Viruses: Structure, Replication, and Impact

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2. Resources Chapter Presentation Visual Concepts Transparencies Standardized Test Prep

3. Viruses and Bacteria Chapter 20 Table of Contents Section 1 Viruses Section 2 Bacteria

4. Section 1 Viruses Chapter 20 Objectives • Describewhy a virus is not considered a living organism. • Summarizethe discovery of the tobacco mosaic virus. • Describethe basic structure of a virus. • Summarizethe steps of viral replication. • Explainhow HIV infects immune system cells.

5. Section 1 Viruses Chapter 20 Is a Virus Alive? • All living things are made of cells, are able to grow and reproduce, and are guided by information stored in their DNA. • Viruses are segments of nucleic acids contained in a protein coat. Viruses are not cells. • Viruses are pathogens—agents that cause disease. • Viruses do not grow, do not have homeostasis, and do not metabolize.

6. Section 1 Viruses Chapter 20 Virus

7. Section 1 Viruses Chapter 20 Pathogen

8. Section 1 Viruses Chapter 20 Is a Virus Alive?, continued Discovery of Viruses • Near the end of the nineteenth century, scientists were trying to find the cause of tobacco mosaic disease, which stunts the growth of tobacco plants. • In 1935, biologist Wendell Stanley of the Rockefeller Institute purified tobacco mosaic virus (TMV) and determined that the purified virus is a crystal. • Stanley concluded that TMV is a chemical rather than an organism.

9. Section 1 Viruses Chapter 20 Viral Structure • The virus protein coat, or capsid, may contain either RNA or DNA, but not both. • Many viruses have a membrane, or envelope, surrounding the capsid. • The envelope helps the virus enter cells. It consists of proteins, lipids, andglycoproteins,which are proteins with attached carbohydrate molecules that are derived from the host cell.

10. Section 1 Viruses Chapter 20 Parts of a Virus

11. Section 1 Viruses Chapter 20 Viral Structure, continued • Some viruses are long rods that form filaments. • Spherical viruses are typically studded with receptors. • A helical virus is rodlike in appearance, with capsid proteins winding around the core in a spiral. • Viruses that infect bacteria, called bacteriophages, have a complicated structure. A T4 bacteriophage, for example, has a polyhedron capsid attached to a helical tail.

12. Section 1 Viruses Chapter 20 Structures of TMV and Influenza Virus

13. Section 1 Viruses Chapter 20 Structures of Adenovirus and Bacteriophage

14. Section 1 Viruses Chapter 20 Bacteriophage

15. Section 1 Viruses Chapter 20 Viral Reproduction • Viruses must rely on living cells (host cells) for replication. • Before a virus can replicate, it must first infect a living cell. • An animal virus enters its host cell by endocytosis. • A bacterial virus, or bacteriophage, punches a hole in the bacterial cell wall and injects its DNA into the cell.

16. Section 1 Viruses Chapter 20 Viral Reproduction, continued Lytic Cycle • In bacterial viruses, the cycle of viral infection, replication, and cell destruction is called the lytic cycle. • After the viral genes have entered the cell, they use the host cell to replicate viral genes and to make viral proteins, such as capsids. • The proteins are then assembled with the replicated viral genes to form complete viruses. The host cell is broken open and releases newly made viruses.

17. Section 1 Viruses Chapter 20 Lytic Cycle

18. Section 1 Viruses Chapter 20 Viral Reproduction, continued Lysogenic Cycle • During an infection, some viruses stay inside the cells but instead of producing virus particles, the viral gene is inserted into the host chromosome and is called a provirus. • Whenever the cell divides, the provirus also divides, resulting in two infected host cells. • In this cycle, called the lysogenic cycle, the viral genome replicates without destroying the host cell.

19. Section 1 Viruses Chapter 20 Prophages and Proviruses

20. Section 1 Viruses Chapter 20 Lysogenic Cycle

21. Section 1 Viruses Chapter 20 Viral Replication in Bacteria

22. Section 1 Viruses Chapter 20 Relationships between the Lytic and Lysogenic Cycles

23. Section 1 Viruses Chapter 20 Viral Reproduction, continued Host Cell Specificity • Viruses are often restricted to certain kinds of cells. • Viruses may have originated when fragments of host genes escaped or were expelled from cells. • The hypothesis that viruses originated from a variety of host cells may explain why there are so many different kinds of viruses. Biologists think there are at least as many kinds of viruses as there are kinds of organisms.

24. Section 1 Viruses Chapter 20 Viral Reproduction, continued Structure of HIV—an Enveloped Virus • The human immunodeficiency virus (HIV) causes acquired immune deficiency syndrome (AIDS). • Within HIV’s envelope lies the capsid, which in turn encloses the virus’s genetic material. • In the case of HIV, the genetic material is composed of two molecules of single-stranded RNA.

25. Section 1 Viruses Chapter 20 How HIV Infects Cells Attachment • Studding the surface of each HIV are spikes composed of a glycoprotein. • This particular glycoprotein precisely fits a human cell surface receptor called CD4. • Thus the HIV glycoprotein can bind to any cell that possesses CD4 receptors.

26. Section 1 Viruses Chapter 20 How HIV Infects Cells, continued Entry into Macrophages • HIV cannot enter a cell merely by docking onto a CD4 receptor. Rather, the glycoprotein must also activate a second co-receptor, called CCR5. • It is this event at CCR5 that starts endocytosis. • Because human macrophages possess both CD4 and CCR5 receptors, HIV can enter macrophages.

