7.3: Viruses
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- Ying Liu
- City College of San Francisco
A virus is a small infectious agent that replicates only inside the living cells of other organisms. Viruses can infect all types of life forms, from animals and plants to microorganisms, including bacteria and archaea.
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- 7.3.1: General Characteristics of Viruses
- Viruses are infectious agents with both living and nonliving characteristics. Living characteristics of viruses include the ability to reproduce – but only in living host cells – and the ability to mutate. Nonliving characteristics include the fact that they are not cells, have no cytoplasm or cellular organelles, and carry out no metabolism on their own and therefore must replicate using the host cell's metabolic machinery.
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- 7.3.2: Size and Shapes of Viruses
- Viruses are usually much smaller than bacteria with the vast majority being submicroscopic, generally ranging in size from 5 to 300 nanometers (nm). Helical viruses consist of nucleic acid surrounded by a hollow protein cylinder or capsid and possessing a helical structure. Polyhedral viruses consist of nucleic acid surrounded by a polyhedral (many-sided) shell or capsid, usually in the form of an icosahedron.
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- 7.3.3: Viral Structure
- Since viruses are not cells, they are structurally much simpler than bacteria. An intact infectious viral particle - or virion - consists of a genome, a capsid, and maybe an envelope. Viruses possess either DNA or RNA as their genome. The genome is typically surrounded by a protein shell called a capsid composed of protein subunits called capsomeres.
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- 7.3.4: Classification of Viruses
- Viruses can store their genetic information in six different types of nucleic acid which are named based on how that nucleic acid eventually becomes transcribed to the viral mRNA. (+) and (-) strands of nucleic acid are complementary. Copying a (+) stand gives a (-) strand; copying a (-) stand gives a (+) strand. Only (+) strands of viral RNA can be translated into viral protein. The "dependent" part of the name refers to the nucleic acid is being copied.
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- 7.3.5: Other Acellular Infectious Agents- Viroids and Prions
- Viroids are small, circular, single-stranded molecules of infectious RNA that cause several plant diseases. Prions are infectious protein particles responsible for a group of transmissible and/or inherited neurodegenerative diseases as a result of prion protein misfolding. Diseases including Creutzfeldt-Jakob disease Gerstmann-Straussler-syndrome, and mad cow disease.
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- 7.3.6: Animal Virus Life Cycles
- Viruses that infect animal cells replicate by what is called the productive life cycle. The productive life cycle is also often referred to as the lytic life cycle, even though not all viruses cause lysis of their host cell during their replication. Some viruses, such as HIV and the herpes viruses are able to become latent in certain cell types. A few viruses increase the risk of certain cancers. We will now look at the life cycles of viruses that infect animal cells.
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- 7.3.7: Bacteriophage Life Cycles- An Overview
- bacteriophages are viruses that only infect bacteria (also see Fig. 1C and Fig. 2E). There are two primary types of bacteriophages: lytic bacteriophages and temperate bacteriophages. Bacteriophages that replicate through the lytic life cycle are called lytic bacteriophages, and are so named because they lyse the host bacterium as a normal part of their life cycle. Bacteriophages capable of a lysogenic life cycle are termed temperate phages.
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- 7.3.8: Pathogenicity of Animal Viruses
- Alteration of host cell function and/or death of the host cell occurs as a result of viruses using an infected host cell as a factory for manufacturing viruses. The body’s immune defenses recognize infected host cells as foreign and destroy infected cells. The body’s adaptive immune defenses produce antibodies against viruses that block viral adsorption to host cells or result in opsonization of the virus.
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- 7.3.9: Bacteriophage-Induced Alterations of Bacteria
- Lytic bacteriophages usually cause the host bacterium to lyse. The added genetic information provided by the DNA of a prophage may enable a bacterium to possess new genetic traits. Some bacteria become virulent only when infected themselves with a specific temperate bacteriophage. The added genetic information of the prophage allows for coding of protein exotoxin or other virulence factors.
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- 7.3.10: Antiviral Agents
- Relatively few antiviral chemotherapeutic agents are currently available and they are only somewhat effective against just a few limited viruses. Many antiviral agents resemble normal DNA nucleosides molecules and work by inhibiting viral DNA synthesis. Some antiviral agents are protease inhibitors that bind to a viral protease and prevent it from cleaving the long polyprotein from polycistronic genes into proteins essential to viral structure and function.
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- 7.3.11: General Categories of Viral Infections
- Acute infections are of relatively short duration with rapid recovery. Persistent infections are where the viruses are continually present in the body. In a latent viral infection the virus remains in equilibrium with the host for long periods of time before symptoms again appear, but the actual viruses cannot be detected until reactivation of the disease occurs. In a chronic virus infection, the virus can be demonstrated in the body at all times and the disease may be present or absent.
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- 7.3.E: Viruses (Exercises)
- These are homework exercises to accompany Kaiser's "Microbiology" TextMap. Microbiology is the study of microorganisms, which are defined as any microscopic organism that comprises either a single cell (unicellular), cell clusters or no cell at all (acellular). This includes eukaryotes, such as fungi and protists, and prokaryotes. Viruses and prions, though not strictly classed as living organisms, are also studied.