Chapter 6 Introduction to Viruses PDF

Chapter 6 Introduction to Viruses Topics Background Structure Classification Multiplication Cultivation and replication Nonviral infectious agents Treatment Introduction and History History 1884 Pasteur -developed first vaccine for rabies -proposed the term “virus” to denote this special group 18...

Chapter 6 Introduction to Viruses Topics Background Structure Classification Multiplication Cultivation and replication Nonviral infectious agents Treatment Introduction and History History 1884 Pasteur -developed first vaccine for rabies -proposed the term “virus” to denote this special group 1892 Iwanowski & 1898 Beijerinck - Discovered the fluids from infected tobacco plants could be filtered off all cells, but still remained contagious. Something smaller than bacteria was causing the tobacco mosaic disease. -“Contagium vividium fluidium” - “Filterable virus” - “Virus” Latin for “poison” (virion) 1935 – Biochemist Wendell Stanley purified and crystallized tobacco mosaic virus. 1940’s – With the advent of the electron microscope we could finally see viral shapes and sizes. Characteristics of Viruses • Virus – Minuscule, acellular infectious agent – Causes many infections of humans, animals, plants, and bacteria – Causes most of the diseases that plague the industrialized world © 2012 Pearson Education Inc. Viruses: Alive or not? Viruses have living & nonliving characters Alive Nucleic acids Reproduction Respond to stimuli Evolve Nonliving DNA or RNA, not both Obligate intracellular parasite Can be crystallized No growth or metabolism No cell structure Structure • • • • • Size and morphology Capsid Envelope Complex Nucleic acid Structure of Viruses Size range -smallest infectious agents -ultramicroscopic (<0.2 micrometers) an electron microscope is necessary to examine them. - 2,000 bacteriophages could fit inside an average bacterial cell. 50 million poliovirus could fit in an average human cell. Figure 13.4 Sizes of selected virions E. coli (bacterium) (1000 nm  3000 nm) Red blood cell (10,000 nm in diameter) Bacterial ribosomes (25 nm) Poliovirus (30 nm) Bacteriophage MS2 (24 nm) Smallpox virus (200 nm  300 nm) Bacteriophage T4 (50 nm  225 nm) Tobacco mosaic virus (15 nm  300 nm) Viral structures - no cell structure so what is it? - 3 possible pieces to viruses 1. Nucleic acid core – one or many strands of DNA or RNA but not both 2. Capsid – protective outer shell made of protein subunits called capsomeres 3. Envelope – optional, not all viruses have this. Usually a modified piece of the hosts cell membrane. There are two major structures of viruses called the naked nucleocapsid virus and the enveloped virus. Fig. 6.4 Generalized structure of viruses Nucleic acid • Unlike cells, viruses contain either DNA or RNA, but not both • Possess only the genes to invade and regulate the metabolic activity of host cells • Ex. Hepatitis B (4 genes) and herpesviruses (100 genes) • No viral metabolic genes, as the virus uses the host’s metabolic resources Nucleic acid core - Viral Nucleic acids must contain instructions for at least 4 things 1. Genes for regulating the actions of the host 2. Genes for synthesizing the viral genetic material 3. Genes for synthesizing the viral capsid and proteins 4. Genes for assembling and packaging the mature virus. -Viruses only contain DNA or RNA but not both. -Viruses may contain dsDNA*, ssDNA, dsRNA, and ssRNA*. -Gene numbers range from 4 in hepatitis B to hundreds in herpesviruses. E. coli has 4,000 and humans have 30-40,000. -Some viruses will have a few enzymes like polymerases, or in the case of HIV reverse transcriptase for making DNA from RNA. Some viruses will steal items from the host cell like ribosomes or tRNA. Figure 13.2 The relative sizes of genomes Partial genome of E. coli Viral genome Characteristics of Viruses • Viral Shapes – Three basic shapes • Helical • Polyhedral • Complex © 2012 Pearson Education Inc. Capsid • Protective outer shell that surrounds viral nucleic acid • Capsid spikes • Composed of capsomer subunits • Two types of capsids – Helical – Icosahedral Capsidprotein coat surrounding nucleic acid core, made