1. 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. Objectives Chapter 24 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. What is happening in the photograph?
Newly assembled HIV viruses are leaving the T cell.
How did the viruses, pictured leaving the cell, enter the cell?
The genetic material from a single HIV particle entered the cell and was responsible for the production of all the viruses pictured.

4. 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.
Virus

5. Characteristics of Viruses
Section 1 Viral Structure and Replication
Chapter 24
Characteristics of Viruses
What are the characteristics of living organisms?
Cellular organization, reproduction, metabolism, homeostasis, heredity, responsiveness, and growth and development.
Viruses do not have all of the characteristics of life and are therefore not considered to be living.

6. Characteristics of Viruses, continued
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Characteristics of Viruses, continued
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.
Parts of a Virus

8. 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.

9. Review What have we learned about the size and structure of viruses?
Very small
Various sizes and shapes (helical, polyhedral, enveloped)
How are viruses classified?
RNA or DNA
Single or double stranded
Linear or circular
Capsid characteristics
Presence or absence of envelope

10. Viral Replication, continued
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Viral Replication, continued
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.

11. 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.
HIV, murine leukemia virus, feline immunodeficiency virus.

12. Reverse Transcriptase
Why is the enzyme contained in retroviruses called reverse transcriptase?
Catalyzes the transcription of DNA from RNA

13. Viral Replication Replication in Viruses That Infect Prokaryotes
Section 1 Viral Structure and Replication
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Viral Replication
Replication in Viruses That Infect Prokaryotes
Bacteriophages are viruses that infect bacteria.

14. Viral Replication, continued
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Viral Replication, continued
Lytic Cycle
Viruses can follow a lytic cycle, making new viral particles immediately.
Occurs when virus is active

15. 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.
Not active

16. The Lytic and Lysogenic Cycles
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The Lytic and Lysogenic Cycles

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

18. Review How do viruses reproduce? Directly produce RNA
Insert into host DNA
Lytic or lysogenic cycles

19. 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.

20. Objectives Chapter 24 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.

21. 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.

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

23. 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.

24. 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.

25. 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.

27. AIDS (Acquired Immunodeficiency Syndrome)
Section 2 Viral Diseases
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AIDS (Acquired Immunodeficiency Syndrome)

28. 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.

29. 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.

30. 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.

31. 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.

32. 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.

33. Section 2 Viral Diseases
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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.

34. Important Viral Diseases
Section 2 Viral Diseases
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Important Viral Diseases