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Download The Genetics of Viruses and Bacteria Chapter 18 PowerPoint Lectures for
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Chapter 18 The Genetics of Viruses and Bacteria PowerPoint Lectures for Biology, Seventh Edition Neil Campbell and Jane Reece Lectures by Chris Romero Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Overview: Microbial Model Systems • Viruses called bacteriophages can infect and set in motion a genetic takeover of bacteria, such as Escherichia coli • E. coli and its viruses are called model systems because of their frequent use by researchers in studies that reveal broad biological principles • Beyond their value as model systems, viruses and bacteria have unique genetic mechanisms that are interesting in their own right Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Bacteria are prokaryotes with cells much smaller and more simply organized than those of eukaryotes • Viruses are smaller and simpler than bacteria Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 18-2 Virus Bacterium Animal cell Animal cell nucleus 0.25 µm Concept 18.1: A virus has a genome but can reproduce only within a host cell • Scientists detected viruses indirectly long before they could see them • The story of how viruses were discovered begins in the late 1800s Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Discovery of Viruses: Scientific Inquiry • Tobacco mosaic disease stunts growth of tobacco plants and gives their leaves a mosaic coloration • In the late 1800s, researchers hypothesized that a particle smaller than bacteria caused the disease • In 1935, Wendell Stanley confirmed this hypothesis by crystallizing the infectious particle, now known as tobacco mosaic virus (TMV) Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Structure of Viruses • Viruses are not cells • Viruses are very small infectious particles consisting of nucleic acid enclosed in a protein coat and, in some cases, a membranous envelope Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Viral Genomes • Viral genomes may consist of – Double- or single-stranded DNA – Double- or single-stranded RNA • Depending on its type of nucleic acid, a virus is called a DNA virus or an RNA virus Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Capsids and Envelopes • A capsid is the protein shell that encloses the viral genome • A capsid can have various structures Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 18-4a Capsomere of capsid RNA 18 250 mm 20 nm Tobacco mosaic virus LE 18-4b Capsomere DNA Glycoprotein 70–90 nm (diameter) 50 nm Adenoviruses • Some viruses have structures have membranous envelopes that help them infect hosts • These viral envelopes surround the capsids of influenza viruses and many other viruses found in animals • Viral envelopes, which are derived from the host cell’s membrane, contain a combination of viral and host cell molecules Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 18-4c Membranous envelope Capsid RNA Glycoprotein 80–200 nm (diameter) 50 nm Influenza viruses • Bacteriophages, also called phages, are viruses that infect bacteria • They have the most complex capsids found among viruses • Phages have an elongated capsid head that encloses their DNA • A protein tailpiece attaches the phage to the host and injects the phage DNA inside Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 18-4d Head Tail sheath Tail fiber DNA 80 225 nm 50 nm Bacteriophage T4 General Features of Viral Reproductive Cycles • Viruses are obligate intracellular parasites, which means they can reproduce only within a host cell • Each virus has a host range, a limited number of host cells that it can infect • Viruses use enzymes, ribosomes, and small host molecules to synthesize progeny viruses Animation: Simplified Viral Reproductive Cycle Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 18-5 Entry into cell and uncoating of DNA VIRUS DNA Capsid Transcription Replication HOST CELL Viral DNA mRNA Viral DNA Capsid proteins Self-assembly of new virus particles and their exit from cell Reproductive Cycles of Phages • Phages are the best understood of all viruses • Phages have two reproductive mechanisms: the lytic cycle and the lysogenic cycle Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Lytic Cycle • The lytic cycle is a phage reproductive cycle that culminates in the death of the host cell • The lytic cycle produces new phages and digests the host’s cell wall, releasing the progeny viruses • A phage that reproduces only by the lytic cycle is called a virulent phage • Bacteria have defenses against phages, including restriction enzymes that recognize and cut up certain phage DNA Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 18-6 Attachment Phage assembly Head Tails Release Entry of phage DNA and degradation of host DNA Tail fibers Assembly Synthesis of viral genomes and proteins Animation: Phage T4 Lytic Cycle Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Lysogenic Cycle • The lysogenic cycle replicates the phage genome without destroying the host • The viral DNA molecule is incorporated by genetic recombination into the host cell’s chromosome • This integrated viral DNA is known as a prophage • Every time the host divides, it copies the phage DNA and passes the copies to