The Genetics of Viruses

1. The Genetics ofViruses Chapter 18

2. Discovery of Viruses • 1796 – Edward Jenner developed the smallpox vaccine • 1897 – Beijerinck coined the term “virus” meaning poison • 1935 – Wendell Stanley crystalized sap from tobacco leaves with TMV and found that viruses were made of nucleic acid and protein

3. Virology • Study of viruses • Deadly viruses are virulent • Viruses couldn’t be seen until the electron microscope was invented

4. Viral Characteristics • Non-living • Non-cellular • Cannot grow or reproduce on its own • No metabolism • Cause disease • AIDS, colds, flu, measles, mono • Cause cancer • Cervical, leukemia

5. Viral Structure • Genetic material • DNA or RNA • Capsid • Protein coat surrounding nucleic acid • Envelope • Some; comes from host cell membrane • Mode of entry • Spikes, receptors, etc.

6. Virus Types

7. Virus Specificity • Host range: • Broad: West Nile • Mosquitoes, birds, humans, horses • Narrow: Measles • Humans • Viruses are small

8. Bacteriophages or T-Phages • Among the most complex viruses • Attack bacterial cells • Composed of a head, tail, base plate, & tail fibers • Long DNA molecule is inside the head  • Tail helps inject the viral DNA into host cell • Tail fibers used to attach to host

9. Lytic Cycle • Viral replication that rapidly kills the host cell causing it to lyse or burst • Involves 5 steps • Adsorption --- phage attaches to cell membrane of host • Injection --- nucleic acid (DNA) of virus injected into host cell • Replication --- viral DNA inactivates host cell's DNA & uses host's raw materials & ribosomes to make viral DNA, capsids, tails, etc. • Assembly --- new viral parts are combined to make new phages • Lysis --- enzymes weaken & destroy the cell membrane causing it to lyse releasing new viruses that infect other cells

10. Lytic Cycle

11. Lysogenic Cycle • Bacteriophage infects bacteria cell and injects its DNA • DNA becomes incorporated into bacterial chromosome • Prophage • Bacteria reproduces, making more copies of viral DNA • Lytic cycle can be triggered

12. Temperate Phages: Lytic & Lysogenic

13. Animal Viruses • Two key variables in classifying viruses that infect animals: • DNA or RNA? • Single-stranded or double-stranded?

18. Retroviruses • Contain RNA • Reverse transcriptase enzyme uses the RNA to make DNA • Use the host cell's ribosomes & raw materials to make viral proteins • Cause some cancers & AIDS

19. HIV Infection

20. Viroids • Smallest particle able to replicate • Made of a short, single strand of RNA with no capsid • Cause disease in plants

21. Prions • Infectious protein • protein particles that have folded incorrectly • No nucleic acid or capsids • Attacks the central nervous system • Cause animal diseases in cows (Mad Cow disease), sheep, & humans

22. Viral control • Interferon are proteins made by cells to fight viruses • Two types of viral vaccines exist --- inactivated & attenuated • Inactivated virus vaccines don't replicate in the host's system • Attenuated viral vaccines have been genetically altered so they can't cause disease • Antiviral drugs (AZT, acyclovir, & azidothymidine) interfere with viral DNA synthesis • Protease Inhibitors interfere with viral capsid production