Talaro's Foundations in Microbiology - Chapter 9: Microbial Genetics - PDF

Because learning changes everything. ® Chapter 9 An Introduction to Microbial Genetics Talaro’s Foundations in Microbiology Twelfth Edition Barry Chess © McGraw Hill LLC. All rights reserved. No reproduction or distribution without the prior written consent of M...

Because learning changes everything. ® Chapter 9 An Introduction to Microbial Genetics Talaro’s Foundations in Microbiology Twelfth Edition Barry Chess © McGraw Hill LLC. All rights reserved. No reproduction or distribution without the prior written consent of McGraw Hill LLC.  Genetics and Genes Genetics – the study of heredity The science of genetics explores: Transmission of biological traits from parent to offspring Expression and variation of those traits Structure and function of genetic material How this material changes © McGraw Hill, LLC 2  Levels of Structure and Function of the Genome (Enterobius vermicularis): Image contributed by the Centre for Tropical Medicine and Imported Infectious Diseases, Bergen, Norway./CDC-DPDX; (Ascaris, 1000X): Richard GrossMcGraw Hill; (Drosophila polytene chromosomes): Keith Maggert/McGraw Hill; (Cell Structure): Designua/Shutterstock Access the text alternative for slide images. © McGraw Hill, LLC 3  Microbial Genomes Genome – sum total of genetic material (DNA) in a cell Most exists as chromosomes Some appear in non-chromosomal sites: Mitochondria Chloroplasts Plasmids Genome of cells – DNA Genome of viruses – DNA or RNA © McGraw Hill, LLC 4  Chromosomes 1 Chromosome - discrete cellular structure composed of a neatly packaged DNA molecule Eukaryotic chromosomes are located in the nucleus and are multiple and linear Bacterial chromosomes are a single circular loop © McGraw Hill, LLC 5  Chromosomes 2 Access the text alternative for slide images. © McGraw Hill, LLC 6  Genotypes and Phenotypes 1 A chromosome is subdivided into genes, the fundamental unit of heredity responsible for a given trait Site on the chromosome that provides information for a certain cell function Segment of DNA that contains the necessary code to make a protein or RNA molecule Three basic categories of genes: Genes that code for proteins – structural genes Genes that code for RNA Genes that control gene expression – regulatory genes © McGraw Hill, LLC 7  Genotypes and Phenotypes 2 All types of genes constitute the genetic makeup – genotype The expression of the genotype creates observable traits – phenotype © McGraw Hill, LLC 8  Size and Packaging of Genomes Smallest virus – 4 to 5 genes E. coli – single chromosome containing 4,288 genes Human cell – 46 chromosomes containing 31,000 genes Sophisticated packaging allows genome to fit inside cell © Dr. Jack Griffith © McGraw Hill, LLC 9  The Packaging of DNA 1 The DNA molecule is compacted in the cell by supercoils, or superhelices: In prokaryotes, by the action of the enzyme DNA gyrase, which coils the chromosome into a tight bundle by introducing reversible series of twists into the DNA molecule In eukaryotes packaging is more complex, with three or more levels of coiling, starting with a chain of nucleosomes (DNA around histone proteins) © McGraw Hill, LLC 10  The Packaging of DNA 2 Access the text alternative for slide images. © McGraw Hill, LLC 11  The Structure of DNA: Double Helix 1 Basic unit of DNA structure is the nucleotide: A deoxyribose sugar A phosphate group A nitrogenous base: adenine (A), guanine (G), thymine (T), cytosine (C) Nucleotides covalently bond to form a sugar-phosphate backbone © McGraw Hill, LLC 12  The Structure of DNA: Double Helix 2 Access the text alternative for slide images. © McGraw Hill, LLC 13  The Structure of DNA: Double Helix 3 Nitrogenous bases covalently bond to the 1′ carbon of each base on the sugar and span the center of the molecule to pair with a complementary strand: Adenine (A) to thymine (T) with 2 hydrogen bonds Guanine (G) to cytosine (C) with 3 hydrogen bonds © McGraw Hill, LLC 14  The Structure of DNA: Double Helix 4 Access the text alternative for slide images. © McGraw Hill, LLC 15  The Structure of DNA: Double Helix 5 Antiparallel arrangement: in one strand, the helix runs in a 5′ to 3′ direction and the other side is oriented from 3′ to 5′ Each strand provides a template for the exact copying of a new strand Order of bases constitutes the DNA code © McGraw Hill, LLC 16  The Structure of DNA: Double Helix 6 Access the text alternative for slide images. © McGraw Hill, LLC 17  The Significance of DNA Structure 1. Maintenance of code