27. Section 1 Viruses Chapter 20 How HIV Infects Cells, continued Replication • Once inside a cell, the HIV particle sheds its capsid. The particle then releases an enzyme called reverse transcriptase. • Reverse transcriptase copies the naked viral RNA into a complementary DNA version. • Translation of the viral DNA by the host cell’s machinery directs the production of many copies of the virus.

28. Section 1 Viruses Chapter 20 Infection of Macrophage by HIV

29. Section 1 Viruses Chapter 20 How HIV Infects Cells, continued AIDS • For years after the initial infection, HIV continues to replicate (and mutate). Eventually and by chance, HIV’s surface glycoproteins change to the point that they now recognize a new cell surface receptor. This receptor is found on the subset of lymphocytes called T cells. • Unlike its activity in macrophages, HIV reproduces in T cells and then destroys them. • It is this destruction of the body’s T cells that blocks the body’s immune response and signals the onset of AIDS.

30. Section 1 Viruses Chapter 20 AIDS (Acquired Immune Deficiency Syndrome)

31. Section 1 Viruses Chapter 20 Viral Diseases • Perhaps the most lethal virus in human history is the influenza virus. • Certain viruses can also cause some types of cancer. • Viruses associated with human cancers include hepatitis B (liver cancer), Epstein-Barr virus (Burkitt’s lymphoma), and human papilloma virus (cervical cancer).

32. Section 1 Viruses Chapter 20 Important Viral Diseases

33. Section 1 Viruses Chapter 20 Viral Diseases, continued Emerging Viruses • Viruses that evolve in geographically isolated areas and are pathogenic to humans are called emerging viruses. • These new pathogens are dangerous to public health. People become infected when they have contact with the normal hosts of these viruses. • Examples of emerging viruses include West Nile virus and hantavirus.

34. Section 1 Viruses Chapter 20 Viral Diseases, continued Prions and Viroids • Prions are composed of proteins but have no nucleic acid. A disease-causing prion is folded into a shape that does not allow the prion to function. • Contact with a misfolded prion will cause a normal prion to misfold, too. In this way the misfolding spreads. • A viroid is a single strand of RNA that has no capsid. Viroids are important infectious disease agents in plants.

35. Section 2 Bacteria Chapter 20 Objectives • Listseven differences between bacteria and eukaryotic cells. • Describethree different ways bacteria can obtain energy. • Describethe external and internal structure of Escherichia coli. • Distinguishtwo ways that bacteria cause disease. • Identifythree ways that bacteria benefit humans.

36. Section 2 Bacteria Chapter 20 Bacteria

37. Section 2 Bacteria Chapter 20 Characteristics of Bacteria

38. Section 2 Bacteria Chapter 20 Bacterial Structure • Bacteria differ from eukaryotes in at least seven ways. • Bacteria are prokaryotes. Unlike eukaryotes, prokaryotes lack a cell nucleus. • Most bacterial cells are about 1 µm in diameter; most eukaryotic cells are more than 10 times that size.

39. Section 2 Bacteria Chapter 20 Bacterial Structure, continued • All bacteria are single cells. • Bacterial chromosomes consist of a single circular piece of DNA. Eukaryotic chromosomes are linear pieces of DNA that are associated with proteins. • Bacteria reproduce by binary fission, a process in which one cell pinches into two cells.

40. Section 2 Bacteria Chapter 20 Bacterial Structure, continued • Bacterial flagella are simple structures composed of a single fiber of protein that spins like a corkscrew to move the cell. • Some bacteria also have shorter, thicker outgrowths called pili. • Bacteria have many metabolic abilities that eukaryotes lack. For example, bacteria perform several different kinds of anaerobic and aerobic processes, while eukaryotes are mostly aerobic organisms.

41. Section 2 Bacteria Chapter 20 Structure of Cilia and Flagella

42. Section 2 Bacteria Chapter 20 Pilus

43. Section 2 Bacteria Chapter 20 Structural Characteristics of a Bacterial Cell

44. Section 2 Bacteria Chapter 20 Parts of a Prokaryotic Cell

45. Section 2 Bacteria Chapter 20 Comparing Organisms That Are Unicellular and Multicellular

46. Section 2 Bacteria Chapter 20 Bacterial Cell Shapes • A bacterial cell is usually one of three basic shapes: bacillus, a rod-shaped cell; coccus, a round-shaped cell; or spirillum, a spiral cell. • Members of the kingdom Eubacteria have a cell wall made of peptidoglycan, a network of polysaccharide molecules linked together with chains of amino acids. • Outside the cell wall and membrane, many bacteria have a gel-like layer called a capsule.

47. Section 2 Bacteria Chapter 20 Three Bacterial Cell Shapes

48. Section 2 Bacteria Chapter 20 Bacterial Capsule

49. Section 2 Bacteria Chapter 20 Bacterial Cell Shapes, continued • Eubacteria can have two types of cell walls, distinguished by a dye staining technique called the Gram stain. • Gram staining is important in medicine because the two groups of eubacteria differ in their susceptibility to different antibiotics. • Antibiotics are chemicals that interfere with life processes in bacteria.

50. Section 2 Bacteria Chapter 20 Gram Staining