Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings PowerPoint Lectures for Biology, Seventh Edition Neil Campbell and Jane Reece Lectures by Chris Romero Chapter 18 The Genetics of Viruses and Bacteria

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 i s the protein shell that encloses the viral genome Capsomere of capsid RNA 18  250 mm Capsomere Glycoprotein 70–90 nm (diameter) DNA Adenoviruses 50 nm

LE 18-4c Glycoprotein 80–200 nm (diameter) RNA Capsid Influenza viruses 50 nm Membranous envelope Bacteriophages, also called phages, are viruses that infect bacteria 80  225 nm DNA Head Tail sheath Tail fiber Bacteriophage T4 50 nm

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings General Features of Viral Reproductive Cycles Viruses are obligate intracellular parasites, which means they can reproduce only within a host cell Animation: Simplified Viral Reproductive Cycle Animation: Simplified Viral Reproductive Cycle DNA VIRUS Capsid HOST CELL Viral DNA Replication Entry into cell and uncoating of DNA Transcription Viral DNA mRNA Capsid proteins Self-assembly of new virus particles and their exit from cell

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 The lytic cycle is a phage reproductive cycle that culminates in the death of the host cell Attachment Entry of phage DNA and degradation of host DNA Synthesis of viral genomes and proteins Assembly Release Phage assembly Head Tails Tail fibers Animation: Phage T4 Lytic Cycle 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 – 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 Animation: Phage Lambda Lysogenic and Lytic Cycles

LE 18-7 Phage DNA The phage attaches to a host cell and injects its DNA. Phage DNA circularizes Bacterial chromosome Lytic cycle The cell lyses, releasing phages. Lytic cycle is induced or Lysogenic cycle is entered Certain factors determine whether Lysogenic cycle Occasionally, a prophage exits the bacterial chromosome, initiating a lytic cycle. The bacterium reproduces normally, copying the prophage and transmitting it to daughter cells. Prophage Many cell divisions produce a large population of bacteria infected with the prophage. Daughter cell with prophage Phage DNA integrates into the bacterial chromosomes, becoming a prophage. New phage DNA and proteins are synthesized and assembled into phages.

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Class/FamilyEnvelopeExamples/Disease I. Double-stranded DNA (dsDNA) AdenovirusNo Respiratory diseases, animal tumors PapovavirusNo Papillomavirus (warts, cervical cancer): polyomavirus (animal tumors) HerpesvirusYes Herpes simplex I and II (cold sores, genital sores); varicella zoster (shingles, chicken pox); Epstein-Barr virus (mononucleosis, Burkitt’s lymphoma) PoxvirusYes Smallpox virus, cowpox virus

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Class/FamilyEnvelopeExamples/Disease II. Single-stranded DNA (ssDNA) ParvovirusNoB19 parvovirus (mild rash) III. Double-stranded RNA (dsRNA) ReovirusNoRotavirus (diarrhea), Colorado tick fever virus

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Class/FamilyEnvelopeExamples/Disease IV. Single-stranded RNA (ssRNA); serves as mRNA PicornavirusNoRhinovirus (common cold); poliovirus, hepatitis A virus, and other enteric (intestinal) viruses CoronavirusYesSevere acute respiratory syndrome (SARS) FlavivirusYesYellow fever virus, West Nile virus, hepatitis C virus TogavirusYesRubella virus, equine encephalitis viruses

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Class/FamilyEnvelopeExamples/Disease V. ssRNA; template for mRNA synthesis FilovirusYes Ebola virus (hemorrhagic fever) OrthomyxovirusYes Influenza virus ParamyxovirusYes Measles virus; mumps virus RhabdovirusYes Rabies virus VI. ssRNA; template for DNA synthesis RetrovirusYes 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 RNA ER Capsid HOST CELL Viral genome (RNA) mRNA Capsid proteins Envelope (with glycoproteins) Glyco- proteins Copy of genome (RNA) Capsid and viral genome enter cell New virus Template

