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Chapter 6 An Introduction to Viruses
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
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The Great Fever video
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1918 Flu 1918 Flu Virus Video (12min) Reviving the virus wks
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6.1 The Search for elusive viruses
Leeuwenhoek’s microscope But there was a limit to magnification Louis Pasteur had an idea that rabies was caused by a “living thing” that was even smaller than bacteria Pasteur coined the term “virus”—means poison
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6.1 The Search for Elusive Viruses
1890s—first virus discovered D. Ivanovski and M. Beijernick showed that a disease in tobacco plants was caused by a virus Tobacco mosaic virus
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6.1 The Search for the Elusive Virus
Frederich Loeffler and Paul Frosch Virus was the causative agent in foot-and-mouth disease in cattle Filtered fluids from host—infectious particles were small enough to pass through Smaller than bacteria
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6.2 The Position of Viruses in the Biological Spectrum
Viruses are a unique group of biological entities known to infect every type of cell, including bacteria, fungi, protozoa, plants, and animals Viruses have been around ever since cells have on this planet
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6.2 The Position of Viruses in the Biological Spectrum
Viruses are considered the most abundant microbes on earth There are 10x more viruses than bacteria!
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6.2 The Position of Viruses in the Biological Spectrum
Viral Terminology Referred to as infectious particles as opposed to “organisms” Refer to them as active or inactive instead of “alive” or “dead” Obligate Intracellular Parasites— cannot multiply unless it invades a specific host cell and instructs its genetic and metabolic machinery to make and release quantities of new viruses
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6.3 The General Structure of Viruses
Viral Components Viruses are made up of many repeating units and can be purified into crystals (Stanley) Further proof that they are non-living
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PROCESS BOX 6-1 If viruses are not considered to be living, why do you think they are studied in microbiology? TWO LINE MINIMUM
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6.3 The General Structure of Viruses
Size Range Viruses are the smallest infectious particles Called ultramicroscopic Less than 0.2 µm Actual range is 20nm-450 nm Need to be seen with an electron microscope Specimens are also usually stained Shadowcasting
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6.3 The General Structure of Viruses
Size Range
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Envelope (not in all viruses)
Covering Capsid Envelope (not in all viruses) Central Core DNA or RNA Matrix Proteins
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6.3 The General Structure of Viruses
Viral Components Protein capsid or shell that surrounds the nucleic acid Nucleic acid + capsid = nucleocapsid Naked viruses—only has nucleocapsid (no envelope) Enveloped viruses– envelope is present
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6.3 The General Structure of Viruses
Viral Components The Viral Capsid Capsids are made of monomers called capsomers Can self-assemble Two main shapes Helical Icosahedral
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6.3 The General Structure of Viruses
The Viral Capsid Helical--continuous helix of capsomers forming a cylindrical nucleocapsid Naked Helical Ex: Tobacco mosaic virus Enveloped Helical Ex: influenza, measles, rabies
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6.3 The General Structure of Viruses
The Viral Capsid Icosahedron 20-sided with 12 corners Naked ex: rotavirus Enveloped ex: herpes simplex
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6.3 The General Structure of Viruses
Nucleic Acids: At the Core of a Viruses Viral genome – either DNA or RNA but never both Unusual nucleic acids Single-stranded DNA (parvoviruses) Double-stranded RNA (reoviruses)
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PROCESS BOX 6-2 What shape and what envelope combination would best describe this virus? TWO LINE MINIMUM
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6.4 Viral Classification DNA viruses RNA viruses