during reproduction - Constancy of base pairing guarantees that the code will be retained. When strands are separated, each strand serves as a template for replication of the molecule into an exact copy. 2. Providing variety - order of bases responsible for RNA and protein synthesis, thus for the phenotype of each organism © McGraw Hill, LLC 18  Concept Check: (1) In DNA, adenine is the complementary base for , and cytosine is the complementary for. A.guanine, thymine B.uracil, guanine C.thymine, guanine D.thymine, uracil © McGraw Hill, LLC 19  Concept Check: (2) In DNA, adenine is the complementary base for , and cytosine is the complementary for. A.guanine, thymine B.uracil, guanine C.thymine, guanine D.thymine, uracil Answer: C © McGraw Hill, LLC 20  The Overall Replication Process 1 Replication occurs on both strands simultaneously Semiconservative process: 1. The parent DNA molecule is uncoiled 2. The two strands are separated exposing the nucleotide sequence to serve as templates 3. Two new complementary strands are synthesized by using each single-strand as template © McGraw Hill, LLC 21  The Overall Replication Process 2 Access the text alternative for slide images. © McGraw Hill, LLC 22  The Overall Replication Process 3 Barry Chess/McGraw Hill Access the text alternative for slide images. © McGraw Hill, LLC 23  The Overall Replication Process 4 The figure of “Replication of a bacterial chromosome” continues on this slide. Replication of chromosomal DNA starts at the origin of replication and proceeds in both directions. Two replication forks move away from one another as they synthesize DNA. Continuing around the molecule, the replication forks meet at a termination site. DNA replication is semiconservative, meaning that each of the two molecules created contains one of the original strands paired with a newly synthesized strand. © McGraw Hill, LLC 24  Events in DNA Replication 1 All chromosomes have a specific origin of replication site as the place where replication will be initiated The origin of replication is AT-rich, thus less energy is required to separate the two strands There are two replication forks where new DNA is being synthesized, each containing its own set of replication enzymes © McGraw Hill, LLC 25  Events in DNA Replication 2 Barry Chess/McGraw Hill Access the text alternative for slide images. © McGraw Hill, LLC 26  Events in DNA Replication 3 The figure of “Replication of a circular bacterial chromosome” continues on this slide. 1. Helicase binds to DNA at the origin and begins to separate the two strands of the molecule, creating a replication bubble and two replication forks. Once a small area of the single-stranded DNA has been made available, primase binds to the DNA and synthesizes a small RNA primer whose sequence is complimentary to the DNA sequence. 2. DNA polymerase III synthesizes new DNA strands using the original DNA as a template, New Strands of DNA are synthesized in the 5` to 3` direction, beginning at the primer. © McGraw Hill, LLC 27  Events in DNA Replication 4 The figure of “Replication of a circular bacterial chromosome” continues on this slide. 3. As the replication bubble continues to grow, primase produces a new RNA primer and DNA polymerase III extends the primer in the 5` to 3` direction. 4. DNA synthesis continues, with all new strands synthesized in the 5` to 3` directions. The two leading strands are synthesized continuously from a single RNA primer, while two lagging strands are made discontinuously from multiple RNA primers. Each segment (RNA primer plus DNA) is known as an Okazaki fragment. © McGraw Hill, LLC 28  Events in DNA Replication 5 The figure of “Replication of a circular bacterial chromosome” continues on this slide. 5. DNA polymerase I removes the RNA nucleotides that make up the primer and replaces them with the corresponding DNA nucleotides. Ligase then seals the gap between the Okazaki fragments, completing synthesis of the new DNA strand. 