of identical subunits called capsomeres, that spontaneously self-assemble Multiple shapes – either Icosahedral or Helical - Icosahedron, 3 dimensional, 20 sided, with 12 evenly spaced corners Icosahedron capsid • Three-dimensional, 20-sided with 12 evenly spaced corners • Variation in capsomer number – Polio virus 32 capsomers – Adenovirus 240 capsomers Helical capsid • Naked helical virus – Ex. Tobacco mosaic virus – Nucleocapsid is rigid and tightly wound into a cylinder-shaped package • Enveloped helical virus – Ex. Influenza, measles, rabies – Nucleocapsid is more flexible Helical tubular rod shaped capsomeres, bonded to form a series of hollow discs, then bound together to form a tube that resembles a bracelet. Complex shape, a more intricate or varied structure Poxviruses lack a capsid and instead has layers of lipoproteins and coarse surface fibers Bacteriophages – combination of icosahedral and helical plus other pieces Figure 13.6 Bacteriophage T4-overview Enveloped vs. Naked viruses - envelope usually made from the host’s cell membrane. - Many or all of the regular proteins are substituted with viral proteins. - Spikes are necessary for recognition and attachment to the next host cell. Figure 13.7 Enveloped virion-overview Icosahedral viruses can be naked or enveloped. Fig. 6.8 Two types of icosahedral viruses Envelope • Lipid and proteins • Envelope spikes • During release of animal viruses, a part of the host membrane is taken • Enable pleomorphic shape of the virus – Spherical – filamentous Function of the capsid/envelope Protection The outer layer protects the virion (nucleic acid) from enzymes, and chemicals outside of the host cell. Example; intestinal viruses are resistant to acids and digestive enzymes in the GI tract. Penetration of host cell These outer layers bind to the host and assist in the insertion of the viral nucleic acid. Activation of Immune system Portions of the capsids and envelopes stimulate the immune system and produce antibodies for future infections. Complex viruses • Structure is more intricate than helical and icosahedral viruses • Pox virus – Several layers of lipoproteins – Course surface fibrils • Bacteriophage – Polyhedral head – Helical tail – Fibers for attachment Comparison of the morphology (helical, icosahedral, complex) of a naked virus, enveloped virus and a complex virus. Fig. 6.9 Morphology of viruses Examples of medically important DNA viruses. Examples of medically important RNA viruses. Classification • • • • • Structure Chemical composition Genetic makeup Host relationship Type of disease Three orders of viruses have been developed for classification. Table 6.4 Examples from the three orders of viruses. Classification of important human viruses. Table 6.5 Important human virus families, genera, common names, and types of viruses. Bacteriophages - Excellent example of virus multiplication cycle - Bacteriophages are viruses that attack bacteria. - Discovered in 1915 - Culturing bacteriophages on a lawn of bacteria they form clear plaques -example of T-even phage that infect E. Coli - Reproduction of bacteriophages can occur in two different cycles. Viral Replication • Dependent on hosts’ organelles and enzymes to produce new virions • Lytic replication – Replication cycle usually results in death and lysis of host cell • Stages of lytic replication cycle – Attachment – Entry – Synthesis – Assembly – Release © 2012 Pearson Education Inc. Lytic Cycle 1. Adsorption - Virus engages host cell by chance collision. Viruses do not move on their own. 2. Penetration – chemical interaction causes the contraction of the tail and it injects DNA into bacterial cell. Leaves ghost phage behind. 3. Synthesis – the bacterial cell replicating viral DNA. Protein synthesis is taking place making capsids, proteins, and nuclease. *Nuclease destroys bacterial DNA so host has only viral DNA. 4. Assembly and maturation – viral parts come together, chemical reactions bring and glue pieces together. 