Chapter 13 MICRO

25 July 2022
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Virus
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Cause many infections of humans, animals, plants, and bacteria. Consists of nucleic acid molecule in a protein coat and is able to multiply only with in the living cells of a host.
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How does a virus differ from a eukaryotic cell?
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Viruses lack a cell structure, they cannot exist independently from a host cell, and they can infect both prokaryote and eukaryote organisms. Eukaryotic cells have a nucleus and organelle.
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Virion
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Complete, infective form of a virus outside a host cell
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What does a virion contain?
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A core of RNA or DNA and a capsid (protein coat)
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What is an enveloped virion?
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A virus with a membrane
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What does an enveloped virion contain?
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Phospholipid bilayer and proteins
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How does a virus appear in the intracellular state?
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The capsid is removed, a virus without its capsid exists solely as nucleic acid but is still referred to as a virus
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Of what does the genome of a virus consist?
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DNA or RNA
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How are viral genomes described?
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Double-stranded DNA, single-stranded DNA, single-standed RNA, and double-stranded RNA
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Identify the range of genes found in viruses
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3 genes to 4000 genes
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What gives viruses specifity?
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The precise affinity of viral surface proteins or glycoproteins for complementary proteins or glycoproteins on the surface of host cell
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What kind of living things can viruses infect?
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Archaea, bacterial, plant, protozoan, fungal, and animal cells
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What is bacteriophage?
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A virus that infects bacteria
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Can viruses be seen through a light microscope?
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No, they are too small
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Describe the composition of a capsid.
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Proteinaceous subunits called capsomeres
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What is the function of a capsid?
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Protection for viral nucleic acid and means of attachment to hosts cells
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Describe the 3 basic capsid shapes.
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Helical - capsomeres that bond together in a serial fashion to form a tube round the nucleic acid Polyhedral - roughly spherical, with a shape similar to a geodesic dome Complex - have capsids of many different shaped that do not readily fit into either of the other two categories
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Describe the shape of a bacteriophage
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Icosahedral heads, which contain the genome, attached to helical tails with tail fibers
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How does a naked virion compare to an enveloped virion?
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A naked virion lacks cell membranes, but an enveloped virion has a cell membrane surrounding their capsids.
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What is the envelope composed of?
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Phospholipid bilayer and proteins; some proteins are virally-coded glycoproteins (spikes)
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How are viruses classified?
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Family than genus
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Contrast viruses to cells. (Table 13.1)
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Viruses - inert macromolecules outside of a cell but become active inside a cell, do not divide or grow, acellular, obligate intracellular parasites, contain either DNA or RNA, with few exceptions, genome can be dsDNA, ssDNA, dsRNA, ssRNA, usually ultramicroscopic in size, have a proteinaceous capsid around genome; some have an envelope around capsid, and replicate in an assembly-line manner using the enzymes and organelles of a host Cells - metabolize on their own, divide and grow, cellular, most are free living, contain both DNA and RNA, genome is dsDNA, surrounded by a phospholipid membrane and often a cell wall, and self replicating by asexual and or sexual means
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Why are viruses not capable of reproduction on their own?
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They lack genes for all the enzymes necessary for replication, nor do they possess functional ribosomes for protein synthesis
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What must viruses rely on for reproduction?
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Their hosts enzymes and organelles to produce new visions
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What are bacteriophages?
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Virus that infects bacteria
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Describe the structure of a bacteriophage.
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Composed of a protein coat surrounding DNA or RNA
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Describe the 5 steps that occur during lytic replication of bacteriophages (Figure 13.8)
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1. Attachment - proteins on tail fibers attach to host cell 2. Entry - nucleic acid is "injected" into the host cell 3. Synthesis - all different parts of the bacteriophage are made proteins and tail fibers 4. Assembly - all parts are put together 5. Release - new phages burst out of cell, which destroys the cell
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Lysogeny
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Dormant stage, not actively making new viruses, modified replication cycle in which infected host cells grow and reproduce normally for many generation before they lyse.
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Temperate phage
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Bacteriophage that does not immediately kill its host
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Prophage
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An inactive bacteriophage, which is inserted into a hosts chromosome.
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Induction
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In virology, excision of a prophage from the host chromosome, at which point the prophage reenters the lytic phase
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Describe the steps that occur during lysogenic replication of bacteriophage (Figure 13.11)
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1. Attachment - proteins on tail fibers attach to host cells 2. Entry - Nucleic acid in "injected" into host cell 3. Prophage - viral DNA incorporated into host DNA in chromosome 4. Replication - of chromosome and cell division 5. Induction - a stress or trigger that will begin lytic cycle
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Describe the events that occur after induction.
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The lytic steps of synthesis, assembly, and release resume from the point at which they stopped.
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Identify the several inductive agents.
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Ultraviolet light, X rays, and carcinogenic chemicals
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Describe the 5 steps that occur during the replication of animal viruses?
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1. Attachment - dependent on the chemical attraction and exact fit between proteins or glycoproteins on the virion and complementary protein or glycoprotein receptors 2. Entry - three different mechanisms: direct penetration, membrane fusion, and endocytosis 3. Synthesis - different strategy for DNA or RNA and whether single-stranded, double-stranded 4. Assembly 5. Release
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How is release different from naked and enveloped viruses?
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Naked viruses - released one of two way either they may be extruded from the cell by exocytosis or they may cause lysis and death of the cell Enveloped viruses - released via budding
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Which release method results in persistent infection? Why?
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Enveloped viruses because the host cell shed viruses slowly and relatively steadily
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What are latent viruses?
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An animal virus that remains inactive for a long period of time before becoming active
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Identify some latent viruses
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HIV, chicken pox, and herpes virus
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How long can latency last?
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Prolonged for years with no viral activity (signs or symptoms)
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Contrast bacteriophage and animal virus replication (Table 13.4)
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Bacteriophage - proteins on tails attach to proteins on cell wall, genome is injected into cell or diffuses into cell, site of synthesis and assembly is in cytoplasm, mechanism of release is lysis, and nature of chronic infection is lysogen always incorporated into host chromosome, may leave host chromosome Animal virus - attachment spikes, capsids, or envelope proteins attach to proteins or glycoproteins on cell membrane, capsid enters cell by direct penetration, fusion, or endocytosis, removal of capsid by cell enzymes, site of synthesis and assembly is RNA viruses in cytoplasm and DNA viruses in nucleus, mechanism of release is naked visions; exocytosis or lysis, enveloped visions; budding, latency with or without incorporation into host DNA; incorporation is permanent.
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Briefly describe the theories related to the role of viruses in cancer.
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As long as protooncogenes (genes that play a role in cell division) are repressed, no cancer results. but when active cancer can develop, several genetic changes muct occur before cancer develops
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What are prions?
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Proteinaceous infective particles
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Identify several prion diseases
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Bovine spongiform encephalitis (mad cow disease), spare in sheep, kuru (human disease that has been eliminated), chronic wasting disease in deer and elk and variant creutzfeldt-jakob disease in humans
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How are prions destroyed?
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By incineration or by autoclaving in concentrated sodium hydroxide
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If viruses are classified as living things, identify 3 observations that indicate that they are complex living entities.
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Viruses use sophisticated methods to invade cells, have the means of taking control of their host cells, and posses genomes containing instructions for replicating themselves