Objectives

1. 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.

2. 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.

3. 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.

4. 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.

5. 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.

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

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

8. 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.

9. 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.

10. 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.

11. Section 1 Viruses Chapter 20 Prophages and Proviruses

12. Section 1 Viruses Chapter 20 Lysogenic Cycle

13. Section 1 Viruses Chapter 20 Viral Replication in Bacteria

14. 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.

15. 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.

16. 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.

17. 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.

18. 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.

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

20. 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.

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

22. 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).

23. Section 1 Viruses Chapter 20 Important Viral Diseases

24. 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.

25. 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.

26. 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.

27. 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.

28. 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.

29. 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.

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

31. 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.

32. Section 2 Bacteria Chapter 20 Three Bacterial Cell Shapes

33. 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.

34. Section 2 Bacteria Chapter 20 Gram Staining

35. Section 2 Bacteria Chapter 20 Bacterial Cell Shapes, continued • Some bacteria form thick-walled endospores around their chromosomes and a small bit of cytoplasm when they are exposed to harsh conditions. • Pili enable bacteria to adhere to the surface of sources of nutrition, such as your skin. Some kinds of pili enable bacteria to exchange genetic material through a process called conjugation. • Conjugation is a process in which two organisms exchange genetic material.

36. Section 2 Bacteria Chapter 20 Escherichia coli

37. Section 2 Bacteria Chapter 20 Obtaining Energy Photosynthesis • Photosynthetic bacteria can be classified into four major groups based on the photosynthetic pigments they contain: purple nonsulfur bacteria, green sulfur bacteria, purple sulfur bacteria, and cyanobacteria. • Green sulfur bacteria and purple sulfur bacteria grow in anaerobic environments. • Cyanobacteria are thought to have made the Earth’s oxygen atmosphere.

38. Section 2 Bacteria Chapter 20 Obtaining Energy, continued Chemoautotrophs • Bacteria called chemoautotrophs obtain energy by removing electrons from inorganic molecules such as ammonia and hydrogen sulfide or from organic molecules such as methane. • In the presence of one of these hydrogen-rich chemicals, chemoautotrophic bacteria can manufacture all their own amino acids and proteins.

39. Section 2 Bacteria Chapter 20 Obtaining Energy, continued Heterotrophs • Most bacteria are heterotrophs. • Many are aerobic, that is, they live in the presence of oxygen. Some other bacteria can live without oxygen. • Together with fungi, heterotrophic bacteria are the principal decomposers of the living world; they break down the bodies of dead organisms and make the nutrients available to other organisms.

40. Section 2 Bacteria Chapter 20 Pathogenic Bacteria Bacteria Can Metabolize Their Host • Heterotrophic bacteria obtain nutrients by secreting enzymes that break down complex organic structures in their environment and then absorbing them. If that environment is your throat or lungs, this can cause serious problems. • Several common bacterial diseases include dental cavities, strep throat, tuberculosis, and acne.

41. Section 2 Bacteria Chapter 20 Important Bacterial Diseases

42. Section 2 Bacteria Chapter 20 Pathogenic Bacteria, continued Bacterial Toxins • The second way bacteria cause disease is by secreting chemical compounds into their environment. These chemicals, called toxins, are poisonous to eukaryotic cells. • When bacteria grow in food and produce toxins, the toxins can cause illness in humans who eat those contaminated foods. • Most bacteria can be killed by boiling water or various chemicals.

43. Section 2 Bacteria Chapter 20 Pathogenic Bacteria, continued Biowarfare • Biowarfare is the deliberate exposure of people to biological toxins or pathogens such as bacteria or viruses. • Biologists are working on new approaches to recognize the onset of an attack with a bioweapon, to treat infected people, and to slow the spread of any outbreak of disease.

44. Section 2 Bacteria Chapter 20 Antibiotics • In 1928, the British bacteriologist Alexander Fleming discovered the antibiotic penicillin. • Today different antibiotics are used to interfere with different cellular processes. • Because these processes do not occur in viruses, antibiotics are not effective against them.

45. Section 2 Bacteria Chapter 20 Antibiotics, continued Antibiotic-Resistant Bacteria • Some bacteria have become resistant to antibiotics. • Susceptible bacteria are eliminated from the population, and resistant bacteria survive and reproduce, thus passing on their resistance traits. • Usually, if the full course of the antibiotic is administered, all the targeted bacteria are killed and there is no chance for a resistant strain to develop. If antibiotic treatment ends prematurely, some of the more-resistant bacteria may survive and reproduce.

46. Section 2 Bacteria Chapter 20 Importance of Bacteria Food and Chemical Production • Many of the foods that we eat, such as pickles, cheese, sauerkraut, olives, vinegar, and sourdough bread, are processed by specific kinds of bacteria. • Humans are able to use different bacteria to produce different kinds of chemicals for industrial uses. • Genetic engineering companies use genetically engineered bacteria to produce their many products, such as drugs for medicine and complex chemicals for research.

47. Section 2 Bacteria Chapter 20 Importance of Bacteria, continued Mining and Environmental Uses of Bacteria • Mining companies can use bacteria to concentrate desired elements from low-grade ore. • Bacteria metabolize different organic chemicals and are therefore used to help clean up environmental disasters such as petroleum and chemical spills. • Powders containing petroleum-metabolizing bacteria are used to help clean oil spills.

48. Standardized Test Prep Chapter 20 Multiple Choice The diagram below illustrates viral replication in bacteria. Use the diagram to answer questions 1–3.

49. Standardized Test Prep Chapter 20 Multiple Choice, continued 1. Which type of cycle is represented in the diagram? A. aerobic B. anaerobic C. lysogenic D. lytic

50. Standardized Test Prep Chapter 20 Multiple Choice, continued 1. Which type of cycle is represented in the diagram? A. aerobic B. anaerobic C. lysogenic D. lytic