6. Gyrase supercoils the replicated DNA. © McGraw Hill, LLC 29  Enzymes Involved in DNA Replication TABLE 9.1 Some Enzymes Involved in DNA Replication and Their Functions Enzyme Function Helicase Separating the two strands of the DNA helix Primase Synthesizing an RNA primer DNA polymerase III Adding bases to the new DNA chain, proofreading the chain for mistakes DNA polymerase I Removing RNA primers, replacing gaps between Okazaki fragments with correct nucleotides, repairing mismatched bases Ligase Final binding of nicks in DNA during synthesis and repair Gyrase Supercoiling © McGraw Hill, LLC 30  DNA Polymerase III Access the text alternative for slide images. © McGraw Hill, LLC 31  Concept Check: (3) Why must the lagging strand of DNA be replicated in short pieces? A.Because of limited space B.To avoid distortion of the helix C.The DNA polymerase works in only one direction D.To make proofreading of the code easier © McGraw Hill, LLC 32  Concept Check: (4) Why must the lagging strand of DNA be replicated in short pieces? A.Because of limited space B.To avoid distortion of the helix C.The DNA polymerase works in only one direction D.To make proofreading of the code easier Answer: C © McGraw Hill, LLC 33  Applications of the DNA Code 1 Genetic information in DNA molecules is conveyed to RNA through the process of transcription The information contained in the RNA molecule is then used to produce proteins in the process of translation A wide variety of specialized RNAs act by regulating gene function © McGraw Hill, LLC 34  Applications of the DNA Code 2 Access the text alternative for slide images. © McGraw Hill, LLC 35  Gene-Protein Connection Each structural gene is an ordered sequence of nucleotides that codes for a protein’s primary structure Groups of three consecutive bases, triplets or codons, on one DNA strand are transcribed into RNA sequence triplets Each triplet of nucleotides on the RNA specifies a particular amino acid A protein’s primary structure (chain of amino acids) determines its shape and function Proteins contribute to the cell phenotype as enzymes and structural proteins © McGraw Hill, LLC 36  DNA-Protein Relationship Access the text alternative for slide images. © McGraw Hill, LLC 37  RNAs: Major Participants in Transcription and Translation The general structure of the ribonucleic acid (RNA) is different than that of the DNA molecule in several ways: 1. RNA is a single-stranded molecule that can assume secondary and tertiary levels of complexity, leading to specialized forms of RNA (mRNA, tRNA, and rRNA) 2. RNA contains uracil (U), no thymine (T) like DNA, as the complementary base-pairing mate for adenine (A) 3. The sugar in RNA is ribose rather than deoxyribose © McGraw Hill, LLC 38  Major Types of RNA TABLE 9.2 Major Types of Ribonucleic Acid Involved In Protein Synthesis RNA Type Contains Codes For Function in Cell Translated Carries the DNA Messenger (mRNA) Sequence of amino master code to the Yes acids in protein ribosomes Carries amino acids Transfer (tRNA) Specifying a given to ribosomes during No amino acid translation Forms the major part Several large of ribosomes and Ribosomal (rRNA) structural rRNA participates in protein No molecules synthesis An RNA that can Primer begin DNA Primes DNA No replication © McGraw Hill, LLC 39  Messenger RNA (mRNA) 1 Transcribed version of a structural gene or genes in DNA Synthesized following complementary-base pairing by a process similar to synthesis of the leading strand during DNA replication Message is in triplets called codons © McGraw Hill, LLC 40  Messenger RNA (mRNA) 2 a) Messenger RNA (mRNA) A short piece of messenger RNA (mRNA) illustrates some of the general structural features of RNA: single strandedness, and the use of the nitrogenous base uracil instead of thymine. Access the text alternative for slide images. © McGraw Hill, LLC 41  Transfer RNA: tRNA 1 Acts as a translator of the mRNA code into protein 75 to 95 nucleotides in length bent into hairpin loops to form a cloverleaf structure further packed into a complex helix Bottom loop of the cloverleaf exposes the tRNA specific anticodon complementary to a mRNA codon Binding site for amino acids is specific for each anticodon © McGraw Hill, LLC 42  Transfer RNA: tRNA 2 The figure of “Transfer RNA.” b) Transfer RNA (tRNA) Left: The tRNA strand loops back on itself to form intrachain hydrogen bonds. The result is a cloverleaf structure, shown here in simplified form. At its bottom is an anticodon that specifies the attachment of a particular amino acid at the 3’ end. Center: A three-dimensional view of tRNA structure. Right: A space-filling model of the molecule. © McGraw Hill, LLC 43  Transfer RNA: tRNA 3 The figure of “Transfer RNA” continues on this slide. Petarg/123RF Access the text alternative for slide images. © McGraw Hill, LLC 44  Ribosomal RNA: rRNA The prokaryotic (70S) ribosome is a particle composed of tightly packaged ribosomal RNA (rRNA) and protein Forms complex three-dimensional figures that contribute to the structure and function of ribosomes: reading the mRNA code, facilitating its interaction with tRNA, and