Introduction to Genetics PDF

Microbial Genetics Part 2: Regulation of Gene Expression, Mutation, and Horizontal Genetic Exchange CHAPTER 9 Toucan Barbet, Ecuador  Regulation of Gene Expression * In prokaryotes usually involves controlling transcription (More complicated in eukaryotes, t...

Microbial Genetics Part 2: Regulation of Gene Expression, Mutation, and Horizontal Genetic Exchange CHAPTER 9 Toucan Barbet, Ecuador  Regulation of Gene Expression * In prokaryotes usually involves controlling transcription (More complicated in eukaryotes, take BIOL 272 and 276!) * Many genes are co-regulated = operon system * 3 types of genes: 1. Constitutive: gene product is always needed so transcription is always “on” * Ex. Glycolytic enzymes in most microbes 2. Repressible: transcription is normally “on” but can be turned “off” * Often the endproduct of metabolic pathway is the repressor * Ex. Tryptophan operon in E.coli 3. Inducible: transcription is normally “off” but can be turned “on” * Ex. lac operon 2 Copyright © 2012 John Wiley & Sons, Inc. All rights reserved. Lac Operon Lac operon: genes for the metabolism of the milk sugar lactose are co-regulated Inducible system Normally “off” as lactose isn’t common Induced when lactose is present Copyright © 2012 John Wiley & Sons, Inc. All rights reserved.  Lac operon – when no lactose is present Regulator gene Repressor protein  Lac operon – lactose is present Regulator gene lactose Lactose-metabolizing enzymes Repressor protein 5  Mutation * A change in DNA sequence * Spontaneous (mistake in DNA pol) or induced * May or may not change a.a. sequence of protein 1. Point Mutation: single base change a. Silent: base change does not result in change in a.a. sequence b. Missense: base change results in a single a.a. change * Ex. Inherited sickle-cell anemia c. Nonsense: base change results in introduction of a premature stop codon 2. Frameshift: insertion or deletion of a base, changes reading frame downstream 6 Copyright © 2012 John Wiley & Sons, Inc. All rights reserved.  C 3. C 2. A 1. DNA: TA C G G G AT G TAA A C T mRNA: protein: 1. type? 2. type? 3. type? 4. type? TA C G G G G AT G TA AA C T..... 7  Causes of Mutation 1. 1. Chemical mutagens a. Base-pair mutagens: alter base-pairing rules * Ex. Nitrous acid causes A to pair with C during DNA replication b. Base analogs: are incorporated as one n’tide but have altered base-pair rules * Ex. 5-bromouracil replaces T but base-pairs with G c. Frameshift mutagens: usually intercollate into DNA and cause DNA pol to make small insertions or deletions * Ex. Aflatoxin from peanut mold Copyright © 2012 John Wiley & Sons, Inc. All rights reserved. 2. Radiation as a mutagen a. a. Ionizing (X-rays, gamma rays, etc.) * Damages DNA, creates free radicals b. b. Nonionizing (UV light) * Creates thymine dimers Thymine dimers can be repaired: 1. Photolyase – activated by light 2. Excision repair – dimer removed and replaced by DNA pol 9 Detecting mutagens: Ames Test * Auxotrophic strain of Salmonella typhimurium * Requires a.a. his in medium * His - strain Copyright © 2012 John Wiley & Sons, Inc. All rights reserved. Genetic Recombination in Prokaryotes Vertical exchange: from one generation to the next All eukaryotes All prokaryotes Horizontal exchange: exchange of genetic info. between members of the same generation Only found in nature in 11 prokaryotes Copyright © 2012 John Wiley & Sons, Inc. All rights reserved.  3 Methods of Horizontal Recombination 1. Transformation 2. Transduction 3. Conjugation 12 Copyright © 2012 John Wiley & Sons, Inc. All rights reserved.  1. Transformation – Griffith’s experiment * Transformation: ability of some bacteria to take up “naked” DNA and incorporate that DNA into it’s own genome See Fig. 9.27 Copyright © 2012 John Wiley & Sons, Inc. All rights reserved.  Transformation * Competent cell: can take up “naked” DNA * Recombination: when DNA seq. is homologous ( alleles) a recombination event may occur, the new DNA seq. replaces the old * cap+ replaces cap- * Recipient cell has new properties Copyright © 2012 John Wiley & Sons, Inc. All rights reserved.  CHEM 201 Transformation * - GFP (green fluorescent protein) gene from a jellyfish added to E. coli plasmid (via recombinant DNA tech.) * Plasmid also contains gene for ampicillin resistance (ampR) * E. coli cells made competent (Ca++ sol’n, on ice) * GFP-plasmid added to E.coli mix, heat shocked (42oC) * E.coli takes up plasmid – only cells that have plasmid can grow on media containing ampicillin * Transformed E.coli cells express GFP gene 🡪 glow in the dark! 15 Copyright © 2012 John Wiley & Sons, Inc. All rights reserved. 2. Transduction * Transfer of DNA from one bacterial cell to another via a bacteriophage * Results in a recipient cell that has a new trait * Focus on generalized transduction (Fig. 9.30) * Ex. E.coli O157:H7 produces a toxin (shiga-like toxin) that it probably received via transduction * The toxin is what makes this strain of E.coli pathogenic Copyright © 2012 John Wiley & Sons, Inc. All rights reserved.  3. Conjugation * Transfer of genetic material via “sex pillus” 17 Copyright © 2012 John Wiley & Sons, Inc. All rights reserved. Standard F+ Conjugation * F factor plasmid codes for sex pillus * F plasmid replicated * Copy is transferred to recipient (F-) cell * Recipient cell now has F plasmid, becomes F+ Copyright © 2012 John Wiley & Sons, Inc. All rights reserved. Hfr Conjugation Hfr: high frequency of recombination * F plasmid has incorporated into bacterial ch’some * DNA replicates and transfers across pillus * Not all of the F factor crosses over so recipient stays F- * Some of DNA ch’some also crosses * Homologous DNA from donor recombines with recipient DNA 19 Copyright © 2012 John Wiley & Sons, Inc. All rights reserved.  R factors * Plasmids that contain genes for antibiotic resistance, resistance to other toxins * R factors are often exchanged via conjugation along with F factor plasmids Copyright © 2012 John Wiley & Sons, Inc. All rights reserved. Golden-collared Toucanet, Ecuador 21