameshift : Changes codon reading frame. Phenotypic effect is Usually strong. • Describe the DNA repair mechanisms that correct mutations. DNA repair of UV damage: - 1 st mechanism : photo-reactivation repair • UV light (<280 nm) is mutagenic • Causes thymine dimerization • Dimers are repaired by visible lightinduced Photolyase enzyme. - 2 nd mechanism : Excision Repair • Three step process: 1. Endonucleases remove damaged DNA 2. DNA polymerase I re-synthesizes
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DNA 3. DNA ligase joins the two fragments - 3 rd mechanism : Recombination Repair - AKA homologous recombination • Important form of repair/exchange involving homologous genes. • Complex process that requires ~25 proteins/enzymes. • Involves recA protein that is involved in DNA repair. - RecA protein – has been shown to be essential in every Homologous recombination pathway • Non-incorporated DNA is degraded. • Describe the effects of insertion sequences, transposons and plasmids. Insertion sequences – small mobile DNA (~ 1 kb), short inverted repeats at the ends (~20 kb), encoded transposase Enzyme Transposons: Larger mobile DNA (<40kb), Can carry additional genes Example: antibiotic resistance
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Widely used tool in microbial genetics. Example: Tn5 mutagenesis) PLASMIDS: • Autonomously replicating genetic elements • Usually double stranded DNA (1-1000 kbp) • Replicate like DNA or by rolling circle • Multiple plasmids per cell can occur • Plasmids encode 2-30 genes that can encode for multiple processes – Antibiotic production – Catabolic processes Example: Oil breakdown – Antibiotic resistance (R plasmids) – Toxin production (pathogenesis) • Grouped by compatibility / incompatibility • Some are conjugative (F plasmid) some are not (cryptic). F Plasmid and conjugation
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• Describe horizontal gene transfer: Transformation, Conjugation and Transduction. Conjugation – Mobilization of DNA between microbes, usually involves plasmids.
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