producing proteins at an impressive rate Access the text alternative for slide images. © McGraw Hill, LLC 45  Transcription: The First Stage of Gene Expression During transcription, an RNA molecule is synthesized using the codes on DNA as a guide or template It proceeds in 3 stages: 1. Initiation: RNA polymerase binds to promoter region upstream of the gene 2. Elongation: RNA polymerase adds nucleotides complementary to the DNA template strand in the 5′ to 3′ direction (Uracil (U) is placed complementary to adenine (A) 3. Termination: RNA polymerase recognizes a “STOP” sign in the DNA and releases the transcript (100 to 1,200 bases long) © McGraw Hill, LLC 46  Major Events in Transcription 1 The figure of “Major Events in Transcription.” 1. Each gene contains a specific promoter region and a leader sequence for guiding the beginning of transcription. Next is the region of the gene that codes for a polypeptide and ends with a series of terminal sequences that stop translation. 2. Initiation: Guided by the sigma factor, the RNA polymerase binds to the DNA at the promoter region. DNA is unwound at the promoter by RNA polymerase. Only one strand of DNA, called the template strand, supplies the codes to be transcribed by RNA polymerase. This strand runs in the 3’ to 5’ direction. © McGraw Hill, LLC 47  Major Events in Transcription 2 The figure of “Major Events in Transcription” continues on this slide. 3. Elongation: The RNA polymerase moves along the DNA strand, adding complementary nucleotides as dictated by the DNA template. The mRNA strands reads in the 5’ to 3’ direction. 4. Termination: The polymerase continues transcribing until it reaches a termination site, and the mRNA transcript is released to be translated. © McGraw Hill, LLC 48  Major Events in Transcription 3 The figure of “Major Events in Transcription” continues on this slide. Access the text alternative for slide images. © McGraw Hill, LLC 49  Translation: The Second Stage of Gene Expression All the elements needed to synthesize protein (mRNA, tRNA, amino acids) are brought together on the ribosomes The process occurs in five stages: initiation, elongation, termination, and protein folding and processing Access the text alternative for slide images. © McGraw Hill, LLC 50  The Master Genetic Code 1 Represented by mRNA codons and their specific amino acids Code is universal among organisms and redundant © McGraw Hill, LLC 51  The Master Genetic Code 2 *This codon initiates translation. **For these codons, which give the orders to stop translation, there are no corresponding tRNAs with amino acids. Access the text alternative for slide images. © McGraw Hill, LLC 52  Interpreting the DNA Code Transcription produces mRNA complementary to the DNA gene During translation, tRNAs use their anticodon to interpret the mRNA codons and bring in the specific amino acids Access the text alternative for slide images. © McGraw Hill, LLC 53  Translation 1 Ribosomes assemble on the 5′ end of an mRNA transcript Ribosome scans the mRNA until it reaches the start codon, usually AUG A tRNA molecule with the complementary anticodon and methionine amino acid enters the P site of the ribosome and binds to the mRNA © McGraw Hill, LLC 54  Translation 2 (1) Entrance of tRNAs 1 and 2 Access the text alternative for slide images. © McGraw Hill, LLC 55  Translation 3 A second tRNA with the complementary anticodon fills the A site (1) Entrance of tRNAs 1 and 2 Access the text alternative for slide images. © McGraw Hill, LLC 56  Translation 4 A peptide bond is formed between the amino acids on the neighboring tRNAs (2) Formation of peptide bond Access the text alternative for slide images. © McGraw Hill, LLC 57  Translation 5 The first tRNA is released and the ribosome slides down to the next codon (3) Discharge of tRNA 1 at E site Access the text alternative for slide images. © McGraw Hill, LLC 58  Translation 6 Another tRNA fills the A site and a peptide bond is formed (4) First translocation; tRNA 2 shifts into P site; tRNA 3 enters ribosome at A Access the text alternative for slide images. © McGraw Hill, LLC 59  Translation 7 (5) Formation of peptide bond Access the text alternative for slide images. © McGraw Hill, LLC 60  Translation 8 This process continues until a stop codon is reached (6) Discharge of tRNA 2; second translocation; tRNA 4 enters ribosome Access the text alternative for slide images. © McGraw Hill, LLC 61  Translation 9 (7) Formation of peptide bond Access the text alternative for slide images. © McGraw Hill, LLC 62  Translation Termination