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ch 19 (19.1 (Bacterial genomes contain about 200 to a few thousand genes…

- - - - In 1883, Adolf Mayer discovered that he could transmit the spot disease from plant to plant by rubbing sap extracted from diseased leaves onto healthy plants
        
        Mayer's then suggested after an unsuccessful search for a infectious microbe in the sap, that the disease was caused by unusually small bacteria that were invisible underneath a microscope
        
        This hypothesis was later tested by Dimitri Ivanowsky who passed sap from infected tobacco leaves through a filter to remove bacteria. Results found that the sap still produced mosaic disease - spot disease
    - - The first phages studied included seven that infect Escherichia coli
      - They were names type 1 (T1) and so forth in the order that they were discovered
        
        T2, T4, and T6 (or the three "T-even" were very similar in structure as their capsids have elongated icosahedral heads that enclose their DNA.
    - - The viral envelopes contain host cell phosophilipds and membrane proteins that also contain proteins and glycoproteins of viral origin
- - - - Viruses replicate only in host cells
        
        The West Nile and equine encephalitis virus is an example of a virus with a broad host range
        
        The measles virus is an example of a virus with a host range so narrow that it can infect only humans
        
        A broad host range would mean that the virus would be able to infect many different types of organisms and wouldn't be limited to only animals or only humans
        
        Viruses are obligate intracellular parasites that can only replicate only within a host cell
        
        Each particular virus can infect cells of only a limited number of hosts species that are called the host range of the virus
      - General features of viral replicative cycles
        
        A viral infection begins when a virus binds to a host cell and the viral genome makes its way inside
        
        Viral reproductive cycle
        
        1. The virus enters the cell and is uncoated,releasing viral DNA and capsid proteins
        
        2.Hosts enzymes replicate the viral genome
        
        3. Meanwhile, host enzymes transcribe the viral genome into viral mRNA, which host ribosomes use to make more capsid proteins
        
        4. Viral genomes and capsid proteins self-assemble into new virus particles which exit the cell
        
        The mechanism of genome entry depends on the type of virus and the type of host cell
        
        Many DNA viruses use the DNA polymerases of the host cell to synthesize new genomes along the templates provided by the viral DNA
        
        The simplest type of viral replicative cycle ends with the exit of hundreds or thousands of viruses from the infected host cell, a process that damages or destroys the cell
        
        the lysogenc cycle allows replication of the phage genome without destroying the host
      - Replicative Cycles of Phages
        
        The lyctic cycle culminates in death of the host cell
        
        A phage that replicates only by a lytic cycle is a virulent phage
        
        Lytic cycle refers to the last stage of infection