Viruses

2. Viruses • Discovery of Viruses • Berijerinck • (1897) • coined the Latin name "virus"  meaning poison for  the substance infecting tobacco plants • Wendell Stanley • (1935) crystallized sap from tobacco leaves infected with Tobacco Mosaic Virus (TMV) • first person to see virus • found virus was made of nucleic acid & protein • Edward Jenner • (1796) developed smallpox vaccine • used pus from cowpox

3. Viruses • Discovery of Viruses - Continued • Louis Pasteur • Developed second vaccine against anthrax • Developed third vaccine against rabies • Used dried spinal cord of a rabbit • Sabin • Developed vaccine against polio that contained live virus • Given orally • Salk • Developed vaccine against polio using killed virus • Given as shot

4. Viruses • Virology - study of viruses • Deadly viruses are said to be virulent • Viruses couldn't be seen until electron microscope invented • Viruses cause disease by • Killing cells • Changing the rate of mitosis (warts, tumors, and cancer)

5. Wendell Stanley and Leaf with TMV

6. Viral Characteristics • Not living organisms • Noncellular • Consist of • a nucleic acid core (DNA or RNA) • a protein coat called the capsid • Some have a plasma membrane • Capsid made of protein subunits called capsomeres • Cannot grow or replicate on their own (inactive particles)

7. Viral Characteristics • Can only reproduce inside of a living host cell using its raw materials & enzymes • Lack ribosomes & enzymes needed for protein synthesis or metabolism • Are extremely small particles ranging from 10 - 1400 nanometers on average • Some can cause disease (smallpox, measles, mononucleosis, influenza, colds, AIDS, Ebola)

8. Viral Characteristics • Some may also cause cancers such as leukemias • Virus free cells are rare • Highly host specific (only infect certain cells) • Referred to as phages • Viruses are classified into 2 main groups by their nucleic acid --- DNA or RNA Viruses •  DNA & RNA viruses are subdivided by capsid shape & whether they do or don't have an envelope

9. Viral Structure • DNA or RNA core surrounded by protein sheath called capsid • Nucleocapsid  includes the viral nucleic acid & its capsid • Some form lipid rich covering around capsid called the envelope • Envelope usually formed from host cell membrane • Envelope may have spikes to help chemically recognize & attach to the host cell

10. TMV is helical Adenovirus & polio viruses are icosohedral (20 sided) Viral Structure

11. Viral Structure T-phages have a head and a tail

12. Bacteriophage or T-phage • Among the most complex viruses • Attack bacterial cells • Composed of a icosohedral 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

13. Retrovirus • Contain RNA • Have an enzyme called reverse transcriptase which helps use the RNA to make DNA • Use the host cell's ribosomes & raw materials to make viral proteins • Cause some cancers & AIDS HIV Virus

14. Lytic Cycle • Phases of the Lytic Cycle of aVirulentVirus: • Absorption: Virus attaches itself to the cell. • Entry: Enzymes weaken the cell wall and nucleic acid is injected into the cell, leaving the empty caspid outside the cell. Many viruses actually enter the host cell intact. • Replication: Viral DNA takes control of cell activity. • Assembly: All metabolic activity of the cell is directed to assemble new viruses. • Release: Enzymes disintegrate the cell in a process called lysis, releasing the new viral particles

15. Lytic Cycle Source: http://science.howstuffworks.com/virus-human.htm

16. Lysogenic Cycle • The Lysogenic Cycle of aTemperateVirus: • The virus attaches itself and injects its DNA into the cell. • The viral DNA attaches itself to the host DNA, becoming a new set of cell genes called a prophage. • When the host cell divides, this new gene is replicated and passed to new cells. This causes no harm to the cell, but may alter its traits. • Now there are two possibilities: • The prophage survives as a permanent part of the DNA of the host organism. • Some external stimuli can cause the prophage to become active, using the cell to produce new viruses.

