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viruses (structure (capsids and envelopes (A capsid is the protein shell…

- - - - Capsids are built from protein subunits called capsomeres
        
        types of structures
        
        influenza viruses
        
        tobacco mosaic virus
        
        bacteriophage [T4]
        
        Adenoviruses
    - - Viral envelopes contain a combination of viral and host cell molecules
- - - - the lytic cycle （溶解循環）
        
        virulent or temperate phage.
        
        Steps:
        1.destruction of host DNA
        2.production of new phages
        3.lysis of host cell causes release of progeny phages
        
        A phage that reproduces only by the lytic cycle is called a virulent phage
        
        The lytic cycle is a phage replicative cycle that culminates in the death of the host cell
      - the lysogenic cycle　（溶原循環）
        
        temperate phage only.
        steps:
        
        genome integrates into bacterial chromosome as prophage.
        
        which is replicated and passed onto daughter cells and
        
        can be induced to leave the chromosome and initiate a lytic cycle
        
        The lysogenic cycle replicates the phage genome without destroying the host
        
        Phages that use both the lytic and lysogenic cycles are called temperate phages
        
        The viral DNA molecule is incorporated into the host cell’s chromosome
      - bacterial defenses against phage infections: various ways including the CRISPR-Cas system
    - - many animal viruses have an envelope
      - retroviruses such as HIV use the enzyme reverse transcriptase to copy their RNA genome into DNA, which can be integrated into the host genome as a provirus.
  - - - Each type of virus has a characteristic host range, affected by whether cell-surface proteins are present that viral surface proteins can bind to.
- - - - e.g. H1N1 2009 flu virus-combination of pig, human, and avian viral genes causing a pandemic.