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Viral Genetics

Viral Genetics

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Viral Genetics

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  1. Viral Genetics Microbiology: The Genetics of Viruses

  2. Viruses &Bacteria: simplest biological systems • Viruses: • smaller/ simpler than bacteria • Contain nucleic acid and protein ONLY • Not considered to be “living” b/c they need a host and NOT make out of a cell • Bacteria: • larger than viruses • Contain ALL marcomolecules • considered living

  3. Virus Bacterium Animal cell Animal cell nucleus 0.25 µm

  4. Bacteria with Bacteriophages infecting the cell:

  5. What is a virus? • Has a genome (RNA or DNA) but… • can reproduceonlywithin a host cell • Scientists detected viruses indirectly long before they could see them • The story of how viruses were discovered begins in the late 1800s: • Tobacco mosaic disease stunts growth of tobacco plants  gives their leaves a mosaic coloration • In late 1800s, researchers hypothesized that a particle smaller than bacteria caused the disease • In 1935, Wendell Stanley confirmed this hypothesis

  6. Tobacco Mosaic Disease

  7. Viral structure Viruses are not (living) cells! • Virus in Latin means “poison” • Made up of infectious particles consisting of a nucleic acid (RNA or DNA) in a protein coat (and sometimes another membrane) • Viruses have a Capsid • protein shell encloses viral genome (nucleic acid) • A capsid can have various structures • Viral genomes may consist of • Double- or single-stranded DNA (dsDNA, ssDNA) • Double- or single-stranded RNA (dsRNA, ssRNA)

  8. Viral Structure Images

  9. Depending on viral type of nucleic acid… • A virus is called a • DNA virus or • an RNA virus • The smallest viruses have only 4 genes, while others have 100’s


  11. Some viruses have membranous envelopes  help them infect hosts • Surround capsids • Viral envelopes (made from host cell’s membrane) contains a combo of viral and host cell molecules • HELPS infect other target host cells • •

  12. Capsid Capsid and viral genome enter cell RNA HOST CELL Envelope (with glycoproteins) Viral genome (RNA) Template mRNA Capsid proteins ER Glyco- proteins Copy of genome (RNA) New virus

  13. General Features of Viral Reproductive Cycles • Viruses are intracellular parasites • they can reproduce only within a host cell • Each virus has a host range, a limited number of host cells that it can infect • Viruses uses HOST molecules to make more (“offspring”) viruses

  14. VIRUS Entry into cell and uncoating of DNA DNA Capsid Transcription Replication HOST CELL • Virus uses HOST: • DNA Polymerase • RNA Polymerase • Ribosomes • tRNA’s • Amino acids Viral DNA mRNA Viral DNA Capsid proteins Self-assembly of new virus particles and their exit from cell

  15. Reproductive Cycles of Phages • Phages = best understood of all viruses lytic cyclevs. lysogenic cycle • Kill infected bacterial host cells BACTERIRAL DEFENCES INCLUDE: • Some bacteria have mutant receptor sites that phage can no longer bind to 2. Some bacteria also produce restriction endonucleases

  16. Viral reproduction: Lytic Cycle • A phage reproductive cycle  ends in DEATH of host cell • Produces new phages and digests the host’s cell wall, releasing the progeny (“offspring”) viruses • A phage that reproduces only by the lytic cycle is called a virulent (strong) phage • EXAMPLES: • Flu, common cold, Rabies

  17. A bigger perspective of the Lytic Cycle: Attachment Entry of phage DNA and degradation of host DNA Phage assembly Release Head Tails Tail fibers Assembly Synthesis of viral genomes and proteins

  18. Viral reproduction: Lysogenic Cycle • Think “S” for “Silent” • Replicates viral genome (DNA) without destroying host cell • Viral DNA becomes incorporatedinto host cell’s DNA (called prophage) • Every time host divides, phage DNA is copied/passed to daughter cells • Virus spread without killing host cells • Occasionally, virus switches to lytic cycle • Temperate virus • Viruses capable of using the lytic and lysogenic cycles. • EXAMPLES: HIV and Herpes

  19. A bigger perspective of the Lysogenic Cycle: Phage DNA Daughter cell with prophage The phage attaches to a host cell and injects its DNA. Many cell divisions produce a large population of bacteria infected with the prophage. Phage Phage DNA circularizes Occasionally, a prophage exits the bacterial chromosome, initiating a lytic cycle. Bacterial chromosome Lytic cycle Lysogenic cycle Certain factors determine whether The bacterium reproduces normally, copying the prophage and transmitting it to daughter cells. Prophage The cell lyses, releasing phages. Lysogenic cycle is entered Lytic cycle is induced or Phage DNA integrates into the bacterial chromosomes, becoming a prophage. New phage DNA and proteins are synthesized and assembled into phages.

  20. RNA viruses = Retroviruses • Retroviruses: transcribe DNA from an RNA template (RNA DNA) • These viruses use Reverse transcriptase (catalyzing enzyme) • Ex: HIV  AIDS

  21. Retrovirus (HIV) Viral envelope Glycoprotein Capsid LE 18-9 RNA (two identical strands) Reverse transcriptase

  22. Membrane of white blood cell HIV HOST CELL Reverse transcription Viral RNA LE 18-10 RNA-DNA hybrid 0.25 µm HIV entering a cell DNA NUCLEUS Provirus Chromosomal DNA RNA genome for the next viral generation mRNA New HIV leaving a cell

  23. Retrovirus Animations • • •

  24. Vaccines • Vaccines • Harmless derivatives (ex: viral proteins) of viruses • stimulates immune system to create antibodies against actual pathogen • Vaccination has eradicated smallpox • Effective vaccines are available against polio, measles, rubella, mumps, hepatitis B, and more

  25. Medical Technology cannot… • Treata viral infection that has already occurred (vaccines do not‘kill’ viruses) • Antibiotics do not treat viral infections • Antiviral drugs resemble nucleoides and interfere with the viral nucleic acid synthesis • Valtrex stops herpes virus reproduction by inhibiting viral DNA polymerase that synthesizes viral DNA • Azidothymidine (AZT) curbs HIV reproduction by interfering with DNA synthesis by reverse transcriptase • Currently the most effective treatment is a ‘cocktail’ multidrug treatment method

  26. Small Pox

  27. Polio Polio

  28. Herpes Simplex

  29. Hepatitis

  30. Varicella Zoster (chicken pox)

  31. Mumps

  32. Measles - Rubeola