daughter cells • Phages that use both the lytic and lysogenic cycles are called temperate phages Animation: Phage Lambda Lysogenic and Lytic Cycles Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 18-7 Phage DNA The phage attaches to a host cell and injects its DNA. Daughter cell with prophage Many cell divisions produce a large population of bacteria infected with the prophage. Phage DNA circularizes Phage Bacterial chromosome Lytic cycle The cell lyses, releasing phages. Occasionally, a prophage exits the bacterial chromosome, initiating a lytic cycle. Lysogenic cycle Certain factors determine whether Lytic cycle or Lysogenic cycle is induced is entered New phage DNA and proteins are synthesized and assembled into phages. The bacterium reproduces normally, copying the prophage and transmitting it to daughter cells. Prophage Phage DNA integrates into the bacterial chromosomes, becoming a prophage. Reproductive Cycles of Animal Viruses • Two key variables in classifying viruses that infect animals: – DNA or RNA? – Single-stranded or double-stranded? Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Class/Family Envelope Examples/Disease I. Double-stranded DNA (dsDNA) No Respiratory diseases, animal tumors Papovavirus No Papillomavirus (warts, cervical cancer): polyomavirus (animal tumors) Herpes simplex I and II (cold sores, genital sores); varicella zoster (shingles, chicken pox); Epstein-Barr virus (mononucleosis, Burkitt’s lymphoma) Smallpox virus, cowpox virus Adenovirus Herpesvirus Yes Poxvirus Yes Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Class/Family Envelope Examples/Disease II. Single-stranded DNA (ssDNA) Parvovirus No B19 parvovirus (mild rash) III. Double-stranded RNA (dsRNA) Reovirus No Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Rotavirus (diarrhea), Colorado tick fever virus Class/Family Envelope Examples/Disease IV. Single-stranded RNA (ssRNA); serves as mRNA Picornavirus No Coronavirus Yes Flavivirus Yes Togavirus Yes Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Rhinovirus (common cold); poliovirus, hepatitis A virus, and other enteric (intestinal) viruses Severe acute respiratory syndrome (SARS) Yellow fever virus, West Nile virus, hepatitis C virus Rubella virus, equine encephalitis viruses Class/Family Envelope Examples/Disease V. ssRNA; template for mRNA synthesis Filovirus Yes Ebola virus (hemorrhagic fever) Orthomyxovirus Yes Influenza virus Paramyxovirus Yes Measles virus; mumps virus Rhabdovirus Yes Rabies virus VI. ssRNA; template for DNA synthesis Retrovirus Yes HIV (AIDS); RNA tumor viruses (leukemia) Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Viral Envelopes • Many viruses that infect animals have a membranous envelope • Viral glycoproteins on the envelope bind to specific receptor molecules on the surface of a host cell Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 18-8 Capsid Capsid and viral genome enter cell RNA HOST CELL Envelope (with glycoproteins) Viral genome (RNA) Template mRNA ER Glycoproteins Capsid proteins Copy of genome (RNA) New virus RNA as Viral Genetic Material • The broadest variety of RNA genomes is found in viruses that infect animals • Retroviruses use reverse transcriptase to copy their RNA genome into DNA • HIV is the retrovirus that causes AIDS Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 18-9 Glycoprotein Viral envelope Capsid Reverse transcriptase RNA (two identical strands) • The viral DNA that is integrated into the host genome is called a provirus • Unlike a prophage, a provirus remains a permanent resident of the host cell • The host’s RNA polymerase transcribes the proviral DNA into RNA molecules • The RNA molecules function both as mRNA for synthesis of viral proteins and as genomes for new virus particles released from the cell Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 18-10 HIV Membrane of white blood cell HOST CELL Reverse transcription Viral RNA 0.25 µm HIV entering a cell RNA-DNA hybrid DNA NUCLEUS Provirus Chromosomal DNA RNA genome for the next viral generation New HIV leaving a cell mRNA Animation: HIV Reproductive Cycle Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Evolution of Viruses • Viruses do not fit our definition of living organisms • Since viruses can reproduce only within cells, they probably evolved as bits of cellular nucleic acid Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Concept 18.2: Viruses, viroids, and prions are formidable pathogens in animals and plants • Diseases caused by viral infections affect humans, agricultural crops, and livestock worldwide • Smaller, less complex entities called viroids and prions also cause disease in plants and animals Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Viral Diseases in Animals • Viruses may damage or kill cells by causing the release of hydrolytic enzymes from lysosomes • Some viruses cause infected cells to produce toxins that lead to disease symptoms Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Vaccines are harmless derivatives of pathogenic microbes that stimulate the immune system to mount defenses against the actual pathogen • Vaccines can prevent certain viral illnesses Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Emerging Viruses • Emerging viruses are those that appear suddenly or suddenly come to the attention of