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings RNA as Viral Genetic Material Retroviruses use reverse transcriptase to copy their RNA genome into DNA Capsid Viral envelope Glycoprotein Reverse transcriptase RNA (two identical strands)

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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

LE HOST CELL Reverse transcription Viral RNA RNA-DNA hybrid DNA NUCLEUS Chromosomal DNA Provirus RNA genome for the next viral generation mRNA New HIV leaving a cell HIV entering a cell 0.25 µm HIV Membrane of white blood cell Animation: HIV Reproductive Cycle Animation: HIV Reproductive Cycle

LE 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.

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 Normal protein New prion Prion Original prion Many prions

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 Many bacteria also have plasmids, smaller circular DNA molecules Origin of replication Replication fork 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 Mutant strain arg + trp – Mutant strain arg + trp – Mixture No colonies (control) No colonies (control) Colonies grew Mutant strain arg – trp + Mutant strain arg – trp +

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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 Transduction - phages carry bacterial genes from one host cell to another A+A+ Phage DNA A+A+ Donor cell B+B+ A+A+ B+B+ Crossing over A+A+ A–A– B–B– Recipient cell A+A+ B–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

LE Sex pilus 5 µm

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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 plasmidBacterial chromosome F + cell Mating bridge F + cell Bacterial chromosome F – cell Conjunction and transfer of an F plasmid from and F + donor to an F – recipient

LE 18-18_2 F plasmidBacterial chromosome F + cell Mating bridge F + cell Bacterial chromosome F – cell Conjunction and transfer of an F plasmid from and F + donor to an F – recipient F + cell Hfr cell F factor

LE 18-18_3 F plasmidBacterial chromosome F + cell Mating bridge F + cell Bacterial chromosome F – cell Conjunction and transfer of an F plasmid from and F + donor to an F – recipient F + cell Hfr cell F factor Hfr cell F – cell

LE 18-18_4 F plasmid Bacterial chromosome F + cell Mating bridge F + cell Bacterial chromosome F – cell Conjunction and transfer of an F plasmid from and F + donor to an F – recipient F + cell Hfr cell F factor Hfr cell F – cell Temporary partial diploid Recombinant F – bacterium Conjugation and transfer of part of the bacterial chromosome from an Hfr donor to an F – recipient, resulting in recombiination

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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

LE 18-21a Promoter DNA trpR Regulatory gene RNA polymerase mRNA 3 5 Protein Inactive repressor Tryptophan absent, repressor inactive, operon on mRNA 5 trpE trpD trpC trpBtrpA Operator Start codon Stop codon trp operon Genes of operon E Polypeptides that make up enzymes for tryptophan synthesis D C B A

LE 18-21b_1 DNA Protein Tryptophan (corepressor) Tryptophan present, repressor active, operon off mRNA Active repressor

LE 18-21b_2 DNA Protein Tryptophan (corepressor) Tryptophan present, repressor active, operon off mRNA Active repressor No RNA made

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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

LE 18-22a DNA lacl Regulatory gene mRNA 5 3 RNA polymerase Protein Active repressor No RNA made lacZ Promoter Operator Lactose absent, repressor active, operon off

LE 18-22b DNAlacl mRNA 5 3 lac operon Lactose present, repressor inactive, operon on lacZ lacYlacA RNA polymerase mRNA 5 Protein Allolactose (inducer) Inactive repressor  -Galactosidase Permease Transacetylase

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. DNA cAMP lacl CAP-binding site Promoter Active CAP Inactive CAP RNA polymerase can bind and transcribe Operator lacZ Inactive lac repressor Lactose present, glucose scarce (cAMP level high): abundant lac mRNA synthesized

LE 18-23b DNA lacl CAP-binding site Promoter RNA polymerase can’t bind Operator lacZ Inactive lac repressor Inactive CAP Lactose present, glucose present (cAMP level low): little lac mRNA synthesized