Ch+13 - Chapter 13 Viruses Viroids and Prions General Characteristics of Viruses Are they alive Obligatory intracellular parasites Require living host

Ch+13 - Chapter 13 Viruses Viroids and Prions General...

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Unformatted text preview: Chapter 13: Viruses, Viroids, and Prions General Characteristics of Viruses • Are they alive? • Obligatory intracellular parasites • Require living host cells to multiply • Contain DNA or RNA (single stranded or double stranded) • Contain a protein coat - capsid • No ribosomes – use cellular machinery to replicate • No ATP-generating mechanism Host Range & Viral Size • The spectrum of host cells a virus can infect • Most viruses infect only specific types of cells in one host • Determined by specific host attachment sites and cellular factors • Bacteriophages—viruses that infect bacteria • Receptor site varies: cell wall, fimbriae or flagella, plasma membrane • Potential to use as a way to treat disease (i.e. phage therapy) • Size range from 20-1000 nm in length Viral Structure • Virion—complete, fully developed, and infectous viral particle • Nucleic acid—DNA or RNA can be single- or double-stranded; linear or circular • Capsid—protein coat made of capsomeres (subunits) • Envelope—lipid, protein, and carbohydrate coating on some viruses • Nonenveloped • Spikes—projections from outer surface Nuclei c acid Capso mere Spi kes Enve lope bacteriaphage General Morphology 65 nm Capsid (head) • Helical viruses—hollow, cylindrical capsid • Polyhedral viruses—many-sided • Enveloped virusesgets the evelope from the host cell • Complex viruses—complicated structures DN A Sheat h Pi n Baseplat e A T-even bacteriophage Nucleic acid Capso mere Cap sid Orthopoxviru s have tail sheath and tail fibers to attach Tail to cell, fiber tail sheath acts like syringe and injects it’s nucleic acid into the bacteria Taxonomy of Viruses • Genus names end in -virus • Family names end in -viridae • Order names end in -ales • Viral species: a group of viruses sharing the same genetic information and ecological niche (host) • Descriptive common names are used for species • Subspecies are designated by a number descriptive common names used for viruses, not bacteria though • Example: Family Herpesviridae, genus Simplexvirus, human herpesvirus 2 Viral Multiplication • Nucleic acids only code for a few genes • For a virus to multiply: • It must invade a host cell • It must take over the host's metabolic machinery • One-step growth curve • One virion can give rise to several or thousands of similar viruses in one host cell • Cells typically die but some remain living and thus produce viruses indefinitely 1. get into cell 2. disassembles once into cell =the eclipse period 3. assemble, break free and grow =one step growth curve 4. acute infection Multiplication of Bacteriophages • Lytic cycle • Phage causes lysis and death of the host cell • Lysogenic cycle • Phage DNA is incorporated in the host DNA • Phage conversion • Specialized transduction Text Bacteria virus vs. animal viruses T-Even Bacteriophages: The Lytic Cycle • Attachment: phage attaches by the tail fibers to the host cell ONLY dna gets into bacteria • Penetration: phage lysozyme opens the cell wall; tail sheath contracts to force the tail core and DNA into the destroy the host dna to eliminate competition cell • Biosynthesis: production of phage DNA and proteins undergo dna replication for their genome • Maturation: assembly of phage particles spontaneous assembly • Release: phage lysozyme breaks the cell wall break free through the cell wall, kills the cell RESULT: cell lysis/death Bacteriophage Lambda (λ): The Lysogenic Cycle • Lysogenic phage = temperate phage • Lysogeny: phage remains latent • Phage DNA incorporates into host cell DNA incorporate their dna into the host dna 1. attach and inject dna 2. genetic recombination • Inserted phage DNA is known as a prophage when its dna is incoporate into the host dna • When the host cell replicates its chromosome, it also replicates prophage DNA host cells keep dividing with the prophage in them=when they replicate, the bacteriophage’s dna is also being replicated=lots of infected cells prophase removed from chromosome from stress conditions. comes out and begins taking over the cell, wait until the stressful condition ADVANTAGE: infects more cells at the same time The Lysogenic Cycle Three benefits of lysogeny: Cells become immune to reinfection from the same phage the phage is converting the Phage conversion cell=cell has new properties Specialized transduction a type of horizontal gene transfer localized transduction-very general, random bacterial dna gets packaged and spread into other cells specialized-only specific parts of dna gets spread, the pieces of dna that are flanking the prophage Multiplication of Animal Viruses • Attachment: viruses attach to the cell membrane • Entry by receptor-mediated endocytosis or fusion doesn’t happen in • Uncoating by viral or host enzymes bacteriophage (dissasembly) • Biosynthesis: production of nucleic acid and proteins • Maturation: nucleic acid and capsid proteins assemble • Release by budding (enveloped viruses) or rupture /lyse steals part of the membane table is good for comparing animal vs bacteria viruses*** The Biosynthesis of DNA Viruses • DNA viruses replicate their DNA in the nucleus of the host using viral enzymes • Synthesize capsid in the cytoplasm using host cell enzymes