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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 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Overview: Microbial Model Systems • E. coli and its viruses are called model systems because of their frequent use by researchers in studies that reveal broad biological principles Virus Bacterium Animal cell Animal cell nucleus 0.25 µm Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Discovery of Viruses: Scientific Inquiry • Tobacco mosaic virus stunts growth of tobacco plants and gives their leaves a mosaic coloration Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Structure of Viruses • Viruses are not cells • Viruses consist of – nucleic acid – protein coat – membranous envelope (in some cases) • are derived from the host cell’s membrane 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 Capsomere Capsomere of capsid DNA RNA Glycoprotein 70–90 nm (diameter) 18 250 mm Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 50 nm Adenoviruses LE 18-4c Membranous envelope Capsid RNA Bacteriophages, also called phages, are viruses that infect bacteria Head Tail sheath Tail fiber DNA Glycoprotein 80–200 nm (diameter) 50 nm Influenza viruses 80 225 nm 50 nm Bacteriophage T4 General Features of Viral Reproductive Cycles Entry into cell and uncoating of DNA • Viruses are obligate intracellular parasites, which means they can reproduce only within a host cell 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 Animation: Simplified Viral Reproductive Cycle Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Reproductive Cycles of Phages • Phages have two reproductive mechanisms: – lytic cycle – lysogenic cycle Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Lytic Cycle Animation: Phage T4 Lytic Cycle • The lytic cycle is a phage reproductive cycle that culminates in the death of the host cell Attachment Phage assembly Release Entry of phage DNA and degradation of host DNA Head Tails Tail fibers Assembly Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Synthesis of viral genomes and proteins The Lysogenic Cycle • The lysogenic cycle replicates the phage genome without destroying the host – viral DNA molecule is incorporated into the host cell’s chromosome • known as a prophage • Every time the host divides, it copies the phage DNA and passes the copies to daughter cells 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. 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 Capsid Capsid and viral genome enter cell RNA HOST CELL Envelope (with glycoproteins) Viral genome (RNA) Template mRNA ER Glycoproteins Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Capsid proteins Copy of genome (RNA) New virus RNA as Viral Genetic Material • Retroviruses use reverse transcriptase to copy their RNA genome into DNA Glycoprotein Viral envelope Capsid Reverse transcriptase Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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 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 mRNA New HIV leaving a cell Animation: HIV Reproductive Cycle 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. 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 Prion Original prion Many prions Normal protein New prion Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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 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 Replication for Origin of replication • Many bacteria also have plasmids, smaller circular DNA molecules Termination of replication Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Mutation and Genetic Recombination as Sources of Genetic Variation • Genetic diversity arises by recombination of DNA from two different bacterial cells Mixture Mutant strain arg+ trp– Mutant strain arg– trp+ Mixture Mutant strain arg+ trp– No Colonies colonies grew Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings (control) Mutant strain arg– trp+ 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 by the uptake of naked, foreign DNA from the surrounding environment Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Transduction Phage DNA A+ B+ • Transduction - phages carry bacterial genes from one host cell to another A+ B+ Donor cell A+ Crossing over A+ A– B– Recipient cell A+ B– Recombinant cell Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Conjugation and Plasmids • Conjugation is the direct transfer of genetic material between bacterial cells that are temporarily joined – The transfer is one-way: (“male”) donates DNA, to its “mate” (“female”) • “Maleness,” the ability to form a sex pilus results from an F (for fertility) factor • 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 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 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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 – individuals with the R plasmid will survive and increase in the overall population Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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 • Corepressor - 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 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 Positive Gene Regulation • Some operons are also subject to positive control through a stimulatory activator protein. Promoter DNA lacl CAP-binding site cAMP lacZ RNA Operator polymerase Active can bind and transcribe CAP Inactive lac Inactive repressor CAP Lactose present, glucose scarce (cAMP level high): abundant lac mRNA synthesized Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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
