Virus

Virus - not considered living, no cells aka not living

  1. Virus: infectious particles of genes packaged in a protein coat

    1. Structure

      1. Genome 

        1. DNA or RNA

        2. Single-stranded or double stranded 

        3. Linear or circular 

        4. Single piece or multiple piece 

      2. Capsid: Protein shell enclosing viral genome

        1. Capsomere: protein subunit of a capsid

        2. Rod-shaped: helical virus

        3. Polyhedral: icosahedral virus (20 side)

        4. Complex: icosahedral head + helical tail

      3. Envelope: membrane derived from host cells that surround many animal viruses

        1. Phospholipids + membrane protein from host

        2. Glycoproteins + enzymes from virus

  2. Viral replicative cycles

    1. Obligate intracellular parasite: can only replicate inside a host cell (has to live in a cell)

      1. Viruses lack enzymes for making macromolecules 

    2. Host range: limited number of species a virus can infect 

      1. Host specificity: evolutionary recognition of receptor proteins on host cell

        1. Lock + key fit of viral proteins to host receptor 

  3. Bacterial virus - Phage (virus that affects bacteria)

    1. Lytic cycle: replicative cycle that ends in death of host cell

      1. Virulent: a virus that only uses the lytic cycle 

      2. Ex. T4 phage

        1. Attachment: T4 phage uses tail fibers to bind to specific receptors on outer shell of E. coli

        2. Entry: T4 phage DNA injected into host cell

          1. Empty capsid left outside

          2. Viral enzymes are expressed and hydrolyze host cell DNA

        3. Synthesis: viral genome/ proteins are produced using host components 

        4. Assembly: virus genomes packaged into capsids (automatic)

        5. Release: viral enzymes break down bacterial cell wall (hypo to hypertonic, water enters the cell and it bursts)

          1. Cell burst and 100-200 viral particles escape

    2. Lysogenic cycle: replicative cycle that incorporates into host genome

      1. Temperate: virus that uses a lysogenic replicative cycle

      2. Does not immediately destroy host cell

      3. Ex. λ phage (lambda phage)

        1. Attachment

        2. Entry

        3. Prophage: viral DNA integrated into host DNA

        4. Replication: normal reproduction of host cell also copies viral genome

          1. Subsequent generations continue to carry infection

          2. Viral genes can be latent: dormant in host

          3. Viral genes can be expressed: changes host phenotype

            1. Normal gut E.coli turning into pathogenic (food poisoning) E.coli

        5. Lytic cycle can be triggered by environmental changes

          1. Virus replicates and escapes when host cell is threatened 

    3. Bacteria defenses

      1. Genetic diversity: not all cells in a population will have the exact surface receptor to match virus (naturally immunity) 

      2. Restriction enzymes: bacterial enzymes that recognize foreign DNA entering and cuts it

        1. General defense against any virus 

      3. CRISPR-Cas system: long-term, targeted defense against specific virus 

        1. Piece of viral DNA incorporated into CRISPR region of bacterial genome

        2. Cas nucleases search out and cut up any DNA sequences identified in CRISPR region

      4. Evolutionary arms race

        1. Bacteria evolve to avoid virus

        2. Virus evolve to beat defenses

  4. Animal viruses

    1. Enveloped virus - have envelope from host cell

    2. Naked virus - do not have envelope

    3. Ex. Human immunodeficiency virus (AID)

      1. Reverse transcriptase: copies viral RNA into DNA

      2. Provirus: viral DNA integrated in host genome

    4. Vaccine: harmless derivative of a pathogen that stimulates immune system to mount robust defense against it

  5. Other infectious agents

    1. Viroid: infectious nucleic acid, no protein coat 

    2. Prion: infectious self-replicating protein, no nucleic acid

    3. Plasmids: small circular DNA that can be shared between bacteria

    4. Transposons: DNA segments that can jump to different sections of the cell’s genome