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VirusesChapter 24

Table of Contents

Section 1 Viral Structure and Replication

Section 2 Viral Diseases



Section 1 Viral Structure and ReplicationChapter 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.




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.



Chapter 24

Virus





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.



Chapter 24

Parts of a Virus





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.




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.




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.





• Replication in Viruses That Infect Prokaryotes– Bacteriophages are viruses that infect bacteria.





• Lytic Cycle– Viruses can follow a lytic cycle, making new viral

particles immediately.



Chapter 24

Lytic Cycle






• Lysogenic Cycle– Viruses can follow a lysogenic cycle, becoming

part of the host genome and making new particles later.



Chapter 24

Lysogenic Cycle




Chapter 24

The Lytic and Lysogenic Cycles






• Viruses: Tools for Biotechnology– Viruses are important tools for biotechnology.




The Origin of Viruses

• Most scientists think viruses originated from fragments of host-cell nucleic-acid material.



Section 2 Viral DiseasesChapter 24

Objectives

• 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.




Vectors of Viral Diseases

• Vectors, or hosts, of viral diseases include humans, animals, and insects.




Human Viral Diseases

• Viruses cause many human diseases, including:– the common cold, – flu, – hepatitis, – rabies, – chickenpox, – certain types of cancer, – and AIDS.




Human Viral Diseases, continued

• Chickenpox and Shingles– Chickenpox and shingles are caused by the same

varicella-zoster herpesvirus.





• 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.





• 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.



Chapter 24

AIDS (Acquired Immunodeficiency Syndrome)






• Viruses and Cancer– Some viruses contain oncogenes that can cause

cancer, while other viruses convert proto-oncogenes, which usually control cell growth, to oncogenes.




Emerging Viral Diseases

• Emerging viruses usually infect animals isolated in nature but can jump to humans when contact occurs in the environment.




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.




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.




Prevention and Treatment, continued

• Drug Therapy– Antibiotics are ineffective against viral diseases. – Viral drugs, such as acyclovir, block specific steps

in viral replication.




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.



Chapter 24

Important Viral Diseases




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

Standardized Test PrepChapter 24



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





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.





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.





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





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





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.





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.





Use the figure below to answer questions 5 - 6.


5. Which of the following does the diagram represent?A. a virusB. a prionC. a viroidD. a bacterium





5. Which of the following does the diagram represent?A. a virusB. a prionC. a viroidD. a bacterium






6. To which of the following is label X pointing?

F. envelope

G. nucleic acid

H. protein coat

J. cell membrane






6. To which of the following is label X pointing?

F. envelope

G. nucleic acid

H. protein coat

J. cell membrane





Complete the following analogy:

7. skin : person :: capsid :

A. virus

B. insect

C. fungus

D. bacterium





Complete the following analogy:

7. skin : person :: capsid :

A. virus

B. insect

C. fungus

D. bacterium




Multiple Choice, continuedUse 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





8. The structure labeled Y represents which of the following?

F. capsid

G. envelope

H. RNA genome

J. reverse transcriptase

Use the figure below to answer question 8. The figure below represents the human immunodeficiency virus.



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.




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.





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.




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.

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