5. Release by lysis – Lysozyme is used to lyse the host bacterial cells, this allows viruses to escape. We also have lysozyme in our own mucous secretions it helps keep us clear of bacterial infection. Burst time – average 20-40 min. to go from penetration to release. Burst size – 50-200 viruses per cell burst. Lytic cycle Viral Replication • Lysogeny – Modified replication cycle – Infected host cells grow and reproduce normally for generations before they lyse – Temperate phages • Prophages – inactive phages – Lysogenic conversion results when phages carry genes that alter phenotype of a bacterium © 2012 Pearson Education Inc. Lysogenic Cycle - will not kill host immediately and results in viral persistence - performed by temporate phages 1. Adsorption –Virus engages host cell by chance collision. Viruses do not move on their own. 2. Penetration – chemical interaction causes the contraction of the tail and it injects DNA into bacterial cell. Leaves ghost phage behind. 3. Lysogenic conversion – the bacteriophage DNA is integrated into the bacterial DNA. As the cell goes through binary fission, the bacteriophage DNA is copied with it. 4.Excision – Due to environmental stimulus the prophage comes out of bacterial DNA, takes over the cell, and the lytic phase of the cycle begins. 5. Synthesis 6. Assembly 7. Release by lysis. Figure 13.17 Viral plaques in a lawn of bacterial growth on the surface of an agar plate Bacterial lawn Viral plaques Lysogenic Cycle Lysogenized conversion – bacterial cell has been changed because Viral DNA has been added to the bacterial DNA - bacterium acquires new traits from its temporate phage 1. Streptococcus pyogenes – If it’s lysogenized ‘strep’ it can result in scarlet fever. 2. Corynebacterium diptheriae – Lysogenized bacteria will result in diptheria, non lysogenized, it’s harmless. Animal viruses Bacteriophages are a nice general model for the workings of viruses however animal viruses do a few things differently. A. Host range B. Method of penetration C. How/where the nucleic acid works in the host D. Method of leaving the host cell. Host range • An exact fit between virus spikes and cell receptors is needed in order for adsorption to occur. • this means the number of cells it can infect is limited, this is known as host range. – Spectrum: hepatitis B infects only human liver cells, Poliovirues infects primate intestinal and nerve cells, and rabies virus infects various cells in all mammals. Method of penetration • In bacteriophages, we have ghost phages left behind after penetration of nucleic acids. • Animal viruses can either enter the cell as the whole virion through endocytosis, or it only introduces its nucleic acid by membrane fusion. – In penetration by endocytosis the whole virus is engulfed by the host cell. – It is enclosed in a vacuole – Enzymes in the vacuole dissolves the envelope and capsid, this is called uncoating. • Entry by membrane fusion involves the virus simply merging with the host cells membrane and uncoating at the same time. Uncoating step Host cell membrane Virus in vesicle (a) Specific attachment Engulfment Vesicle, envelope and capsid break down Host cell membrane Receptors Free RNA Uncoating of nucleic acid RNA Spike Envelope Irreversible attachment (b) Free DNA Receptorspike complex Membrane fusion Entry of nucleocapsid How/where the nucleic acid is working Once the viral nucleic acid gets into the host cell depending on what type of nucleic acid it is, it will begin work in either the nucleus or the cytoplasm. -DNA viruses enter the host cell nucleus and are replicated and assembled there -RNA viruses are replicated and assembled in the cytoplasm. Some have positive sense strands and some have negative sense strands. -Retroviruses are a little different Uncoating and synthesis of viruses rely on the host’s metabolic systems. Fig. 6.11 General feature in the multiplication cycle of an enveloped animal virus. Different forms of viral nucleic acids arrive in the cell and accomplish their tasks by different methods!!! Method of leaving the host cell • While bacteriophages generally always lyse the bacterial cell they infect, animal viruses, are not so predictable. - Non-enveloped and complex viruses will lyse or rupture the host cell. - Enveloped viruses are released by budding, also known as exocytosis (or