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Download Chapter 19: Viruses 1. Viral Structure & Reproduction What exactly is a Virus?
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11/7/2015 Chapter 19: Viruses 1. Viral Structure & Reproduction 2. Bacteriophages 3. Animal Viruses 4. Viroids & Prions 1. Viral Structure & Reproduction Chapter Reading – pp. 393-396 What exactly is a Virus? Viruses are extremely small entities that are obligate intracellular parasites with no metabolic capacity of their own. • have none of the characteristics of living cells • do NOT reproduce or metabolize on their own • do NOT respond to their environment or maintain homeostasis in any way **It’s hard to “kill” something that’s not really alive, so antibiotics that kill bacteria, fungi, etc, do NOT harm viruses** • depend on host cells for their reproduction (which are typically destroyed in the process) 1 11/7/2015 What’s a Virus made of? All viruses consist of at least 2 components: Genetic Material • usually a single DNA or RNA molecule • can be single or double stranded, linear or circular • contains the viral genes A Capsid • a hollow protein capsule which houses the genetic material Some viruses also contain: An Envelope • membrane from host cell with viral proteins (spikes) that surrounds the capsid The Viral Capsid Capsids are hollow, protein “shells” that: • are an array of protein subunits called capsomeres • consist of >1 type of protein • house the genetic material (DNA/RNA) • are frequently involved in host recognition & entry • vary in shape, size among viruses The Viral Envelope The capsid of some viruses is enclosed in a phospholipid membrane called an envelope containing viral proteins called “spikes”: • membrane comes from host cell • “spike” proteins involved in attachment and entry into host cell 2 11/7/2015 Viral Genetic Material Viral genomes range from ~4000 to 250,000 bp (or nt) and can be: DNA or RNA Double- (ds) or single-stranded (ss) • if single-stranded, it is referred to as “+” or “–” + strand = sense or coding strand – strand = antisense or template strand Viral Morphology Viruses come in 4 basic morphological types: 1. Icosahedral Viruses • capsomeres in capsid have a “polyhedral” arrangement 2. Helical Viruses • capsomeres in capsid have a “helical” arrangement 3. Enveloped Viruses 4. Complex Viruses • consist of multiple types of structures Variety of Viral Structure RNA Capsomere DNA Membranous RNA envelope Capsid DNA Head Tail sheath Capsomere of capsid Tail fiber Glycoprotein 18 250 nm 70–90 nm (diameter) 50 nm 20 nm (a) Tobacco (b) Adenoviruses mosaic virus Glycoproteins 80–200 nm (diameter) 80 225 nm 50 nm (c) Influenza viruses 50 nm (d) Bacteriophage T4 3 11/7/2015 The Viral Reproductive Cycle 1 Attachment & Entry DNA VIRUS Capsid 2 Replication of Viral Gemone 3 Expression of Viral Genes HOST CELL Viral DNA mRNA Viral DNA Capsid proteins • attachment requires highly specific interactions between viral capsid or envelope proteins and host cell surface molecules • attachment determines the host range of virus 4 Self-assembly of new virus particles 5 Release from the host cell 2. Bacteriophages Chapter Reading – pp. 396-398 What’s a Bacteriophage? A bacteriophage is a virus that infects and destroys bacterial cells. Bacteriophages are of many different types (some w/DNA or RNA, etc), however 2 types are of particular interest due to decades of study: “T-even” bacteriophages (T2, T4, T6) bacteriophage lambda (l) ***More is known about the biology of these viruses than any other type of virus!*** 0.5 mm 4 11/7/2015 Bacteriophage Reproductive Cycle 1 Attachment T-even bacteriophages have an exclusively lytic reproductive cycle such as this. 2 Entry of phage DNA and degradation of host DNA 5 Release Phage assembly 4 Assembly Head Tail 3 Synthesis of viral genomes and proteins Tail fibers Lytic vs Lysogenic Cycle Bacteriophage l has 2 options: lytic vs lysogenic Phage DNA Daughter cell with prophage The phage injects its DNA. Cell divisions produce a population of bacteria infected with the prophage. Phage DNA circularizes. Phage Bacterial chromosome Occasionally, a prophage exits the bacterial chromosome, initiating a lytic cycle. Lytic cycle The cell lyses, releasing phages. Lysogenic cycle Certain factors determine whether lytic cycle is induced New phage DNA and proteins are synthesized and assembled into phages. or lysogenic cycle is entered Prophage The bacterium reproduces, copying the prophage and transmitting it to daughter cells. Phage DNA integrates into the bacterial chromosome, becoming a prophage. 