Viruses Viroids and Prions Chapter 13 notes A Glimpse of History Tobacco mosaic disease 1890s D M Iwanowsky Martinus Beijerinck determined caused by filterable virus too small to be seen with light microscope passed through filters for bacteria Decade later F W Twort and F d Herelle discovered filterable virus that destroyed bacteria Previously many bacteria fungi protozoa identified as infectious diseases Virus means poison Viruses have many features more characteristic of complex chemicals e g still infective following precipitation from ethyl alcohol suspension or crystallization Viruses Obligate Intracellular Parasites Viruses simply genetic information DNA or RNA contained within protective coat Inert particles no metabolism replication motility Genome hijacks host cell s replication machinery Inert outside cells inside direct activities of cell Infectious agents but not alive Can classify generally based on type of cell they infect eukaryotic or prokaryotic Bacteriophages May provide alternative to antibiotics phages infect prokaryotes General Characteristics of Viruses Most viruses notable for small size Smallest 10 nm 10 genes Largest 500 nm viral particle is nucleic acid protein coat Virion Protein coat is capsid protects nucleic acids Carries required enzymes Composed of identical subunits called capsomers Capsid plus nucleic acids called nucleocapsid Enveloped viruses have lipid bilayer envelope Matrix protein Naked viruses between nucleocapsid and envelope lack envelope more resistant to disinfectants Viral genome either DNA or RNA never both Useful for classification i e DNA or RNA viruses Genome linear or circular Double or single stranded affects replication strategy Viruses have protein components for attachment Phages have tail fibers Many animal viruses have spikes Allow virion to attach to specific receptor sites Generally three different shapes Icosahedral helical or complex International Committee on Viral Taxonomy ICVT publishes classification of viruses 2009 report 6 000 viruses 2 288 species 348 genera 87 families 6 orders Key characteristics include genome structure nucleic acid and strandedness and hosts infected Other characteristics e g viral shape disease symptoms also considered Virus families end in suffix viridae Names follow no consistent pattern Some indicate appearance e g Coronaviridae from corona meaning crown Others named for geographic area from which first isolated e g Bunyaviridae from Bunyamwera in Uganda Africa Genus ends in virus e g Enterovirus Species name often name of disease e g poliovirus causes poliomyelitis though viruses commonly referred to only by species name Viruses often referred to informally Groups of unrelated viruses sharing routes of infection Oral fecal route enteric viruses Respiratory route respiratory viruses Zoonotic viruses cause zoonoses animal to human Arboviruses from arthropod borne are spread by arthropods often can infect widely different species important diseases yellow fever dengue fever West Nile encephalitis La Crosse encephalitis Bacteriophages Three general types of bacteriophages based on relationship with host Lytic phages Temperate phages Filamentous phages or virulent phages exit host Lytic Phage Infections Lytic Cell is lysed productive infection new particles formed T4 phage dsDNA as model entire process takes 30 minutes Five step process attachment genome entry synthesis assembly release Attachment phage exploits bacterial receptors Genome entry T4 lysozyme degrades cell wall Tail contracts injects genome through cell wall and membrane Synthesis of proteins and genome Early proteins host s RNA polymerase to not recognize its own promoters Late proteins are structural proteins capsid tail produced toward end of cycle translated within minutes nuclease degrades host DNA protein modifies Assembly maturation some components spontaneously assemble others require protein scaffolds Release Temperate Phage Infections Lysozyme produced late in infection digests cell wall Cell lyses releases phage Burst size of T4 is 200 Option of lytic infection or incorporation of DNA into host cell genome Lysogenic infection Infected cell is lysogen Lambda phage as model Lambda phage linear chromosome complementary single stranded overhangs at ends join inside host Resulting circular molecule either directs lytic infection or integrates into E coli chromosome Phage enzyme integrase inserts DNA at specific site Site specific recombination Integrated phage DNA termed prophage Replicates with host chromosome Can be excised by phage encoded enzyme Results in lytic infection A repressor prevents maintains lysogenic state Lambda phage DNA excised from chromosome only about once per 10 000 divisions of lysogen If DNA damaged e g UV light exposure SOS repair system turns on activates a protease Protease destroys repressor allows prophage to be excised enter lytic cycle Called phage induction allows phage to escape damaged host Lysogen immune to superinfection infection by same phage and lysogenic conversion Change in phenotype of lysogen from prophage E g toxins encoded by phage genes only strains carrying prophage produce the toxins Repressor maintaining integrated prophage also binds to operator on incoming phage DNA prevents gene expression immunity to superinfection Filamentous Phages Single stranded DNA phages used to produce only single stranded recombinant DNA Look like long fibers Cause productive infections host cells not killed but grow more slowly M13 phage as model Attaches to protein on F pilus of E coli Single stranded DNA genome enters cytoplasm DNA polymerase synthesizes complementary strand Called replicative form RF one strand used as template for synthesis of mRNA copies of genome M13 phage coat protein molecules inserted into cytoplasmic membrane Other proteins form pores As phage DNA excreted through pores coat proteins coat the DNA form nucleocapsids The Roles of Bacteriophages in Horizontal Gene Transfer Generalized Transduction Results from packaging error during phage assembly Some phages degrade host chromosome fragments can be mistakenly packaged into phage head These phages cannot direct phage replication cycle Termed generalized transducing particles Following release can bind to new host inject DNA DNA may integrate via homologous recombination replacing host DNA Any gene from donor cell can be transferred Specialized Transduction Excision mistake during transition from lysogenic to lytic