1. Chapter 24 Table of Contents Section 1 Viral Structure and Replication
Viruses
Chapter 24
Table of Contents
Section 1 Viral Structure and Replication
Section 2 Viral Diseases

2. Chapter 24 Objectives Summarize the discovery of viruses.
Section 1 Viral Structure and Replication
Chapter 24
Objectives
Summarize the discovery of viruses.
Describe why viruses are not considered living organisms.
Describe the basic structure of viruses.
Compare the lytic and lysogenic cycles of virus replication.
Summarize the origin of viruses.

3. Chapter 24 Discovery of Viruses
Section 1 Viral Structure and Replication
Chapter 24
Discovery of Viruses
Researchers in the late 1800s discovered that something smaller than bacteria could cause disease.
In 1935, Wendell Stanley demonstrated that viruses were not cells when he crystallized TMV, the virus that causes tobacco mosaic disease in tobacco and tomato plants.

4. Section 1 Viral Structure and Replication
Chapter 24
Virus

5. Characteristics of Viruses
Section 1 Viral Structure and Replication
Chapter 24
Characteristics of Viruses
Viruses do not have all of the characteristics of life and are therefore not considered to be living.
Viral Size and Structure
Viruses are nonliving particles containing DNA or RNA and are surrounded by a protein coat called a capsid.
Some viruses also have an envelope that is derived from a host cell’s nuclear membrane or cell membrane.

6. Section 1 Viral Structure and Replication
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Parts of a Virus

7. Characteristics of Viruses, continued
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Characteristics of Viruses, continued
Classification of Viruses
Viruses can be classified based on whether they have
RNA or DNA,
whether the RNA or DNA is single or double stranded and circular or linear,
by capsid shape,
and whether or not they have an envelope.

8. Chapter 24 Viral Replication Replication in DNA Viruses
Section 1 Viral Structure and Replication
Chapter 24
Viral Replication
Replication in DNA Viruses
DNA viruses can enter host cells
and directly produce RNA,
or they can insert into a host’s chromosome,
where they are transcribed to RNA along with the host’s DNA.

9. Viral Replication, continued
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Viral Replication, continued
Replication in RNA Viruses
The RNA genome of some RNA viruses can be directly translated to make viral proteins.
Retroviruses use reverse transcriptase and RNA as a template to make DNA, which is then used to produce viral RNA and proteins.

10. Viral Replication, continued
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Viral Replication, continued
Replication in Viruses That Infect Prokaryotes
Bacteriophages are viruses that infect bacteria.

11. Viral Replication, continued
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Viral Replication, continued
Lytic Cycle
Viruses can follow a lytic cycle, making new viral particles immediately.

12. Section 1 Viral Structure and Replication
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Lytic Cycle

13. Viral Replication, continued
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Viral Replication, continued
Lysogenic Cycle
Viruses can follow a lysogenic cycle, becoming part of the host genome and making new particles later.

14. Section 1 Viral Structure and Replication
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Lysogenic Cycle

15. The Lytic and Lysogenic Cycles
Section 1 Viral Structure and Replication
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The Lytic and Lysogenic Cycles

16. Viral Replication, continued
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Viral Replication, continued
Viruses: Tools for Biotechnology
Viruses are important tools for biotechnology.

17. Chapter 24 The Origin of Viruses
Section 1 Viral Structure and Replication
Chapter 24
The Origin of Viruses
Most scientists think viruses originated from fragments of host-cell nucleic-acid material.

18. Chapter 24 Objectives Section 2 Viral Diseases
Name several vectors of viral diseases.
Identify four viral diseases that result in serious human illnesses.
Discuss the relationship between viruses and cancer.
Name three examples of emerging viral diseases.
Compare the effectiveness of vaccination, vector control, and drug therapy in fighting viruses.
Contrast viroids, prions, and viruses.

19. Vectors of Viral Diseases
Section 2 Viral Diseases
Chapter 24
Vectors of Viral Diseases
Vectors, or hosts, of viral diseases include humans, animals, and insects.

