1. Lesson Overview 20.1 Viruses

2. Discovery of Viruses • In 1892, Dmitri Ivanovski demonstrated that the cause of tobacco mosaic disease was found in the liquid extracted from infected plants. • In 1897, Martinus Beijerinck suggested that tiny particles in the juice caused the disease, and he named these particles viruses, after the Latin word for “poison.” • In 1935, Wendell Stanley isolated crystals of tobacco mosaic virus. Since living organisms do not crystallize, Stanley inferred that viruses were not truly alive.

3. Discovery of Viruses • A virus is a nonliving particle made of proteins, nucleic acids, and sometimes lipids. • Viruses can reproduce only by infecting living cells.

4. Structure and Composition • Viruses differ widely in terms of size and structure. • Most viruses are so small they can be seen only with the aid of a powerful electron microscope.

5. Structure and Composition • The protein coat surrounding a virus is called a capsid. • Some viruses, such as the influenza virus, have an additional membrane that surrounds the capsid. • The simplest viruses contain only a few genes, whereas the most complex may have more than a hundred genes.

6. Structure and Composition • Most viruses have proteins on their surface membrane or capsid that bind to receptor proteins on the host cell. • The proteins “trick” the cell to take the virus, or in some cases just its genetic material, into the cell. • Once inside, the viral genes are eventually expressed and may destroy the cell.

7. Structure and Composition • Most viruses infect only a very specific kind of cell. • Plant viruses infect plant cells; most animal viruses infect only certain related species of animals; viruses that infect bacteria are called bacteriophages.

8. Viral Infections • What happens after a virus infects a cell?

9. Viral Infections • What happens after a virus infects a cell? • Inside living cells, viruses use their genetic information to make multiple • copies of themselves. Some viruses replicate immediately, while others • initially persist in an inactive state within the host.

10. Lytic Infections • In a lytic infection, a virus enters a bacterial cell, makes copies of itself, and causes the cell to burst, or lyse. • Bacteriophage T4 is an example of a bacteriophage that causes such an infection.

11. Lytic Infections • Bacteriophage T4 has a DNA core inside a protein capsid that binds to the surface of a host cell.

12. Lytic Infections • The virus injects its DNA into the cell.

13. Lytic Infections • The cell then begins to make messenger RNA (mRNA) from the viral genes.

14. Lytic Infections • The viral mRNA is translated into viral proteins that chop up the cell’s DNA.

15. Lytic Infections • Controlled by viral genes, the host cell’s metabolic system makes copies of viral nucleic acid.

16. Lytic Infections • The host cell’s metabolic system also makes copies of capsid proteins.

17. Lytic Infections • The viral nucleic acid and capsid proteins are then assembled into new virus particles.

18. Lytic Infections • The host cell lyses, releasing hundreds of virus particles that go on to infect other cells.

19. Lytic Infections • A lytic virus is similar to an outlaw in the Wild West of the American frontier in the demands the virus makes on its host. • First, the outlaw eliminates the town’s existing authority. • In a lytic infection, the host cell’s DNA is chopped up.

20. Lytic Infections • Next, the outlaw demands to be outfitted with new equipment from the local townspeople. • In a lytic infection, the viruses use the host cell to make viral DNA and viral proteins.

21. Lytic Infections • Finally, the outlaw forms a gang that leaves the town to attack new communities. • In a lytic infection, the host cell bursts, releasing hundreds of virus particles.

22. Lysogenic Infection • Some bacterial viruses cause a lysogenic infection.

23. Lysogenic Infection • In a lysogenic infection a host cell is not immediately taken over.

24. Lysogenic Infection • The viral nucleic acid is inserted into the host cell’s DNA.

25. Lysogenic Infection • The viral DNA is then copied along with the host DNA without damaging the host.

26. Lysogenic Infection • Viral DNA multiplies as the host cells multiply.

27. Lysogenic Infection • In this way, each generation of daughter cells derived from the original host cell is infected.

28. Lysogenic Infection • Bacteriophage DNA that becomes embedded in the bacterial host’s DNA is called a prophage.

29. Lysogenic Infection • The prophage may remain part of the DNA of the host cell for many generations.

30. Lysogenic Infection • Influences from the environment—radiation, heat, etc—trigger the prophage to become active.

31. Lysogenic Infection • It then removes itself from the host cell DNA, directs the synthesis of new virus particles, and now becomes an active lytic infection.

32. A Closer Look at Two RNA Viruses • About 70 percent of viruses contain RNA rather than DNA. • In humans, RNA viruses cause a wide range of infections, from relatively mild colds to severe cases of HIV. • Certain kinds of cancer also begin with an infection by viral RNA.

33. The Common Cold • Cold viruses attack with a very simple, fast-acting infection. • A capsid settles on a cell, typically in the host’s nose, and is brought inside, where a viral protein makes many new copies of the viral RNA.

34. The Common Cold • The host cell’s ribosomes mistake the viral RNA for the host’s own mRNA and translate it into capsids and other viral proteins. • The new capsids assemble around the viral RNA copies, and within 8 hours, the host cell releases hundreds of new virus particles to infect other cells.

35. HIV • The deadly disease called acquired immune deficiency syndrome (AIDS) is caused by an RNA virus called human immunodeficiency virus (HIV). • HIV belongs to a group of RNA viruses that are called retroviruses. • The genetic information of a retrovirus is copied from RNA to DNA instead of from DNA to RNA.

36. HIV • When a retrovirus infects a cell, it makes a DNA copy of its RNA.

37. HIV • The copy inserts itself into the DNA of the host cell.

38. HIV • Retroviral infections are similar to lysogenic infections of bacteria. Much like a prophage in a bacterial host, the viral DNA may remain inactive for many cell cycles before making new virus particles and damaging the cells of the host’s immune system.

39. Viruses and Cells • All viruses are parasites. Parasites depend entirely upon other living organisms for their existence, harming these organisms in the process. • Viruses must infect living cells in order to grow and reproduce, taking advantage of the nutrients and cellular machinery of their hosts.

40. Viruses and Cells • Viruses have many of the characteristics of living things. After infecting living cells, viruses can reproduce, regulate gene expression, and even evolve.

41. Viruses and Cells • Some of the main differences between cells and viruses are summarized in this chart.

42. Viruses and Cells • Although viruses are smaller and simpler than the smallest cells, it is unlikely that they were the first living organisms. • Because viruses are dependent upon living organisms, it seems more likely that viruses developed after living cells. • The first viruses may have evolved from the genetic material of living cells. Viruses have continued to evolve, along with the cells they infect, for billions of years.