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Chapter 19: Viruses (Viral Genetic Material (Class I. (Double-stranded DNA…

- - - - Enzyme reverse transcriptase that transcribes RNA to DNA flow that is opposite of usual direction.
      - HIV reverse transcriptase molecules release into cytoplasm, synthesize viral DNA, provirus, never leaves host's genome.
  - - - Plasmids are small, circular DNA molecules found in bacteria and yeasts.
      - Plasmids replicate independently, occasionally transferred between cells.
      - Transposons are DNA segments move from one location to another within cell's genome.
      - Mobile genetic elements.
    - - Largest virus, Mimivirus, genome included genes previously only found in cellular genomes.
      - Some code for proteins involved in translation, DNA repair, protein folding, and polysaccharide synthesis.
      - Even larger viruses discovered in 2013 that can't be classified within any known virus.
        
        Pithovirus sivericum discovered in permanently frozen soil in Siberia. Able to infect amoeba after being frozen for 30,000 years.
- - - - T-even phages use tail apparatus to inject DNA in bacterium.
    - - Lytic Cycle
        
        Results in death of the host cell.
        
        Bacterium lyses break open and release phages produced within cell.
        
        Infect healthy cell, can destroy entire bacterial population in a few hours.
        
        Virulent phage.
      - Lysogenic Cycle
        
        Coexist with host cell.
        
        Replication of phage without destroying host.
        
        Temperate phages.
        
        Break both circular DNA molecules and join them together, prophage.
        
        One may code for protein that prevents transcription of most other prophage genes.
        
        Signifies prophages capable of generating active phages that lyse in host cell.
      - Bacterial Defenses
        
        Lyosgeny spares bacteria of extinction.
        
        Natural selection favors bacterial mutants with surface proteins that phages no longer recognize as receptors.
        
        Restriction enzymes may cut up viral DNA and recognize them as foreign.
        
        CRISPR-Cas System
        
        Discovered during study of repetitive DNA sequences in genomes of prokaryotes.
        
        Clustered Regularly Interspaced Short Palindromic Repeats. Read same forward and backwards.
        
        Every spacer sequence corresponds to DNA from particular phage in infected cell.
        
        Can identify and cut phage DNA.
        
        RNA of CRISPR identify and cut phage DNA to destruct it.
    - - Two variables: viral genome and presence of membranous envelope.
      - Viral Envelopes
        
        Used to enter host cell.
        
        Viral glycoproteins bind to specific receptor molecules outside host cell.
        
        Ribosomes bound to ER make protein parts of envelope glycoproteins.
        
        Resulting viral glycoproteins transported to cell surface.
        
        Free to infect other cells.
        
        Some have envelopes not derived from host cell membrane.
        
        Double-stranded DNA genomes and replicate within host cell nucleus.
        
        Combination of viral and cellular enzymes to replicate and transcribe DNA.
- - - - Viruses that suddenly become apparent.
      - HIV
        
        Appeared in San Francisco in early 1980s out of nowhere.
        
        Uncovered a case in Belgian Congo in 1959.
      - West Nile Virus
        
        Appeared in North America in 1999, spread to all 48 continuous states.
        
        Over 40,000 cases and 2,000 deaths.
        
        Mosquito-borne.
      - Ebola Virus
        
        Recognized in 1976 in central Africa.
        
        Cause Hemorrhagic fever that is often fatal. Fever, vomiting, massive bleeding, and circulatory system collapse.
        
        Outbreak in 2014, 27,000 illnesses and 11,000 deaths.
      - Chikungunya
        
        Acute illness; fever, rashes, and persistent joint pain.
      - Zika Virus
        
        Caused outbreak of disease in Spring 2015 in Brazil.
        
        Infection of pregnant women correlated with increase in babies with abnormally small brains, microcephaly.
        
        Mosquito-borne.
      - Infleunza
        
        2009, widespread epidemic appeared in Mexico and United States.
        
        H1N1
        
        Declared global epidemic, or pandemic, shortly thereafter.
        
        207 countries, infected 600,000 people and killed almost 8,000.
        
        Three Types
        
        B and C infect only humans, never cause an epidemic.
        
        A infects wide range of animals.
        
        4 major flu epidemics in the last 100 years.
        
        "Spanish flu" killed 40 - 50 million people from 1918 - 1919.
        
        Hemagglutinin
        
        16 different types.
        
        Protein that helps flu virus attach to host cells.
        
        Neuraminidase
        
        9 different types.
        
        Enzyme that helps release new virus particles from infected cells.
        
        Waterbirds carry all possible combinations.
        
        H5N1
        
        Those infected have more than 50% mortality rate.
        
        Birds to humans in Hong Kong in 1997.
      - Three Processes
        
        Mutation of existing viruses.
        
        Viral RNA polymerases don't proofread and correct errors in RNA genomic replication.
        
        New genetic varieties (strains), genetically different for people to have little immunity to them.
        
        Dissemination of viral disease from small, isolated human population.
        
        AIDS started from small population and spread around the world.
        
        Technological and economic factors.
        
        Spread of existing viruses from other animals.
        
        3/4 of human diseases originate this way.
        
        H1N1 virus likely passed to humans from pigs. "Swine flu."
  - - - Plant infected from external source of virus.
      - More susceptible is damaged by wind, injury, or herbivores.
      - Herbivores can act as carriers of viruses.
    - - Plant inherits viral infection from parent.
      - Occur in asexual or sexual production.
- - - - 1883: Adolf Mayer rubbed sap from diseased leaves to healthy leaves of plants.
        
        Suggested disease caused by unusually small bacteria.
      - Dmitri Ivanowsky passed sap through a filter, resulting leaf still produced mosaic disease.
        
        Sap still cause mosaic disease even when filtered.
      - Martinus Beijerinck carried out series of experiments showing infectious agent in filtered sap replicated.
        
        Showed bacteria could not be cultivated alone.
        
        First scientist to voice concept of a virus.
      - Wendell Stanley (1935) crystallized the infectious particle.
        
        Tobacco mosaic virus and other viruses seen under electron microscope.
  - - - Genomes have double-stranded DNA, single-stranded DNA, double-stranded RNA, and single-stranded RNA.
      - Called DNA or RNA virus based on its nucleic acid making up genome.
      - Some contain multiple molecules of nucleic acid, smallest have 3, largest have several hundred to 2,000.
    - - Protein shell enclosing virus called a capsid.
        
        May be rod-shaped, polyhedral, or more complex.
        
        Built from protein subunits called capsomeres.
        
        Rod-shaped viruses called helical viruses.
        
        Polyhedral viruses called icosahedral viruses.
        
        Membraneous envelopes (viral envelopes) contain host cell phospholipids and membrane proteins.
        
        Most complex capsids among bacteria viruses, bacteriophages.
        
        First 7 phages named T1, T2, T3, and so on...
        
        Elongated icosahedral heads with DNA enclosed and protein tail with fibers where they attach to bacterial cell.