Termination codons – UAA, UAG, and UGA – are codons for which there is no corresponding tRNA When this codon is reached, the ribosome falls off and the last tRNA is removed from the polypeptide © McGraw Hill, LLC 63  Concept Check: (5) Transfer RNA (tRNA) is the molecule that A. contributes to the structure of ribosomes B. acts in the transcription of DNA C. transfers the DNA code to mRNA D. acts as a translator of the mRNA code into protein © McGraw Hill, LLC 64  Concept Check: (6) Transfer RNA (tRNA) is the molecule that A. contributes to the structure of ribosomes B. acts in the transcription of DNA C. transfers the DNA code to mRNA D. acts as a translator of the mRNA code into protein Answer: D © McGraw Hill, LLC 65  Polyribosomal Complex (c) Steven McKnight and Oscar L Miller, Department of Biology, University of Virginia Access the text alternative for slide images. © McGraw Hill, LLC 66  Eukaryotic Transcription and Translation 1. Do not occur simultaneously – transcription occurs in the nucleus and translation occurs in the cytoplasm 2. Eukaryotic start codon is AUG, but it does not use formyl- methionine 3. Eukaryotic mRNA encodes a single protein, unlike bacterial mRNA which encodes many 4. Eukaryotic DNA contains introns– intervening sequences of noncoding DNA – which have to be spliced out of the final mRNA transcript © McGraw Hill, LLC 67  Splicing of Eukaryotic pre-mRNA 1 Eukaryotes gene coding sequences, or exons, are interrupted by segments called introns Introns are transcribed but not translated, they are removed before translation Splicing does not occur in prokaryotes © McGraw Hill, LLC 68  Splicing of Eukaryotic pre-mRNA 2 Access the text alternative for slide images. © McGraw Hill, LLC 69  Regulation of Protein Synthesis and Metabolism Genes are regulated to be active only when their products are required In prokaryotes this regulation is coordinated by operons, a set of genes, all of which are regulated as a single unit © McGraw Hill, LLC 70  Operons 2 types of operons: Inducible – operon is turned ON by substrate: catabolic operons - enzymes needed to metabolize a nutrient are produced when needed Repressible – genes in a series are turned OFF by the product synthesized; anabolic operon –enzymes used to synthesize an amino acid stop being produced when they are not needed © McGraw Hill, LLC 71  Lactose (lac) Operon: Inducible Operon Made of 3 segments: Regulator – gene that codes for repressor Control locus – composed of promoter and operator Structural locus – made of 3 genes each coding for an enzyme needed to catabolize lactose : -galactosidase – hydrolyzes lactose Permease – brings lactose across cell membrane -galactosidase transacetylase – uncertain function © McGraw Hill, LLC 72  Lac Operon 1 Normally off In the absence of lactose, the repressor binds with the operator locus and blocks transcription of downstream structural genes © McGraw Hill, LLC 73  Lac Operon 2 1) Operon Off: No Lactose In the absence of lactose, a repressor protein (the product of a regulatory gene located elsewhere on the bacterial chromosome) attaches to the operator region of the operon. This blocks transcription of structural genes downstream (to its right). Suppression of transcription (and translation) prevents the unnecessary synthesis of enzymes for processing lactose. Access the text alternative for slide images. © McGraw Hill, LLC 74  Lac Operon 3 Lactose turns the operon on by acting as the inducer Binding of lactose to the repressor protein changes its shape and causes it to fall off the operator. RNA polymerase can bind to the promoter. Structural genes are transcribed The figure of “Lac Operon on.” 2) Operon on: Lactose Present Upon entering the cell, the substrate (lactose) becomes a genetic inducer by attaching to the repressor, which is rendered inactive and falls away. The operator is no longer closed off and its DNA becomes accessible to the RNA polymerase. The RNA polymerase transcribes the structural genes, and the mRNA is translated into enzymes that can digest lactose. © McGraw Hill, LLC 75  Lac Operon 4 The figure of “Lac Operon on” continues on this slide. Access the text alternative for slide images. © McGraw Hill, LLC 76  Arginine Operon: Repressible 1 Normally on and will be turned off when the product of the pathway is no longer required The figure of “Arginine Operon on.” 1) Operon On: Arginine being used by cell A repressible operon remains on when its nutrient products (here, the amino acid arginine) are in great demand by the cell. The repressor has the