17. Lysogenic Cycle Source: http://science.howstuffworks.com/virus-human.htm

18. 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 • New viruses emerge as rain forests are cleared (Ebola virus)

19. Adenovirus Common cold Influenza Virus Note-plasma membrane

20. HIV Coronavirus SARS

21. Classification • By Nucleic acids • By Structure • By Speed of Infection • By Specificity

22. Classify By Nucleic Acid in Core DNA Viruses • DNA inserted into host cell is incorporated into the host genome. • Host cell does not differentiate • Viral DNA commandeers control and signals host to replicate viral DNA and to produce viral protein. • Host uses itself up to produce these • Result is a hollow sack of viral particles.

23. Classify By Nucleic Acid in Core RNA Viruses • RNA is inserted into the cell • Host cell is signaled to make viral RNA and viral protein • Result is the same for host cell DEATH

24. Polio virus is a RNA virus

25. Classify By Nucleic Acid in Core Retrovirus Viruses • Nucleic acid is RNA but with the addition of enzyme-Reverse Transcriptase • Enzyme allows production of DNA from viral RNA • RNA->DNA->RNA->Protein • Does this disprove the Central Dogma? • Lab use of reverse transcriptase

26. RNA Reverse transcriptase HIV is a retrovirus

27. Classify By Specificity • Viruses are extremely specific as to which kinds of cells they parasitize • Identify protein receptors on the outside of host cells • Tobacco mosaic virus does not infect animals or bacteria. • The viral protein determines the specificity

28. Classify By Speed • Lytic viruses attack the host cell quickly • Lysogenic or latent viruses insert the nucleic acid and lie dormant. • Unknown chemical signals activate them • They may never be activated • If host cell divides the viral genome is replicated along with other DNA

29. Mutation • Viral nucleic acids can mutate • Sometimes pick up parts of host DNA • Sometimes pick up host membrane • Three types of pathogens that have no vaccines Retroviruses Those with multiple antigens Those that mutate rapidly • HIV is a retrovirus with many antigens that mutates rapidly.

30. Origin of Viruses • Hypothesis 1. Viruses predate first cells • Hypothesis 2. Viruses are cellular fragments • Evidence for 2 is that some viruses wrap their DNA around histones to form nucleosomes as eukaryotic cells • Viruses have followed own evolutionary path.

31. Viroids and Prions: The Ultimate in Simplicity Plant Viroid Prion

32. Viroids • Naked RNA found in plant cells • Infectious agents • Process of infection a mystery • As is replication • May interfere with gene regulation

33. Prions • Naked protein not associated with a nucleic acid • Infectious agent • Action unknown • Identified as cause of scrapie--sheep mad cow--bovine C-J—humans

34. Bacteria Unicellular Prokaryotes Point of a straight pin with thousands of bacterial cells

35. History • Leeuwenhoek • Probably first person to see bacteria • Louis Pasteur • Father of bacteriology • First person to study bacteria extensively • Robert Petri • Developed the petri dish • Used to culture bacteria

36. Koch’s Postulates • A formalized group of criteria for establishing the cause of an infectious disease • The microorganism must be present in every case of a disease. • The organism must be grown in pure culture from diseased hosts.

37. Koch’s Postulates continued • The same disease must be produced when a pure culture of the organism is introduced into susceptible hosts. • The organism must be recovered from the experimentally infected host.

38. Koch’s bacterial discovery • 1876 • Convincing proof of “germ theory” of disease • Showed that Bacillus anthracis is cause of anthrax • Often fatal disease in humans, sheep and other animals

39. Koch’s procedure • Under microscope, observed B. anthracis cells in blood and spleen of dead sheep • Inoculated mice with the infected sheep blood • Recovered B. anthracis from blood of mice • Grew bacteria in pure culture and showed that they caused anthrax when injected into healthy mice. • From this and later work with Mycobacterium tuberculosis, Koch’s Postulates formalized

40. Types of Immunity • Naturally acquired passive • From mother at birth • Temporary • Naturally acquired passive • Recover from disease • Make antibodies • Permanent in most cases • Artificially acquired passive • From a serum • Some other animal makes the antibodies • Requires booster shot • Example tetanus • Artificially acquired active • From vaccine • You make the antibodies • Contains dead or weakened pathogens

41. References • http://spa.k12.ar.us/massengale/virus_notes_b1.htm