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An Introduction to Viruses. Chapter 6. 6.1 The Search for elusive viruses. Leeuwenhoek’s microscope was extremely important in discovering small life forms But there was a limit to magnification
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An Introduction to Viruses Chapter 6
6.1 The Search for elusive viruses • Leeuwenhoek’s microscope was extremely important in discovering small life forms • 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
6.1 The Search for Elusive Viruses • 1890s—first virus discovered • D. Ivanovski and M. Beijerinck showed that a disease in tobacco plants was caused by a virus • Tobacco mosaic virus
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 • By 1950s, much more was known about viruses
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
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!
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—entity that cannot multiply unless it invades a specific host cell and instructs its genetic and metabolic machinery to make and release quantities of new viruses
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
6.3 The General Structure of Viruses • Viral Components • Viruses are made up of many repeating units and can be purified into crystals • Further proof that they are non-living
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
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
6.3 The General Structure of Viruses • The Viral Capsid • Helical capsids are more simple • Naked Helical Ex: Tobacco mosaic virus • Enveloped Helical Ex: influenza, measles, rabies
6.3 The General Structure of Viruses • The Viral Capsid • Icosahedron capsule—3D, 20-sided figure • Naked ex: rotavirus • Enveloped ex: herpes simplex
6.3 The General Structure of Viruses • Nucleic Acids: At the Core of a Viruses • Viruses contain either DNA or RNA, but not both • Unusual nucleic acids • Single-stranded DNA (parvoviruses) • Double-stranded RNA (reoviruses)
6.5 Modes of Viral Replication • Multiplication Cycles in Animal Viruses • General phases are • Adsorption • Penetration • Uncoating • Synthesis • Assembly • Release
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 Lysisof cell & release of virions ~ 200/cell
6.5 Modes of Viral Multiplication • Adsorption: The virus attaches to the host cell by specific binding of the spikes to cell receptors • Viruses are both host-specific and even cell-type specific • EX: Hepatitis B only infects human liver cells • EX: polio virus infects intestinal and nerve cells of primates
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**
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
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
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
Lytic Cycle Release/Lysis Attachment at Receptor site Entry/Penetration Assembly/Maturation Replication
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 And Another More! And…the last one
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
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 leaksthe 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
6.5 Modes of Viral Multiplication • Some viruses change host DNA (can lead to cancer) • Transduction is process of DNA change
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.
Generalized Transduction Episome
6.5 Modes of Viral Multiplication • Oncovirus– virus that can cause tumors to form • EX: HPV leads to cervical cancer • EX: Epstein-Barr viruses causes Burkitt’slymphoma • Tumor—a swelling that results from abnormal cell growth • Benign – causing no harm. Can be removed • Malignant – Metastasizes and spreads. Cancer • Cells are transformed by virus
6.5 Modes of Viral Multiplication • Bacteriophage multiplcation 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
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
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
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
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”)
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
How to detect a viral infection Symptoms Presence of antigens Amplify viral DNA using PCR Cell culturing Screening test (HIV) 6.8 Detection and Treatment of Animal Viral Infections
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
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!