dna travels to nucleus to be copied, other dna gets transcribed into rna to make proteirns The Biosynthesis of RNA Viruses • Virus multiplies in the host cell's cytoplasm using RNA-dependent RNA polymerase RNA dependent RNA Polymerase-makes RNA from RNA, the virus has to bring this with it • ssRNA; + (sense) strand have to have 2 versions of ss RNA-either +sense or -antisense • Viral RNA serves as mRNA for protein synthesis • ssRNA; – (antisense) strand • Viral RNA is transcribed to a + strand to serve as mRNA for protein synthesis • dsRNA—double-stranded RNA • They type of RNA genome, single stranded (+) or (-) sense or double stranded, determines how the virus is replicated Multiplication of ssRNA (+) sense Viruses • • (+) RNA strand acts as mRNA and translation occurs directly (+) strand serves as template for transcription of (-) strand which can be used to produce more (+) strands for tsl and insertion into progeny viruses Attachment Capsid Nucleus RNA Host cell Cytoplasm Entry and uncoating Maturation and release Translation and synthesis of viral proteins RNA replication by viral RNA– dependent RNA polymerase Uncoating releases viral RNA and proteins. – strand is transcribed from + viral genome. KEY Capsid protein + or sense strand of viral genome Viral genome (RNA) – or antisense strand of viral genome ss = single-stranded ds = double-stranded + strand mRNA is transcribed from the – strand. ssRNA; + or sense strand; Picornaviridae Viral protein Multiplication of ssRNA (-) sense Viruses Txn of (+) strand to serve as mRNA tsl occurs New (+) strand is transcribed to make more (-) strands for incorporation into capsid Attachment Capsid RNA Nucleus Cytoplasm Host cell Entry and uncoating Maturation and release Translation and synthesis of viral proteins Capsid protein KEY RNA replication by viral RNA– dependent RNA polymerase Uncoating releases viral RNA and proteins. The + strand (mRNA) must first be transcribed from the – viral genome before proteins can be synthesized. Viral genome (RNA) + or sense strand of viral genome – or antisense strand of viral genome ss = single-stranded ds = double-stranded – strands are incorporated into capsid Additional – strands are transcribed from mRNA. ssRNA; – or antisense strand; Rhabdoviridae Viral protein Multiplication of dsRNA Viruses mRNA produced in cytoplasm where it undergoes tsl Make more (-) strands via RNA dependent RNA Polymerase mRNA and (-) strand form dsRNA that is incorporated into capsid Attachment Capsid RNA Nucleus Cytoplasm Host cell Entry and uncoating Maturation and release Translation and synthesis of viral proteins RNA polymerase initiates production of – strands. The mRNA and – strands form the dsRNA that is incorporated as new viral genome. RNA replication by viral RNA– dependent RNA polymerase Uncoating releases viral RNA and proteins. mRNA is produced inside the capsid and released into the cytoplasm of the host. Viral genome (RNA) KEY + or sense strand of viral genome – or antisense strand of viral genome ss = single-stranded ds = double-stranded Capsid proteins and RNAdependent RNA polymerase dsRNA; + or sense strand with – or antisense strand; Reoviridae Viral protein Retroviruses start as an RNA virus and become a DNA virus=stick with us for life ex: HIV RNA virus that uses DNA Reverse transcriptase enzyme the makes RNA from DNA, unique to retroviruses Viral genome integrates into host as provirus Virus DNA intergrates into our DNA using the enzyme INTEGRASE =PROVIRUS a provirus never leaves Viral Maturation and Release • • • Spontaneous assembly of capsid Budding if enveloped virus Ruptures membrane if noneveloped Categories of Animal Virus Infections • Acute viral infections: virulent infection results in death of host cell that may or may not lyse with release of virions • Rapid onset • Short duration • Persistent infections: viruses or their genomes are continually present in body and virions are released from infected cells by budding • • Continue for years or lifetime May or may not have symptoms • Some viruses exhibit both Consequences of Virus Infection in Animal Cells • Persistent infections: occurs gradually over a long period of time • Latent infections: delay between infection by the virus and lytic events • Provirus integrated into host chromosome or replicates separately, much like plasmid • Cannot be eliminated • Remains in asymptomatic host cell for long periods • May reactivate due to changes in immunity Prions • Proteinaceous infectious particles • Infectious proteins, no nucleic acid! • Inherited and transmissible by ingestion, transplant, and surgical instruments • Linked to slow, fatal animal and human diseases "Mad cow disease" • • • • Creutzfeldt-Jakob disease (CJD) Gerstmann-Sträussler-Scheinker syndrome Fatal familial insomnia Sheep scrapie Prions PrPS c PrP c PrPc produced by cells is secreted to the cell surface. PrPSc may be acquired or produced by an altered PrPc gene. PrPSc reacts with PrPc on the cell surface. PrPSc converts the PrPc to PrPSc. Lysosome Endosome The new PrPSc converts more PrPc. The new PrPSc is taken in, possibly by receptormediated endocytosis. PrPSc accumulates in endosomes. PrPSc continues to accumulate as the endosome contents are transferred to lysosomes. The result is cell death. • PrPC: normal cellular prion protein, on the cell surface • PrPSc: scrapie protein; abnormal protein that is infectious ...
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