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Chapter 19 Viruses (A borrowed life (Capsids and envelopes (The…

- - - - An animal virus equipped with an envelope that is a membrane is outer layer uses it to enter the host cell
      - Protruding from the outer surface of this envelope or viral glycoproteins that bind to specific receptor molecules on the surface of a host cell
      - Ribosomes Bound to the endoplasmic reticulum of the host so make the protein parts of the envelope glycoproteins cellular enzymes in the ER and Golgi apparatus then add sugars
    - - The RNA animal viruses with the most complicated replicative cycles are the retroviruses.
      - These viruses have an enzyme called reverse transcriptase that transcribes an RNA template into DNA and RNA -> DNA information flow that is the opposite of the usual direction
      - Of particular medical importance is HIV the retrovirus that causes AIDS.
      - HIV and other retroviruses are enveloped viruses that contain two identical molecules of single stranded RNA and two molecules of reverse transcriptase
      - The integrated viral DNA called a provirus never leaves the host genome remaining a permanent resident of the cell
    - - Viruses do not really fit our definition of living organisms
      - And isolated virus is biologically in her unable to replicate its jeans or we generate its own ATP
      - Although viruses cannot replicate or carry out metabolic activities independently their use of the genetic code makes it hard to deny their evolutionary connection to the living world
      - Viruses have been found that infect every form of life not just bacteria animals and plants but also archaea fungi and algae and other protists
      - Most molecular biologist favor the hypothesis that viruses originated from naked bits of cellular nuclear acids that moved from one cell to another perhaps via injured cell surfaces
      - The evolution of jeans calling for Said proteins may have a loud viruses to Byam cell membranes those facilitating the infection of uninjured cells
      - Candidates for the original source of viral genome is include plus minutes and transposons
      - Plasmids are small circular DNA molecules found in bacteria in in the unicellular eukaryotes called yeast
      - Transposons are do you need segmented I can move from one location to another within a cells genome
      - Thus plasmids transposons and viruses all share an important feature they are mobile genetic elements