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Chapter Presentation
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Visual Concepts
Standardized Test Prep
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Chapter 24
Viruses
Table of Contents
Section 1 Viral Structure and Replication
Section 2 Viral Diseases
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Chapter 24
Section 1 Viral Structure and
Replication
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.
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Chapter 24
Section 1 Viral Structure and
Replication
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.
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Chapter 24
Section 1 Viral Structure and
Replication
Virus
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Chapter 24
Section 1 Viral Structure and
Replication
Characteristics of Viruses
• Viruses do not have all of the characteristics of life
and are therefore not considered to be living.
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Chapter 24
Section 1 Viral Structure and
Replication
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.
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Chapter 24
Section 1 Viral Structure and
Replication
Parts of a Virus
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Chapter 24
Section 1 Viral Structure and
Replication
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.
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Chapter 24
Section 1 Viral Structure and
Replication
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.
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Chapter 24
Section 1 Viral Structure and
Replication
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.
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Chapter 24
Section 1 Viral Structure and
Replication
Viral Replication, continued
• Replication in Viruses That Infect Prokaryotes
– Bacteriophages are viruses that infect bacteria.
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Chapter 24
Section 1 Viral Structure and
Replication
Viral Replication, continued
• Lytic Cycle
– Viruses can follow a lytic cycle, making new viral
particles immediately.
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Chapter 24
Section 1 Viral Structure and
Replication
Lytic Cycle
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Chapter 24
Section 1 Viral Structure and
Replication
Viral Replication, continued
• Lysogenic Cycle
– Viruses can follow a lysogenic cycle, becoming
part of the host genome and making new particles
later.
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Chapter 24
Section 1 Viral Structure and
Replication
Lysogenic Cycle
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Chapter 24
Section 1 Viral Structure and
Replication
The Lytic and
Lysogenic
Cycles
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Chapter 24
Section 1 Viral Structure and
Replication
Viral Replication, continued
• Viruses: Tools for Biotechnology
– Viruses are important tools for biotechnology.
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Chapter 24
Section 1 Viral Structure and
Replication
The Origin of Viruses
• Most scientists think viruses originated from
fragments of host-cell nucleic-acid material.
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Chapter 24
Section 2 Viral Diseases
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.
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Chapter 24
Section 2 Viral Diseases
Vectors of Viral Diseases
• Vectors, or hosts, of viral diseases include humans,
animals, and insects.
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Chapter 24
Section 2 Viral Diseases
Human Viral Diseases
• Viruses cause many human diseases, including the
common cold, flu, hepatitis, rabies, chickenpox,
certain types of cancer, and AIDS.
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Chapter 24
Section 2 Viral Diseases
Human Viral Diseases, continued
• Chickenpox and Shingles
– Chickenpox and shingles are caused by the same
varicella-zoster herpesvirus.
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Chapter 24
Section 2 Viral Diseases
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.
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Chapter 24
Section 2 Viral Diseases
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.
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Chapter 24
Section 2 Viral Diseases
AIDS (Acquired Immunodeficiency Syndrome)
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Chapter 24
Section 2 Viral Diseases
Human Viral Diseases, continued
• Viruses and Cancer
– Some viruses contain oncogenes that can cause
cancer, while other viruses convert protooncogenes, which usually control cell growth, to
oncogenes.
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Chapter 24
Section 2 Viral Diseases
Emerging Viral Diseases
• Emerging viruses usually infect animals isolated in
nature but can jump to humans when contact occurs
in the environment.
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Chapter 24
Section 2 Viral Diseases
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.
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Chapter 24
Section 2 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.
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Chapter 24
Section 2 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.
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Chapter 24
Section 2 Viral Diseases
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.
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Chapter 24
Section 2 Viral Diseases
Important
Viral
Diseases
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Chapter 24
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
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Chapter 24
Standardized Test Prep
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
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Chapter 24
Standardized Test Prep
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.
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Chapter 24
Standardized Test Prep
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.
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Chapter 24
Standardized Test Prep
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
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Chapter 24
Standardized Test Prep
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
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Chapter 24
Standardized Test Prep
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.
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Chapter 24
Standardized Test Prep
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.
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Chapter 24
Standardized Test Prep
Multiple Choice, continued
Use the figure below to
answer questions 5 - 6.
5. Which of the following
does the diagram
represent?
A. a virus
B. a prion
C. a viroid
D. a bacterium
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Chapter 24
Standardized Test Prep
Multiple Choice, continued
Use the figure below to
answer questions 5 - 6.
5. Which of the following
does the diagram
represent?
A. a virus
B. a prion
C. a viroid
D. a bacterium
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Chapter 24
Standardized Test Prep
Multiple Choice, continued
Use the figure below to
answer questions 5 - 6.
6. To which of the
following is label X
pointing?
F. envelope
G. nucleic acid
H. protein coat
J. cell membrane
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Chapter 24
Standardized Test Prep
Multiple Choice, continued
Use the figure below to
answer questions 5 - 6.
6. To which of the
following is label X
pointing?
F. envelope
G. nucleic acid
H. protein coat
J. cell membrane
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Chapter 24
Standardized Test Prep
Multiple Choice, continued
Complete the following analogy:
7. skin : person :: capsid :
A. virus
B. insect
C. fungus
D. bacterium
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Chapter 24
Standardized Test Prep
Multiple Choice, continued
Complete the following analogy:
7. skin : person :: capsid :
A. virus
B. insect
C. fungus
D. bacterium
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Chapter 24
Standardized Test Prep
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
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Chapter 24
Standardized Test Prep
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
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Chapter 24
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.
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Chapter 24
Standardized Test Prep
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.
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Chapter 24
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.
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Chapter 24
Standardized Test Prep
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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