wrong shape to bind to the DNA operator without a corepressor, so that RNA polymerase is free to actively transcribe the genes and translation actively proceeds. © McGraw Hill, LLC 77  Arginine Operon: Repressible 2 The figure of “Arginine Operon on” continues on this slide. Access the text alternative for slide images. © McGraw Hill, LLC 78  Arginine Operon: Repressible 3 When excess arginine is present, it binds to the repressor and changes it. Then the repressor binds to the operator and blocks arginine synthesis. Arginine is the corepressor The figure of “Arginine Operon off.” 2) Operon Off: Arginine building up The operon is repressed when (1) arginine builds up and, serving as a corepressor, activates the repressor protein. (2) The activated repressor binds to the operator and blocking RNA polymerase and further transcription of genes for arginine synthesis. © McGraw Hill, LLC 79  Arginine Operon: Repressible 4 The figure of “Arginine Operon off” continues on this slide. Access the text alternative for slide images. © McGraw Hill, LLC 80  RNA and Gene Expression 1 Riboswitch: short nucleotide segment of mRNA that regulates translation of the same mRNA it is part of. RNA Interference: noncoding RNA molecules – including microRNA (miRNA), antisense RNA (aRNA), and small interfering RNA (siRNA), regulate gene expression in eukaryotes. © McGraw Hill, LLC 81  RNA and Gene Expression 2 Barry Chess/McGraw Hill Access the text alternative for slide images. © McGraw Hill, LLC 82  Concept Check: (7) If an operon’s repressor is in its active form that means: A.Transcription from the operon is occurring B.Transcription from the operon is not occurring © McGraw Hill, LLC 83  Concept Check: (8) If an operon’s repressor is in its active form that means: A.Transcription from the operon is occurring B.Transcription from the operon is not occurring Answer: B © McGraw Hill, LLC 84  Mutations: Changes in the Genetic Code A change in phenotype due to a change in genotype (nitrogen base sequence of DNA) is called a mutation A natural, nonmutated characteristic is known as a wild type (wild strain) An organism that has a mutation is a mutant strain, showing variance in morphology, nutritional characteristics, genetic control mechanisms, resistance to chemicals, etc. © McGraw Hill, LLC 85  Isolating Mutants Replica plating technique allows identification of mutants: (a) Culture is exposed to mutagen (b) Isolated colonies are transferred on a master plate (c) tiny clump of cells are picked up and transferred to a plate with complete medium and another with incomplete medium (d) Colonies in the complete medium plate missing from the incomplete one are mutant colonies that can be subcultured for further use © McGraw Hill, LLC 86  Causes of Mutations 1 Spontaneous mutations – random change in the DNA due to errors in replication that occur without known cause Induced mutations – result from exposure to known mutagens, physical (primarily radiation) or chemical agents that interact with DNA in a disruptive manner © McGraw Hill, LLC 87  Causes of Mutations 2 TABLE 9.3 Selected Mutagenic Agents and Their Effects Agent Effect Chemical Nitrous acid, bisulfite Remove an amino group from some nitrogen bases Ethidium bromide Inserts between the paired bases Acridine dyes Cause frameshifts due to insertion between base pairs Base analogs Compete with natural bases for sites on replicating DNA Radiation Ionizing (gamma rays, X rays) Form free radicals that cause single or double breaks in DNA Ultraviolet Causes cross-links between adjacent pyrimidines © McGraw Hill, LLC 88  Categories of Mutations Point mutation – addition, deletion, or substitution of a few bases Missense mutation – causes change in a single amino acid Nonsense mutation – changes a normal codon into a stop codon Silent mutation – alters a base but does not change the amino acid Back-mutation – when a mutated gene reverses to its original base composition Frameshift mutation – when the reading frame of the mRNA is altered © McGraw Hill, LLC 89  Effect of Major Types of Mutations 1 TABLE 9.4 Classification of Major Types of Mutations. I. Wild-type (nonmutated) sequence Example: THE BIG BAD CAT ATE THE FAT RED BUG The wild-type sequence of a gene is the DNA sequence found in most organisms and is generally considered the “normal” sequence. © McGraw Hill, LLC 90  Effect of Major Types of Mutations 2 The “TABLE 9.4” continues on this slide. II. Categories of mutations based on type of DNA alteration A. Substitution mutations 1. Missense: Example: THE BIG MAD CAT ATE THE FAT RED BUG A missense mutation causes a different amino acid to be incorporated into a protein. Effects range from unnoticeable to severe, based on how the new amino acids alters protein function. © McGraw Hill, LLC 91  Effect of Major Types of Mutations 3 The “TABLE 9.4” continues on this slide. 