3. Animal Viruses Chapter Reading – pp. 398-401 5 11/7/2015 Life Cycle of Animal Viruses The basic life cycle stages of animal viruses differ from bacteriophages in some key ways: 1) attachment and entry • requires specific interactions between host cell membrane proteins & viral “spike” proteins (enveloped) or capsid proteins (non-enveloped) • entry by direct penetration, endocytosis or fusion of the envelope with the cytoplasmic membrane • uncoating of the virus (release of DNA, RNA) 2) replication of viral genome • in nucleus (DNA or RNA) or cytoplasm (RNA) 3) expression of viral genes • can occur entirely in the cytoplasm for some RNA viruses 4) assembly • RNA viruses typically assemble in cytoplasm • DNA viruses typically assemble in nucleus 5) release • via lysis (rupture of plasma membrane), budding or exocytosis • host cell is not necessarily killed Reproductive Cycle of a DNA Virus 6 11/7/2015 Unique Features of RNA Viruses Copying of viral RNA poses a unique problem: 1) viral RNA must be converted to DNA which can then be transcribed to produce more RNA OR 2) viral RNA must somehow serve as a template to produce more RNA In reality, RNA viruses use both approaches: • retroviruses use reverse transcriptase to make DNA from an RNA template • all other RNA viruses use RNA-dependent RNA polymerase to transcribe from an RNA template Reproductive Cycle of RNA Viruses Capsid Capsid and viral genome enter the cell RNA Envelope (with glycoproteins) • copying of the viral RNA genome requires RNA-dependent RNA polymerase HOST CELL Template Viral genome (RNA) mRNA Capsid proteins ER • is commonly packaged in viral capsid (essential for RNA – strand viruses) Copy of genome (RNA) Glycoproteins New virus Glycoprotein Retroviral Cycle Viral envelope Capsid Reverse transcriptase RNA (two identical strands) HIV Membrane of white blood cell HOST CELL HIV Reverse transcriptase Viral RNA RNA-DNA hybrid 0.25 m DNA HIV entering a cell NUCLEUS Provirus Chromosomal DNA RNA genome for the next viral generation mRNA New virus New HIV leaving a cell 7 11/7/2015 Latent Viral Infections Some DNA viruses and retroviruses integrate viral DNA into host cell chromosomal DNA : • analogous to the lysogeny of bacteriophage l • the inserted viral DNA is considered a provirus which can remain dormant indefinitely • such an infection is considered to be latent • the provirus can become active due to various conditions of stress in the cell and re-enter the standard viral life cycle Important Animal Viruses 4. Viroids & Prions Chapter Reading – pp. 402-406 8 11/7/2015 What are Viroids? Viroids are very small, circular RNA molecules (hundreds of nucleotides) that infect plants: • viroid RNA does NOT encode any proteins • the viroid RNA is copied in the host cell • RNA silencing via miRNA is thought to perturb host plant gene expression causing the pathology of viroid infection • transmission appears to be mechanical: • grazing animals • cutting tools What are Prions? Prions are unique, infectious proteins that cause spongiform encephalopathies: • e.g., “mad cow” disease, kuru, Creutzfelt-Jacob disease (CJD), scrapie • involves NO nucleic acid (DNA or RNA) • involves aberrant folding of a normal protein (PrP) expressed in neural tissue • normal = PrPC; aberrant & infectious = PrPSC • PrPSC is extremely stable, forms insoluble aggregates • consumed PrPSC induces host PrPC to become PrPSC Model of Prion based Illness • contact between PrPC & PrPSC induces PrPSC • insoluble PrPSC accumulates, kills cells 9 11/7/2015 Prion Pathology Normal vs Abberrant Prp PrpC Spongiform Encephalopathy PrpSC Key Terms for Chapter 19 • capsid, envelope, spikes • icosahedral, helical, enveloped, complex viruses • attachment, entry, release, budding • bacteriophage, lytic vs lysogenic • prophage, lysogen • uncoating, latent, provirus • RNA-dependent RNA polymerase Relevant Chapter Questions 1, 4-6, 8 • reverse transcriptase, retrovirus • viroid, prion, spongiform encephalopathy 10