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Chapter 10 Viruses • Viruses contain DNA or RNA • And a protein coat called a • The capsid is composed of individual • Some are enclosed by an envelope • May have spikes protruding from envelope • Most viruses infect only specific types of cells in one host • Host range is determined by specific host attachment sites and cellular factors Viruses Polyhedral Viruses Helical Viruses Causes systemic infection often with hemorrhagic fever Complex Viruses Viral Taxonomy • Family names end in -viridae • Genus names end in -virus • A group of viruses sharing the same genes and host. Common names are used for species • Subspecies are designated by a number Viral Taxonomy • Family : Herpesviridae • Genus: Simplexvirus • Species/Subspecies: Human herpes virus 1, HHV 2 • Family: Retroviridae • Genus: Lentivirus • Species/Subspecies: Human Immunodeficiency Virus 1, HIV 2 Growing Animal Viruses • Animal viruses may be grown • in living animals • In embryonated eggs • • or in cell culture. Growing Viruses • Animal (and plant) viruses may be grown in cell culture. • For research purposes • For clinical identification of virus type • By observing Virus Identification • Some cytopathic effects (not comfirmatory) • Inclusion bodies - rabies virus • Syncytia formation – measles, common cold • Transformation – human papillomavirus Virus Identification • Serological tests • Detect antibodies against viruses in a patient • Use antibodies to identify viruses • Western blot • Viral nucleic acids: RFLPs (i.e. DNA Fingerprinting), PCR tests Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings Virus Identification – a cytopathic effect Viral Multiplication • Viruses require host cell “machinery” for replication • • Some viruses have some of their own enzymes: mainly for viral nucleic acid replication • Most, if not all, enzymes come from host cell • These enzymes are involved in building new virions Multiplication of Bacteriophages (Lytic Cycle) • Phage causes lysis and death of host cell • Attachment Phage attaches by tail fibers to host cell • Penetration Phage lysozyme opens cell wall; DNA is then “injected” into cell • Biosynthesis Production of phage DNA and proteins • Maturation Assembly of phage particles • Release Phage lysozyme breaks cell wall Bacterial cell wall Bacterial chromosome Capsid DNA Capsid Sheath Tail fiber 1 Attachment: Phage attaches to host cell. Base plate Pin Cell wall Tail Plasma membrane 2 Penetration: Phage penetrates host cell and injects its DNA. Sheath contracted Tail core 3 Biosynthesis: phage DNA directs synthesis of viral components by host cell. Tail DNA 4 Maturation: Viral components are assembled into virions. Capsid 5 Release: Host cell lyses and new virions are released. Tail fibers The Lysogenic Cycle: Phage DNA incorporated into host DNA Specialized Transduction Prophage gal gene Bacterial DNA 1 Prophage exists in galactose-using host (containing the gal gene). Galactose-positive donor cell gal gene 2 Phage genome excises, carrying with it the adjacent gal gene from the host. gal gene 3 Phage matures and cell lyses, releasing phage carrying gal gene. 4 Phage infects a cell that cannot utilize galactose (lacking gal gene). Galactose-negative recipient cell 5 Along with the prophage, the bacterial gal gene becomes integrated into the new host’s DNA. 6 Lysogenic cell can now metabolize galactose. Galactose-positive recombinant cell Multiplication of Animal viruses • Attachment Viruses attach to cell membrane • Penetration By endocytosis or fusion • Uncoating By viral or host enzymes • Biosynthesis Production of nucleic acid and proteins • Maturation Nucleic acid and capsid proteins assemble • Release By budding (enveloped viruses) or rupture (non-enveloped viruses) Attachment, Penetration, and Uncoating Release of an enveloped virus by budding Nonenveloped DNA virus • • Can cause cancer (cervical cancer) • Hand warts • Genital warts • Commonly sexually transmitted • Prevention: vaccine for HPV • • Respiratory infections in humans • Usually sudden onset and short duration • Tumors in animals Enveloped DNA virus • • Can cause chronic liver disease • Can lead to liver cancer • Body fluid transmisson • Prevention: vaccine • Human Herpes Virus 1 and HHV 2 – “cold sores” and genital herpes • Sexually transmitted • Other direct contact transmission • Prevention: condoms (not 100%), abstinence Enveloped DNA virus • Varicella-Zoster virus (HHV 3) – • Chicken pox is initial infection • Shingles may occur later • Contracted by inhaling virus • Prevention: vaccine • Epstein-Barr virus (HHV 4) – Infectious Mononucleosis • Saliva transmission • Burkitt’s lymphoma in African children • Prevention: avoid infected body fluids (mainly saliva) Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings Multiplication of a Retrovirus Capsid Reverse transcriptase Envelope Virus Two identical strands of RNA 1 Retrovirus penetrates host cell. Host cell DNA of one of the host cell’s chromosomes 5 Mature retrovirus leaves host cell, acquiring an envelope as it buds out. Viral RNA Identical strands of RNA 4 Transcription of the Viral proteins RNA Reverse transcriptase provirus may also occur, producing RNA for new retrovirus genomes and RNA that codes for the retrovirus capsid and envelope proteins. Provirus 2 Its RNA is uncoated; reverse transcription takes place. 3 The new viral DNA is tranported into the host cell’s nucleus and integrated as a provirus. The provirus may divide indefinitely with the host cell DNA. Retroviruses – enveloped RNA viruses • Use reverse transcriptase to produce DNA from RNA viral genome • HIV - AIDS • Oncogenic viruses • Some retroviruses can cause cancer Cancer • The genetic material of oncogenic viruses becomes integrated into the host cell's DNA. “transform” normal cells into • cancerous cells. • An oncovirus can promote oncogene expression • may contain oncogenes Viral Infections • • Virus remains in host cell for long periods, asymptomatically • Subsequent activation may lead to: i.e. cold sores, shingles • Persistent Viral Infections • Disease progresses over a long period, generally fatal • Subacute sclerosing panencephalitis, SSPE (measles virus) • AIDS dementia complex (HIV) Prions • Infectious proteins • Inherited, and transmissible by ingestion, transplant, & surgical instruments • Spongiform encephalopathies (fatal): Sheep Scrapie, Creutzfeldt-Jakob disease, mad cow disease • Cause: • normal cellular prion protein on cell surface, • converts to • scrapie protein, accumulates in brain cells forming plaques Prions PrPSc PrPc 1 2 3 4 Lysosome Endosome 5 6 7 8 Plant viruses and viroids • Plant Viruses • enter through wounds or via insects • are infectious RNA • One causes potato spindle tuber disease Potato Spindle Tuber Viroids RNA virus, nonenveloped • Poliovirus • May be paralytic in ~1% of cases • Transmission: • Prevention: vaccine • Human • Number one cause of the common cold • • Attacks liver, kidneys, spleen • Transmission: fecal-oral route • Usually not fatal • Prevention: vaccine (long-term), immune globulin (short-term) RNA virus, enveloped • Influenza viruses A, B, and C • Type A have caused • Both type A and B cause seasonal flu • Subtypes differ based on H and N spike variation • Influenza viruses continued: • Mutation of H and N spike can lead to epidemics or pandemics • Contracted by inhaling virus • Prevention: vaccine may prevent Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings RNA virus, enveloped • • Infects birds and humans • Transmitted by mosquitos • First appeared in U.S. in 1999 in NYC area; now coast to coast • Symptoms: Usually “flu-like;” less than 1% get West Nile Encephalitis • Prevention: avoid mosquito bites Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings