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VIRUSES (Replication Only in Host Cells (general features of viral…

- - - - hence why they need a host cell
        
        viruses can infect cells of only a limited number of host species
        
        which is called the host range of a virus
        
        broad host ranges
        
        for example,
        
        west nile virus
        
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        can infect a multitude of species
        
        narrow host ranges
        
        some viruses can only infect one species
        
        viral infections of multicellular eukaryotes
        
        are limited to particular tissues
  - - - lead to the discovery that some double-stranded DNA viruses could replicate by two alternative mechanisms: lytic and lysogenic
        
        lytic cycle
        
        a phage replicative cycle that culminates in the death of a host cell
        
        it refers as the last stage of infection
        
        during which the bacterium lyses (breaks open)
        
        and releases the phages that were produced within the cell
        
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        lysogenic cycle
        
        allows replication of the phage genome without destroying the host
        
        most of the phages coexist with them in a state of lysogeny
        
        temperate phages
        
        also known as lambda (λ)
        
        are phages that are capable of using both modes of replication within a bacterium
        
        for example,
        
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        within the host,
        
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  - - - the the viral genome makes it way inside
        
        the mechanism of genome entry depends on the type of virus and the type of host cell
        
        T-even phages
        
        use their elaborate tail apparatus to inject DNA into a bacterium
        
        other viruses
        
        taken up by endocytosis, or in the case of envelope viruses, by fusion of the viral envelope with the host's plasma membrane
        
        once the viral genome is inside
        
        the proteins it encodes can commandeer the host
        
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        DNA viruses
        
        use DNA polymerases of the host cell to synthesize new genomes along the templates provided by the viral DNA
        
        to replicate the genomes
        
        RNA viruses use virally encoded RNA polymerases that can use RNA as a template
        
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  - - - another variable is the presence or absence of a membranous envelope
        
        nearly all animal viruses with RNA genomes have an envelope
        
        same with DNA genomes
        
        viral envelopes
        
        membranous outer layer
        
        used to enter a host cell
        
        glycoproteins
        
        are on the outer surface of the envelopes
        
        transported to the surface of the host's outer surface
        
        new viral capsids are wrapped in membrane
        
        as they bud from the cell
        
        usually derived from the cell's plasma membrane
        
        the enveloped viruses are now free to infect other cells
    - - is the single or double-stranded DNA or RNA
        
        RNA genomes
        
        are in some phages and most plant viruses
        
        the broadest variety are in animal viruses
        
        three types of single-stranded RNA genomes of animal viruses
        
        the genome of class IV viruses
        
        can serve as mRNA
        
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        types of class IV
        
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        retroviruses (class VI)
        
        the most complicated replicative cycles
        
        they can have an enzyme called reverse transcriptase
        
        which transcribes an RNA template into DNA
        
        DNA genomes
- - - - viruses can damage or kill cells by causing the release of hydrolytic enzymes from lysosomes
        
        some infect cells to produce toxins that lead to disease
    - - poliovirus
        
        the damage it inflicts to mature nerve cells
        
        cannot be repaired
        
        since the cells do not divide
        
        and cannot be replaced
      - temporary symptoms
        
        such as fever and body aches
        
        is a result from the body's own efforts to defend itself against infection
        
        instead of dying off from the virus
      - immune system
        
        a critical part of the body's natural defenses
        
        vaccine
        
        a harmless derivative of a pathogen
        
        that stimulates the immune system
        
        to mount defenses against the harmful pathogen
        
        antibiotics
        
        helps people recover from bacterial infections
        
        but are powerless against viruses
        
        by killing bacteria from the inhibited enzymes specific to bacteria
        
        but has no effect on eukaryotic or virally encoded enzymes
        
        antiviral drugs
        
        resembles nucleosides
        
        and interferes with viral nucleic acid synthesis
  - - - appeared seemingly out of nowhere
        
        and spread quickly
    - - appeared in North America in 1999
        
        and spread to all 48 contiguous states
        
        resulted in 40,000 cases
        
        and 2,000 deaths
    - - widespread outbreak
    - - a global epidemic
  - - - and accounted for an estimated annual loss of $15 billion worldwide
        
        due to their destruction of agriculture and horticultural crops
    - - have the same basic structure
        
        and mode of replication as animal viruses
        
        has RNA genomes
        
        either helical capsids
        
        or icosahedral capsids
      - spreads through two routes
        
        vertical transmission
        
        a plant inherits a viral infection from a parent
        
        occurs in asexual propagation
        
        or in sexual reproduction via infected seeds
        
        horizontal transmission
        
        plant is infected through external source
        
        the invading virus must get past the plant's outer protective layer of cells
  - - - are proteins that cause a number of degenerative brain diseases in various animal species
      - characteristics
        
        first, it acts slowly
        
        with an incubation period of at least ten years before symptoms develop
        
        prevents infections from being identified
        
        which allows many more infections to occur
        
        secondly, prions are virtually indestructible
        
        are not destroyed or deactivated by heating to normal cooking temperatures
      - created from misfolded forms of proteins
- - - - genomes could consist of
        
        double-stranded DNA
        
        single-stranded DNA
        
        double-stranded RNA
        
        single-stranded RNA
      - nucleic acid determines what type of virus
        
        organized as a single linear or circular molecule of nucleic acid
        
        although, the genomes of some viruses consist of multiple molecules of nucleic acids
        
        smallest viruses
        
        three genes in gemones
        
        largest viruses
        
        several hundred to 2,000 genes
    - - capsids
        
        the protein shell that encloses the viral genome
        
        built from a large number of proteins called capsomeres
        
        types
        
        rod-shape
        
        known as helical viruses
        
        due to the protein being arranged in a helix
        
        polyhedral
        
        exists in icosahedral viruses
        
        which gets its name from an icosahedron
        
        an polyhedral capsid with 20 triangular facets
        
        more complex shapes
        
        could be found in
        
        bacteriophages, or simply phages
        
        which infects bacteria
        
        capsids has elongated icosahedral heads surrounding their DNA
        
        attached to the head is a protein tail piece with fibers
        
        which enables the phages to attach to a bacterial cell
      - viral envelopes
        
        are what enables viruses to infect host cells
        
        derives from the membranes of the host cell
        
        and contains
        
        host cell phospholipids and membranes proteins
        
        proteins and glycoproteins of viral origin
        
        glycoproteins
        
        are proteins with carbohydrates covalently attached
        
        some viruses carry viral enzymes molecules with their capsids
  - - - a german scientist
        
        in 1883
        
        discovered the ability to transmit tobacco mosaic disease from plant to plant
        
        he found this out by
        
        rubbing the sap extract from diseased leaves onto healthy ones
        
        he tried to search for infectious microbe in the sap
        
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    - - tested Adolf Mayer's hypothesis
        
        passed the sap through a filter designed to remove bacteria
        
        the sap still produced mosaic disease
        
        clung onto hypothesis that bacteria caused tobacco mosaic disease
        
        he reasoned that
        
        bacteria were small enough to pass through the filter or made a toxin that could do so
    - - a dutch botanist
        
        in future experiments,
        
        he showed the mysterious agent of mosaic disease could not be cultivated on nutrient media in test tubes or petri dishes
        
        imagined a replicating particle much smaller and simpler than a bacterium
        
        credited for
        
        the first scientist to voice the concept of a virus
        
        ruled out Ivanosky's reasoning of the disease being made out of a toxin
        
        he done this by
        
        doing classic series of experiments that showed the infectious agents in the filtered sap could replicate
    - - american scientist
        
        in 1935, he confirmed Beijerinck's suspscious
        
        he crystallized the infectious particle, now known as tobacco mosaic virus (TMV)
        
        which could be seen with an electron microscope