20. Chapter 24 Human Viral Diseases
Section 2 Viral Diseases
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Human Viral Diseases
Viruses cause many human diseases, including:
the common cold,
flu,
hepatitis,
rabies,
chickenpox,
certain types of cancer,
and AIDS.

21. Human Viral Diseases, continued
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Human Viral Diseases, continued
Chickenpox and Shingles
Chickenpox and shingles are caused by the same varicella-zoster herpesvirus.

22. Human Viral Diseases, continued
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Human Viral Diseases, continued
Viral Hepatitis
Hepatitis, or inflammation of the liver, can be caused by at least five viruses:
Hepatitis A and Hepatitis E can be spread by fecally contaminated food and water.
Hepatitis B, C, and D are spread by sexual contact, by contact with infected blood and serum,and by the use of contaminated needles.

23. Human Viral Diseases, continued
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Human Viral Diseases, continued
Acquired Immune Deficiency Syndrome (AIDS)
The human immunodeficiency virus (HIV) is an RNA virus spread by:
sexual contact,
by contact with infected body fluids,
and from mother to fetus.
HIV targets macrophages and thus damages the body’s immune system. The disease called acquired immune deficiency syndrome (AIDS) results.

24. AIDS (Acquired Immunodeficiency Syndrome)
Section 2 Viral Diseases
Chapter 24
AIDS (Acquired Immunodeficiency Syndrome)

25. Human Viral Diseases, continued
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Human Viral Diseases, continued
Viruses and Cancer
Some viruses contain oncogenes that can cause cancer, while other viruses convert proto-oncogenes, which usually control cell growth, to oncogenes.

26. Emerging Viral Diseases
Section 2 Viral Diseases
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Emerging Viral Diseases
Emerging viruses usually infect animals isolated in nature but can jump to humans when contact occurs in the environment.

27. Prevention and Treatment
Section 2 Viral Diseases
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Prevention and Treatment
Vaccinations
A vaccine contains a harmless version of a virus, bacterium, or a toxin that causes an immune response when introduced to the body.
Vaccines have helped to greatly reduce certain viral diseases.

28. Prevention and Treatment, continued
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Prevention and Treatment, continued
Vector Control
Control efforts, including killing mosquitoes and other vectors and quarantining ill patients, have helped reduce the spread of certain viral diseases.

29. Prevention and Treatment, continued
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Prevention and Treatment, continued
Drug Therapy
Antibiotics are ineffective against viral diseases.
Viral drugs, such as acyclovir, block specific steps in viral replication.

30. Chapter 24 Virods and Prions
Section 2 Viral Diseases
Chapter 24
Virods and Prions
Viroids are short, circular, single strands of RNA lacking a capsid that infect plant cells.
Prions are infectious particles containing protein but no nucleic acids.
Prions cause mad cow disease and similar degenerative brain diseases.

31. Important Viral Diseases
Section 2 Viral Diseases
Chapter 24
Important Viral Diseases

32. Chapter 24 Multiple Choice 1. What are viruses made of?
Standardized Test Prep
Multiple Choice
1. What are viruses made of?
A. enzymes and fats
B. carbohydrates and ATP
C. protein and nucleic acids
D. mitochondria and lysosomes

33. Multiple Choice, continued
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Multiple Choice, continued
1. What are viruses made of?
A. enzymes and fats
B. carbohydrates and ATP
C. protein and nucleic acids
D. mitochondria and lysosomes

34. Multiple Choice, continued
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Multiple Choice, continued
2. How do viroids differ from viruses?
F. Viroids are larger in size.
G. Viroids do not have a capsid.
H. Viroids do not have nucleic acids.
J. Viroids can cause disease in plants.

35. Multiple Choice, continued
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Multiple Choice, continued
2. How do viroids differ from viruses?
F. Viroids are larger in size.
G. Viroids do not have a capsid.
H. Viroids do not have nucleic acids.
J. Viroids can cause disease in plants.

