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MICROBIOLOGY WITH DISEASES BY TAXONOMY, THIRD EDITION Chapter 7 Microbial Genetics Lecture prepared by Mindy Miller-Kittrell, University of Tennessee, Knoxville Copyright © 2011 Pearson Education Inc. Gene Mutation Gene Mutation – a change in the sequence of bases within a gene May have neutral, beneficial, or harmful effects (Almost always harmful) May result in a change in amino acid or a nonfunctional protein Some mutations are repaired by excising the mutant nucleotides Rarely leads to a protein having a novel property that improves ability of organism and its descendents to survive and reproduce Copyright © 2011 Pearson Education Inc. Types of Gene Mutations Point mutations (most common) – –A single base at one point in the DNA sequence is inserted, deleted, or substituted by another base Frameshift mutations – (a type of point mutation) –One or several base pairs are deleted or inserted into the DNA sequence –Shifts the “translational reading frame” Copyright © 2011 Pearson Education Inc. Types of Gene Mutations Point mutations – 3 types 1. Silent Mutation - the change in the codon but no change in amino acid; due to code redundancy 2.Missense Mutation - the change in the codon changes the amino acid thus the protein 3.Nonsense Mutation - the change in the codon change amino acid to Stop codon Copyright © 2011 Pearson Education Inc. The effects of the various types of point mutations Copyright © 2011 Pearson Education Inc. Figure 7.23 The Frequency of Mutation • Spontaneous mutation rate 1 in 109 (10-9) replicated base pairs or 1 in 106 (10-6) replicated genes • Mutagens (agents that cause a mutation) increase mutation rate to 10–5 or 10–3 per replicated gene Copyright © 2011 Pearson Education Inc. Mutations of Genes Animation: Mutagens Copyright © 2011 Pearson Education Inc. Mutagens • Chemicals: – Nitrous acid – – mutates adenine to guanine – Base analogs – – mimic nitrogenous base; disrupt DNA and RNA replication and cause point mutations – ex. 5 bromo uracil – Smoke/ethidium bromide – causes insertion or deletion of base pairs – Results in frameshift mutations Copyright © 2011 Pearson Education Inc. Nitrogen Base Analog Copyright © 2011 Pearson Education Inc. Mutagens • Environment: – Ionizing radiation – (X rays and gamma rays) – Causes ion formations that react with bases and deoxyribosephosphate backbone – induces breaks in chromosomes – Nonionizing radiation –(UV rays) – causes thymine to crosslink resulting in thymine dimers – Cause errors in replication and transcription – The most common type of mutation Copyright © 2011 Pearson Education Inc. The action of a frameshift mutagen Copyright © 2011 Pearson Education Inc. Figure 7.26 Transfer of Genetic Information Horizontal gene transfer Copyright © 2011 Pearson Education Inc. Gene transfer mechanisms –Vertical Gene Transfer – –Occurs in cell division; passing on gene’s to offspring –Horizontal Gene Transfer – –Exchanging gene’s within the same generation –Results in a recombinant cell that has received DNA from another cell Copyright © 2011 Pearson Education Inc. Horizontal Gene Transfer 1. 2. 3. 4. Transformation- transfer of naked DNA fragments Transduction- mediated through a bacteriophage Conjugation- transfer of DNA through a pilus Transposition- migration of DNA “jumping genes” Copyright © 2011 Pearson Education Inc. Griffith’s Experiments of Transformation Copyright © 2011 Pearson Education Inc. Figure 7.30 Transformation •Transfer of naked DNA fragments (from environment) to cell •Cells must be competent (able to take up the DNA) Copyright © 2011 Pearson Education Inc. Transduction Mediated through a bacteriophage Copyright © 2011 Pearson Education Inc. Figure 7.31 Bacterial Conjugation Transfer of DNA through a pilus Copyright © 2011 Pearson Education Inc. Figure 7.32a Plasmids Nucleoid region Plasmid Copyright © 2011 Pearson Education Inc. Figure 4.6a, b Types of Plasmids – F (fertility) factor – Carries genes for sex pili and transfer of the plasmid – R (resistance) factor – Encodes antibiotic resistance – Bacteriocin factor – – Encodes for toxin that kills bacterial cell of the same or similar species that lack that factor – Virulence factor – – Encode for enzymes, structures or toxins that make bacteria pathogenic Copyright © 2011 Pearson Education Inc. Conjugation Copyright © 2011 Pearson Education Inc. Conjugation Hfr plasmid Conjugation Hfr plasmid incorporates directly into the host genome so increases probability of recombination Copyright © 2011 Pearson Education Inc. Figure 7.33 Transposition Transposons are segments of DNA that can move from one region of DNA to another They contain insertion sequences (palindromic) for cutting and resealing DNA http://highered.mcgrawhill.com/sites/0072556781/student_view0/chapter13/animation_quiz_5.html Copyright © 2011 Pearson Education Inc. Figure 7.36 Transposons a. Insertion sequence b. Simple Transposon c. Complex Transposon Copyright © 2011 Pearson Education Inc. Figure 7.36a-c