2. Nonsense Example: THE BIG BAD XXX (stop) A nonsense mutation converts a codon to a stop codon, resulting in premature termination of protein synthesis. Effects of this type of mutation are almost always severe. © McGraw Hill, LLC 92  Effect of Major Types of Mutations 5 The “TABLE 9.4” continues on this slide. B. Frameshift Mutations 1. Insertion: Example: THE BIG BAB DCA TAT ETH EFA TRE DBU G  2. Deletion: Example: THE BIG * ADC ATA TET HEF ATR EDB UG  Insertion (addition of nucleotide) and deletion (removal of nucleotide) mutations cause a change in the reading frame of the mRNA, resulting in a protein in which every amino acid after the mutation can be affected. Because of this, frameshift mutations almost always result in a nonfunctional protein. © McGraw Hill, LLC 93  Repair of Mutations Since mutations can be potentially fatal, the cell has several enzymatic repair mechanisms in place to find and repair damaged DNA DNA polymerase – proofreads nucleotides during DNA replication Mismatch repair – locates and repairs mismatched nitrogen bases that were not repaired by DNA polymerase Light repair – for UV light damage Excision repair – locates and repairs incorrect sequence by removing a segment of the DNA and then adding the correct nucleotides © McGraw Hill, LLC 94  The Ames Test 1 Any chemical capable of mutating bacterial DNA can similarly mutate mammalian DNA Agricultural, industrial, and medicinal compounds are screened using the Ames test Indicator organism is a mutant strain of Salmonella enterica that has lost the ability to synthesize histidine This mutation is highly susceptible to back-mutation © McGraw Hill, LLC 95  The Ames Test 2 (d): Courtesy of Xenometrix AG Access the text alternative for slide images. © McGraw Hill, LLC 96  Positive and Negative Effects of Mutations Mutations leading to nonfunctional proteins are harmful, possibly fatal Organisms with mutations that are beneficial in their environment can readily adapt, survive, and reproduce – these mutations are the basis of change in populations Any change that confers an advantage during selection pressure will be retained by the population © McGraw Hill, LLC 97  Concept Check: (9) Which of the following mutations would cause a frameshift mutation? A.Silent mutation B.Missense mutation C.Nonsense mutation D.Deletion mutation © McGraw Hill, LLC 98  Concept Check: (10) Which of the following mutations would cause a frameshift mutation? A.Silent mutation B.Missense mutation C.Nonsense mutation D.Deletion mutation Answer: D © McGraw Hill, LLC 99  DNA Recombination Events Genetic recombination – occurs when an organism acquires and expresses genes that originated in another organism 3 means for genetic recombination in bacteria: Conjugation Transformation Transduction © McGraw Hill, LLC 100  Conjugation 1 Transfer of a plasmid or chromosomal fragment from a donor cell to a recipient cell via direct contact Gram-negative cell donor has a fertility plasmid (F factor) that allows the synthesis of a conjugative pilus Donor (F+ cell) transfers fertility plasmid through pilus to recipient (F− cell), which becomes F+ cell Some F+ cells become Hfr cells (high frequency of recombination) © McGraw Hill, LLC 101  Conjugation 2 Physical Conjugation (1) The pilus of donor cell (top) attaches to receptor on recipient cell and retracts to draw the two cells together. This is the mechanism for gram-negative bacteria. Access the text alternative for slide images. © McGraw Hill, LLC 102  Conjugation 3 High-frequency recombination – donor’s fertility plasmid is integrated into the bacterial chromosome When conjugation occurs, a portion of the chromosome and a portion of the fertility plasmid are transferred to the recipient © McGraw Hill, LLC 103  Conjugation 4 F Factor Transfer (2) Transfer of F factor, or conjugative plasmid Access the text alternative for slide images. © McGraw Hill, LLC 104  Conjugation 5 Hfr Transfer (3) High frequency (Hfr) transfer involves transmission of chromosomal genes from a donor cell to a recipient cell. The donor chromosome is duplicated and transmitted in part to a recipient cell, where it is integrated into the chromosome. Access the text alternative for slide images. © McGraw Hill, LLC 105  