Usually double stranded (ds) but may be single stranded (ss) Circular or linear RNA viruses Usually single stranded, may be double stranded, may be segmented into separate RNA pieces ssRNA genomes ready for immediate translation are positive-sense RNA ssRNA genomes that must be converted into proper form are negative-sense RNA
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Human Viruses & Viral Diseases
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Viral Replication
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6.5 Modes of Viral Replication
Multiplication Cycles in Animal Viruses General phases are Adsorption Penetration Uncoating Synthesis Assembly Release
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6.5 Modes of Viral Multiplication
Adsorption: The virus attaches to the host cell by specific binding of the spikes to cell receptors Spectrum of cells a virus can infect – host range EX: Hepatitis B only infects human liver cells EX: polio virus infects intestinal and nerve cells of primates
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6.5 Modes of Viral Multiplication
Penetration: The virus is engulfed into a vesicle Uncoating: The envelope of the cell is uncoated, which frees the viral genetic material into the cytoplasm **These two steps basically happen at the same time**
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Variety in Penetration and Uncoating
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Host cell membrane Free RNA Receptors Uncoating of nucleic acid Receptor-spike complex Entry of nucleocapsid Irreversible attachment Membrane fusion (a) Uncoating step Host cell membrane Free DNA Virus in vesicle Vesicle, envelope and capsid break down Specific attachment Engulfment (b) Capsid RNA Nucleic acid Receptor (c) Adhesion of virus to host receptors Engulfment into vesicle Viral RNA is released from vesicle
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6.5 Modes of Viral Multiplication
Synthesis: Replication and Protein Production Under the control of viral genes, the cell synthesizes the basic components of new viruses, RNA molecules, capsomers, and spikes
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6.5 Modes of Viral Multiplication
Assembly: Viral spike proteins are inserted into the cell membrane for the viral envelope; nucleocapsid is formed from RNA and capsomere
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6.5 Modes of Viral Multiplication
Release: Enveloped viruses bud off of the membrane (exocytosis), carrying away an envelope with the spikes. This complete virus or virion is ready to infect another cell. Naked viruses cause the cell to erupt or lyse
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6.5 Modes of Viral Multiplication
Virion—fully formed, extracellular virus particle that is virulent Number releases varies from virus to virus but is typically high Poliovirus—100,000 virions!! Virus Cycle Animation
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Putting the steps together
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PROCESS BOX 6-3 In your own words, sum up the animal viral multiplication cycle. THREE LINE MINIMUM
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6.5 Modes of Viral Multiplication
Damage to the Host Cell and Persistent Infections Cytopathic effects—virus-induced damage to the cell that alters its microscopeic appearance
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6.5 Modes of Viral Multiplication
Damage to Host Cell and Persistent Infection Productive Responses – viruses are produced in host cells Lysis – host cell bursts, releasing virions. Dies Non-lysis – the host cell doesn’t burst. It slowly leaks the new virons. Latent State – Viral nucleic acid becomes integrated into host’s chromosomes, replicating as part of the host genome. Persistent – Usually bud off from cell & does not damage cell – Herpes
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6.5 Modes of Viral Multiplication
Some viruses change host DNA (can lead to cancer) Oncoviruses Transduction is process of DNA change
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Transduction DNA transferred from 1 bacterial cell to another by way of a phage Generalized – Phages that can transfer any bacterial gene. Transferred to new bacterial cell once phage is incorporated. Bacterial genes (DNA) are integrated into recipient’s DNA Specialized – only a few specific genes are transferred to recipient bacteria Episomes – pieces of a chromosome that can replicate as part of a bacterial chromosome (as a prophage) or independently of it (virulent phage). Such as temperate bacteriophages.