36. Multiple Choice, continued
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Multiple Choice, continued
3. During which of the following processes does a phage kill its host?
A. conjugation
B. transcription
C. the lytic cycle
D. the lysogenic cycle

37. Multiple Choice, continued
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Multiple Choice, continued
3. During which of the following processes does a phage kill its host?
A. conjugation
B. transcription
C. the lytic cycle
D. the lysogenic cycle

38. Multiple Choice, continued
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Multiple Choice, continued
4. Which of the following is one reason why viruses are not considered living organisms?
F. Viruses are able to grow.
G. Viruses do not metabolize.
H. Viruses can reproduce by splitting.
J. Viruses are too small to be easily observed.

39. Multiple Choice, continued
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Multiple Choice, continued
4. Which of the following is one reason why viruses are not considered living organisms?
F. Viruses are able to grow.
G. Viruses do not metabolize.
H. Viruses can reproduce by splitting.
J. Viruses are too small to be easily observed.

40. Multiple Choice, continued
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Multiple Choice, continued
Use the figure below to answer questions
5. Which of the following does the diagram represent?
A. a virus
B. a prion
C. a viroid
D. a bacterium

41. Multiple Choice, continued
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Multiple Choice, continued
Use the figure below to answer questions
5. Which of the following does the diagram represent?
A. a virus
B. a prion
C. a viroid
D. a bacterium

42. Multiple Choice, continued
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Multiple Choice, continued
Use the figure below to answer questions
6. To which of the following is label X pointing?
F. envelope
G. nucleic acid
H. protein coat
J. cell membrane

43. Multiple Choice, continued
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Multiple Choice, continued
Use the figure below to answer questions
6. To which of the following is label X pointing?
F. envelope
G. nucleic acid
H. protein coat
J. cell membrane

44. Multiple Choice, continued
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Multiple Choice, continued
Complete the following analogy:
7. skin : person :: capsid :
A. virus
B. insect
C. fungus
D. bacterium

45. Multiple Choice, continued
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Multiple Choice, continued
Complete the following analogy:
7. skin : person :: capsid :
A. virus
B. insect
C. fungus
D. bacterium

46. Multiple Choice, continued
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Multiple Choice, continued
Use the figure below to answer question 8. The figure below represents the human immunodeficiency virus.
8. The structure labeled Y represents which of the following?
F. capsid
G. envelope
H. RNA genome
J. reverse transcriptase

47. Multiple Choice, continued
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Multiple Choice, continued
Use the figure below to answer question 8. The figure below represents the human immunodeficiency virus.
8. The structure labeled Y represents which of the following?
F. capsid
G. envelope
H. RNA genome
J. reverse transcriptase

48. Chapter 24 Short Response
Standardized Test Prep
Short Response
Reverse transcriptase is an enzyme that catalyzes the synthesis of DNA from RNA.
Explain why RNA viruses must have reverse transcriptase to replicate.

49. Short Response, continued
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Short Response, continued
Reverse transcriptase is an enzyme that catalyzes the synthesis of DNA from RNA.
Explain why RNA viruses must have reverse transcriptase to replicate.
Answer: Retroviruses need reverse transcriptase to make DNA that can be inserted into the host cell’s genome for replication.

50. Chapter 24 Extended Response
Standardized Test Prep
Extended Response
Base your answers to parts A & B on the information below.
Viruses share several characteristics of living organisms. However, viruses are not considered to be living.
Part A Compare the characteristics viruses share with living organisms to the characteristics they do not share with living organisms.
Part B Would you anticipate more or fewer emerging viral diseases to appear in the future? Explain.

51. Extended Response, continued
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Extended Response, continued
Answer:
Part A Viruses have genetic material and proteins, as living organisms do. However, viruses cannot reproduce on their own and lack cytoplasm, organelles, metabolism and homeostasis, all of which are characteristics of living things.
Part B Sample answer: I would anticipate more emerging diseases due to increasing human population, which increases the chance of contact between humans and infected animals.