budding). • The process of exocytosis begins with the nucleocapsid binds to the membrane • The membrane forms a pouch and then that pouch pinches off the host cell and forms the envelope. • This is a gradual process of shedding viruses. Figure 13.14 The process of budding in enveloped viruses Enveloped virion Budding of enveloped virus Viral glycoproteins Viral capsid Cytoplasmic membrane of host After effects of viral infection • No matter how the virus leaves the cell, irreversible damage has occurred and is ultimately lethal due to damage to metabolism, organelles, and cell membrane. • Burst size for animal cells range from 3-4,000 for pox viruses to 100,000+ for polioviruses Cytopathic effects • Damage to the host cell due to a viral infection – Inclusion bodies – Syncytia – Chronic latent state – Transformation Examples of syncytia and inclusion bodies. Fig. 6.16 Cytopathic changes in cells and cell cultures infected by viruses Latency • Some cells maintain a carrier relationship with the infecting virus – The cell harbors the virus and is not immediately lysed, these are considered persistent infections and can last from a few weeks to lifelong. – Several viruses remain in a chronic latent state, meaning they are inactive over long periods of time. Herpes simplex virus and herpes zoster virus go into latency in nervous tissue and reappear due to stimuli. When animal viruses remain dormant in host cells – Some latent viruses do not become incorporated into host chromosome – Incorporation of provirus into host DNA is permanent Oncoviruses • Some animal viruses permanently alter that cells genetic material, leading to cancer. • The effect on those cells is called transformation, the viral nucleic acid is consolidated into the host DNA • Transformed cells have increased growth, chromosomal alteration, cell surface alteration, and indefinite cell division. Other noncellular infectious agents Prions - small protein fibrils found deposited in the brain tissue of animals infected with spongiform encephalopathies. - Prions are composed primarily of protein (no nucelic acids). Thought that proteins meet up with existing prions and “shape flip” to match. Diseases Caused By Prions 1) Creutzfeld-Jakob Disease - Affects the nervous system, gradual degeneration and death. - Transmissible but mechanism unknown. 2) Bovine spongiform encephalopathy, or “mad cow disease” – Can be acquired by consumption of contaminated beef – First incidence of prion disease transmission from animal to human. With Both Diseases: - Infections have a long period of latency (several years). - Signs include mental derangement, loss of muscle control. Diseases are progressive and universally fatal. Satellite viruses Viruses that are dependent on other viruses for replication 1. AAV, adeno associated virus. Replicates only in cells infected with adenovirus. 2. Delta agent, a naked strand of RNA expressed only in the presence of hepatitis B virus. Treatment of viral infections • The uncommon nature of viruses make them difficult to deal with • Many antiviral drugs block viral replication by blocking host cell function. Leads to severe side effects. – Azidothymide (AZT) targets the synthesis stage of the life cycle. – Protease inhibitors interrupt the assembly phase. Antiviral drugs • Selective toxicity is almost impossible to achieve with viruses. Why? • Three major modes of viral drug actions are: 1. Halting penetration of virus into host cell 2. Blocking transcription and/or translation 3. Preventing assembly, or maturation of viral pieces • Antiviral drugs protect uninfected cells, but don’t do much for extracellular viruses or latent viruses Interferon • Natural method of virus control is interferon. • At first limited by the limited quantities acquired from blood. Now produced by recombinant DNA technology. • Interferon benefits include: – Reducing time in healing and complications of certain infections – Preventing/reducing symptoms of cold and papillomaviruses – Slowing certain cancers, bone, cervical, certain leukemias and lymphomas – Treating hepatitis C, genital warts, Kaposi’s sarcoma, and a rare cancer; hairy-cell leukemia