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Generalized Transduction
Episome
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PROCESS BOX 6-4 How do generalized and specialized transduction compare (include a similarity and a difference)? THREE LINE MINIMUM
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6.5 Modes of Viral Multiplication
Bacteriophage multiplication cycles Viruses that target bacteria Double-stranded DNA T2 & T4 bacteriophages target E. coli Stages overview Adsorption Penetration (of nucleic acid only) Synthesis of viral parts Assembly Release
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6.5 Modes of Viral Multiplication
Lysogeny: The Silent Virus Infection Virus enters cell but is not replicated and released right away Viral DNA gets replicated every time with host Allows DNA to spread without killing host Eventually, viral DNA will be triggered (induction), and viral components will be replicated and assembled
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Attachment at Receptor site
Lytic Cycle Release/Lysis Attachment at Receptor site Entry/Penetration Assembly/Maturation Replication
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The Lytic Cycle of Virus infection
Nuclease destroys host DNA, phage DNA translated phage structures Chance meeting & attachment Lysozyme degrades hole & DNA injected in. Capsid remains outside Assembly of virions - Maturation Lysis of cell & release of virions ~ 200/cell
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PROCESS BOX 6-5 What differences do you notice between the animal and bacterial viral multiplication cycles? TWO LINE MINIMUM
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6.6 Techniques in Cultivating and Identifying Animal Viruses
Primary purposes of viral cultivation Isolate and identify viruses in clinical specimens To prepare viruses for vaccines To do detailed research on viral structure, multiplication cycles, genetics, and effects on host cells
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6.6 Techniques in Cultivating and Identifying Animal Viruses
Cell culturing Growing a thin layer of cells to do research with Often study viral-host interactions Can detect growth by looking at a plaque
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6.6 Techniques in Cultivating and Identifying Animal Viruses
Using bird embryos Bird embryos contained within an egg Isolated system great for studying viral propagation (“growth”) Inoculation of amniotic cavity Inoculation of embryo Air sac Inoculation of chorioallantoic membrane Amnion Shell Inoculation of yolk sac Allantoic cavity Albumin (b)
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6.7 Medical Importance of Viruses
AIDS The Cold Measles Mumps Rubella Chicken pox/Shingles Small Pox Hepatitis SARS The Flu Ebola HPV Bird Flu Polio Herpes
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6.8 Detection and Treatment of Animal Viral Infections
How to detect a viral infection Symptoms Presence of antigens Amplify viral DNA using PCR (Lyme, HPV) Cell culturing Screening test for specific antibodies (HIV)
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6.8 Detection and Treatment of Animal Viral Infections
How to prevent a viral infection Antiviral drugs – not a lot since viruses aren’t living. Basically change the receptor sites & prevent attachment Vaccines – either inactivated (dead viral particles) or attenuated (weakened or altered viral particles) are injected into organism. Body starts the production of antibodies and memory cells to combat viral invaders when needed. Jonas Salk with live Polio vaccine
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6.8 Detection and Treatment of Animal Viral Infections
How to treat a viral infection Very difficult! Not living No “antibiotics” Viral drugs often are not specific Target host cells too!
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PROCESS BOX 6-6 Compare methods of viral infection detection, prevention, and treatment. THREE LINE MINIMUM
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6.9 Prions and Other Nonviral Infectious Particles
Prions—protinaceous infectious particle Piece of naked protein that can cause infection NO DNA!!! Disease: transmissible spongiform encephalopathies (TSEs) Spread by Direct contact Contaminated food Turns brain/nervous tissue into spongy matter
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6.9 Prions and Other Nonviral Infectious Agents
Examples of TSEs (animal) Scrapie (sheep) Bovine spongiform encephalophathy—(“mad cow disease”) (cattle) Wasting disease (elk, deer, mink) Examples of TSEs (human) Creutzfeldt-Jackob syndrome (CJS)
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6.9 Prions and Other Nonviral Infections Particles
Viroids Very very small (1/10 normal virus size) Naked strands of RNA Only infect plants Tomatoes Potatoes Cucumbers Citrus trees
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PROCESS BOX 6-7 Why do you think prions and viroids are not considered true viruses? TWO LINE MINIMUM
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Chapter Overview Viruses are a unique group of tiny infectious particles that are obligate parasites of cells Viruses do not exhibit the characteristics of life but can regulate the function of host cells They infect all groups of living things and produce a variety of diseases They are not cells but resemble complex molecules composed of protein and nucleic acid They are encased in an outer shell or envelope and contain either DNA or RNA as their genetic material Viruses are genetic parasites that take over the host cell’s metabolism and synthetic machinery
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Chapter Overview Viruses can instruct the cell to manufacture new virus parts and assemble them They are released in a mature, infectious form, followed by destruction of the host cell Viruses may persist in cells, leading to slow progressive diseases and cancer They are identified by structure, host cell, type of nucleic acid, outer coating, and type of disease They are among the most common infectious agents, causing serious medical and agricultural impact
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