scientists • Severe acute respiratory syndrome (SARS) recently appeared in China • Outbreaks of “new” viral diseases in humans are usually caused by existing viruses that expand their host territory Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 18-11 Young ballet students in Hong Kong wear face masks to protect themselves from the virus causing SARS. The SARS-causing agent is a coronavirus like this one (colorized TEM), so named for the “corona” of glyco-protein spikes protruding form the envelope. Viral Diseases in Plants • More than 2,000 types of viral diseases of plants are known • Some symptoms are spots on leaves and fruits, stunted growth, and damaged flowers or roots Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Plant viruses spread disease in two major modes: – Horizontal transmission, entering through damaged cell walls – Vertical transmission, inheriting the virus from a parent Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Viroids and Prions: The Simplest Infectious Agents • Viroids are circular RNA molecules that infect plants and disrupt their growth • Prions are slow-acting, virtually indestructible infectious proteins that cause brain diseases in mammals • Prions propagate by converting normal proteins into the prion version Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 18-13 Prion Original prion Many prions Normal protein New prion Concept 18.3: Rapid reproduction, mutation, and genetic recombination contribute to the genetic diversity of bacteria • Bacteria allow researchers to investigate molecular genetics in the simplest true organisms • The well-studied intestinal bacterium Escherichia coli (E. coli) is “the laboratory rat of molecular biology” Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Bacterial Genome and Its Replication • The bacterial chromosome is usually a circular DNA molecule with few associated proteins • Many bacteria also have plasmids, smaller circular DNA molecules that can replicate independently of the chromosome • Bacterial cells divide by binary fission, which is preceded by replication of the chromosome Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 18-14 Replication fork Origin of replication Termination of replication Mutation and Genetic Recombination as Sources of Genetic Variation • Since bacteria can reproduce rapidly, new mutations quickly increase genetic diversity • More genetic diversity arises by recombination of DNA from two different bacterial cells Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 18-15 Mixture Mutant strain arg+ trp– Mutant strain arg– trp+ Mixture Mutant strain arg+ trp– Mutant strain arg– trp+ No colonies (control) Colonies grew No colonies (control) Mechanisms of Gene Transfer and Genetic Recombination in Bacteria • Three processes bring bacterial DNA from different individuals together: – Transformation – Transduction – Conjugation Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Transformation • Transformation is the alteration of a bacterial cell’s genotype and phenotype by the uptake of naked, foreign DNA from the surrounding environment • For example, harmless Streptococcus pneumoniae bacteria can be transformed to pneumonia-causing cells Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Transduction • In the process known as transduction, phages carry bacterial genes from one host cell to another Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 18-16 Phage DNA A+ B+ A+ B+ Donor cell A+ Crossing over A+ A– B– Recipient cell A+ B– Recombinant cell Conjugation and Plasmids • Conjugation is the direct transfer of genetic material between bacterial cells that are temporarily joined • The transfer is one-way: One cell (“male”) donates DNA, and its “mate” (“female”) receives the genes Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • “Maleness,” the ability to form a sex pilus and donate DNA, results from an F (for fertility) factor as part of the chromosome or as a plasmid • Plasmids, including the F plasmid, are small, circular, self-replicating DNA molecules Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 18-17 Sex pilus 5 µm The F Plasmid and Conjugation • Cells containing the F plasmid, designated F+ cells, function as DNA donors during conjugation • F+ cells transfer DNA to an F recipient cell • Chromosomal genes can be transferred during conjugation when the donor cell’s F factor is integrated into the chromosome • A cell with a built-in F factor is called an Hfr cell • The F factor of an Hfr cell brings some chromosomal DNA along when transferred to an F– cell Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 18-18_1 F plasmid F+ cell Mating bridge F– cell Bacterial chromosome F+ cell F+ cell Bacterial chromosome Conjunction and transfer of an F plasmid from and F+ donor to an F– recipient LE 18-18_2 F plasmid Bacterial chromosome F+ cell Mating bridge F– cell F+ cell F+ cell Bacterial chromosome Conjunction and transfer of an F plasmid from and F+ donor to an F– recipient Hfr cell F+ cell F factor LE 18-18_3 F plasmid Bacterial chromosome F+ cell Mating bridge F– cell F+ cell F+ cell Bacterial chromosome Conjunction and transfer of an F plasmid from and F+ donor to an F– recipient Hfr cell F+ cell F factor Hfr cell F– cell LE 18-18_4 F plasmid Bacterial chromosome F+ cell Mating bridge F– cell F+ cell F+ cell Bacterial