Transformation 1 Chromosome fragments from a lysed cell are accepted by a recipient cell; the genetic code of the DNA fragment is acquired by the recipient Donor and recipient cells can be unrelated Useful tool in recombinant DNA technology © McGraw Hill, LLC 106  Transformation 2 The figure of “Transformation.” Access the text alternative for slide images. © McGraw Hill, LLC 107  Transformation 3 The figure of “Transformation” continues on this slide. DNA fragment (blue) delivering cap+ gene for capsule formation (red) binds to a surface receptor on a competent recipient cell. DNA is converted to one strand and transported into the cell, by the DNA transport system. The DNA strand aligns itself with a compatible region on the recipient chromosome. The DNA strand is incorporated into the recipient chromosome. Recipient is now transformed with gene for synthesizing a capsule. © McGraw Hill, LLC 108  Griffith’s Work on Transformation Barry Chess/McGraw Hill Access the text alternative for slide images. © McGraw Hill, LLC 109  Transduction Bacteriophage serves as a carrier of DNA from a donor cell to a recipient cell Two types: Generalized transduction – random fragments of disintegrating host DNA are picked up by the phage during assembly; any gene can be transmitted this way Specialized transduction – a highly specific part of the host genome is regularly incorporated into the virus © McGraw Hill, LLC 110  Generalized versus Specialized Transduction Access the text alternative for slide images. © McGraw Hill, LLC 111  Recombination: Intermicrobial DNA Exchange TABLE 9.5 Types of Genetic Recombination in Bacteria Mode Factors Involved Direct of Examples of Genes Transferred Indirect* Conjugation Donor cell with pilus Direct Drug resistance; resistance to metals; toxin Fertility plasmid in donor production; enzymes; adherence Both donor and recipient alive molecules; degradation of toxic substance; Bridge forms between cells to transfer uptake of iron DNA. Transformation Free donor DNA (fragment or plasmid) Indirect Polysaccharide capsule; metabolic Live, competent recipient cell, donor enzymes; drug resistance; unlimited with usually dead cloning techniques Transduction Donor is lysed bacterial cell. Indirect Exotoxins; enzymes for sugar fermentation; Defective bacteriophage is carrier of donor drug resistance DNA. Live recipient cell of same species as donor *Direct means the donor and recipient are in contact during exchange; indirect means they are not. © McGraw Hill, LLC 112  Transposons 1 Special DNA segments that have the capability of moving from one location in the genome to another – “jumping genes” Cause rearrangement of the genetic material Can move from one chromosome site to another, from a chromosome to a plasmid, or from a plasmid to a chromosome May be beneficial or harmful © McGraw Hill, LLC 113  Transposons 2 Access the text alternative for slide images. © McGraw Hill, LLC 114  Genetics of Animal Viruses Viral genome - one or more pieces of DNA or RNA that contain only genes needed for production of new viruses Viruses require access to host cell’s genetics and metabolic machinery to instruct the host cell to synthesize new viral particles Most viruses contain normal double-stranded (ds) DNA or single- stranded (ss) RNA, but other patterns exist With few exceptions, replication of the viral DNA occurs in the nucleus. The genome of most RNA viruses is replicated in the cytoplasm In all viruses, viral mRNA is translated into viral proteins on host cell ribosomes using host tRNA © McGraw Hill, LLC 115  Replication of dsDNA Viruses Access the text alternative for slide images. © McGraw Hill, LLC 116  Replication of ssRNA Viruses Access the text alternative for slide images. © McGraw Hill, LLC 117  Concept Check: (11) Which of the following terms describes the genetic transfer of bacterial DNA that occurs by direct cell-to-cell contact and is mediated by bacterial plasmids? A. Transformation B. Conjugation C. Generalized Transduction D. Specialized Transduction © McGraw Hill, LLC 118  Concept Check: (12) Which of the following terms describes the genetic transfer of bacterial DNA that occurs by direct cell-to-cell contact and is mediated by bacterial plasmids? A. Transformation B. Conjugation C. Generalized Transduction D. Specialized Transduction Answer: B © McGraw Hill, LLC 119  End of Main Content Because learning changes everything. ® www.mheducation.com © McGraw Hill LLC. All rights reserved. No reproduction or distribution without the prior written consent of McGraw Hill LLC.