chromosome Conjunction and transfer of an F plasmid from and F+ donor to an F– recipient Hfr cell F+ cell F factor Hfr cell F– cell Temporary Recombinant F– partial bacterium diploid Conjugation and transfer of part of the bacterial chromosome from an Hfr donor to an F– recipient, resulting in recombiination R plasmids and Antibiotic Resistance • R plasmids confer resistance to various antibiotics • When a bacterial population is exposed to an antibiotic, individuals with the R plasmid will survive and increase in the overall population Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Transposition of Genetic Elements • The DNA of a cell can also undergo recombination due to movement of transposable elements within the cell’s genome • Transposable elements, often called “jumping genes,” contribute to genetic shuffling in bacteria Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Insertion Sequences • The simplest transposable elements, called insertion sequences, exist only in bacteria • An insertion sequence has a single gene for transposase, an enzyme catalyzing movement of the insertion sequence from one site to another within the genome Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 18-19a Insertion sequence 5 3 3 5 Inverted repeat Transposase gene Inverted repeat Transposons • Transposable elements called transposons are longer and more complex than insertion sequences • In addition to DNA required for transposition, transposons have extra genes that “go along for the ride,” such as genes for antibiotic resistance Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 18-19b Transposon Insertion sequence Antibiotic resistance gene Insertion sequence 5 3 3 5 Inverted repeat Transposase gene Concept 18.4: Individual bacteria respond to environmental change by regulating their gene expression • A bacterium can tune its metabolism to the changing environment and food sources • This metabolic control occurs on two levels: – Adjusting activity of metabolic enzymes – Regulating genes that encode metabolic enzymes Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 18-20 Regulation of enzyme activity Precursor Regulation of enzyme production Feedback inhibition Enzyme 1 Gene 1 Enzyme 2 Gene 2 Regulation of gene expression Enzyme 3 Gene 3 Enzyme 4 Gene 4 Enzyme 5 Tryptophan Gene 5 Operons: The Basic Concept • In bacteria, genes are often clustered into operons, composed of – An operator, an “on-off” switch – A promoter – Genes for metabolic enzymes • An operon can be switched off by a protein called a repressor • A corepressor is a small molecule that cooperates with a repressor to switch an operon off Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 18-21a trp operon Promoter Promoter Genes of operon DNA Regulatory gene mRNA trpE trpR 3 trpC trpB trpA C B A Operator Start codon Stop codon RNA polymerase mRNA 5 5 E Protein trpD Inactive repressor D Polypeptides that make up enzymes for tryptophan synthesis Tryptophan absent, repressor inactive, operon on LE 18-21b_1 DNA mRNA Active repressor Protein Tryptophan (corepressor) Tryptophan present, repressor active, operon off LE 18-21b_2 DNA No RNA made mRNA Active repressor Protein Tryptophan (corepressor) Tryptophan present, repressor active, operon off Repressible and Inducible Operons: Two Types of Negative Gene Regulation • A repressible operon is one that is usually on; binding of a repressor to the operator shuts off transcription • The trp operon is a repressible operon • An inducible operon is one that is usually off; a molecule called an inducer inactivates the repressor and turns on transcription • The classic example of an inducible operon is the lac operon, which contains genes coding for enzymes in hydrolysis and metabolism of lactose Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 18-22a Promoter Regulatory gene Operator lacl DNA lacZ No RNA made 3 mRNA 5 Protein RNA polymerase Active repressor Lactose absent, repressor active, operon off LE 18-22b lac operon DNA lacZ lacl 3 mRNA 5 lacA Permease Transacetylase RNA polymerase mRNA 5 -Galactosidase Protein Allolactose (inducer) lacY Inactive repressor Lactose present, repressor inactive, operon on • Inducible enzymes usually function in catabolic pathways • Repressible enzymes usually function in anabolic pathways • Regulation of the trp and lac operons involves negative control of genes because operons are switched off by the active form of the repressor Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Positive Gene Regulation • Some operons are also subject to positive control through a stimulatory activator protein, such as catabolite activator protein (CAP) • When glucose (a preferred food source of E. coli ) is scarce, the lac operon is activated by the binding of CAP • When glucose levels increase, CAP detaches from the lac operon, turning it off Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 18-23a Promoter DNA lacl lacZ CAP-binding site Active CAP cAMP Inactive CAP RNA Operator polymerase can bind and transcribe Inactive lac repressor Lactose present, glucose scarce (cAMP level high): abundant lac mRNA synthesized LE 18-23b Promoter DNA lacl CAP-binding site Inactive CAP lacZ Operator RNA polymerase can’t bind Inactive lac repressor Lactose present, glucose present (cAMP level low): little lac mRNA synthesized