20.1 Viruses

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Wendell Meredith Stanley
Wendell Meredith Stanley

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20.1 Viruses
Lesson Objectives
Explain how viruses reproduce.
Explain how viruses cause infection.
Lesson Summary
The Discovery of Viruses In 1935, the American biochemist Wendell Stanley isolated a
virus for the first time.
A virus is a particle made of nucleic acid, protein, and, in some cases, lipids.
A typical virus is composed of a core of DNA or RNA surrounded by a protein coat called a
Viruses that infect bacteria are called bacteriophages. They enter living cells and, once
inside, use the machinery of the infected cell to produce more viruses.
Viral Infections Viruses have two methods of infection once inside a host cell.
In a lytic infection, a virus enters a cell, makes copies of itself, and causes the cell to burst,
releasing new virus particles that can attack other cells. In the case of bacteriophage T4, viral
DNA directs the synthesis of new viruses using materials in the cell.
In a lysogenic infection, a virus integrates part of its DNA called a prophage into the DNA
of the host cell. The viral genetic information replicates along with the host cell’s DNA.
Eventually, the prophage will remove itself from the host cell DNA and make new virus
In a retrovirus, the genetic information is copied backward—from RNA to DNA instead of from
DNA to RNA. The virus that causes the disease AIDS is a retrovirus.
Viruses must infect a living cell in order to reproduce. Although viruses are parasites, they are not
made of cells and are not considered living things.
The Discovery of Viruses
1. What is a bacteriophage?
2. What are viruses?
3. What is a capsid?
4. How does a typical virus get inside a cell?
5. What occurs when viruses get inside cells?
Viral Infections
6. VISUAL ANALOGY In the visual analogy, why is the outlaw
locking up the sheriff, instead of the other way around?
7. THINK VISUALLY The diagram below shows the lytic cycle of a
viral infection. Label the bacterial DNA, host bacterium, viral DNA,
and virus. Then, circle the step that shows lysis of the host cell.
8. In a lysogenic infection, how can one virus infect many cells?
9. How is the common cold like the HIV virus?
Apply the Big idea
10. What would happen to a virus that never came in contact with a living cell? Explain your
20.2 Prokaryotes
Lesson Objectives
Explain how the two groups of prokaryotes differ.
Describe how prokaryotes vary in structure and function.
Explain the role of bacteria in the living world.
Lesson Summary
Classifying Prokaryotes The smallest and most common microorganisms are
prokaryotes, which are unicellular organisms that lack a nucleus. Prokaryotes are classified
either in domain Bacteria or domain Archaea.
They can be surrounded by a cell wall, which contains peptidoglycan. Inside the cell wall is a
cell membrane surrounding the cytoplasm.
Archaea look similar to bacteria, but are genetically closer to eukaryotes. Archaea lack
peptidoglycan and have different membrane lipids than bacteria.
Structure and Function Prokaryotes are identified by characteristics such as shape, the
chemical nature of their cell walls, the way they move, and the way they obtain energy.
Bacilli are rod-shaped. Cocci are spherical. Spirilla are spiral or corkscrew-shaped.
Most prokaryotes are heterotrophs. Others are autotrophs. Autotrophs may be
photoautotroph, or chemoautotrophs.
Prokaryotes that require a constant supply of oxygen to live are called obligate aerobes.
Those that cannot survive in oxygen are called obligate anaerobes. Organisms that can
survive without oxygen when necessary are called facultative anaerobes.
Prokaryotes reproduce asexually by binary fission, which results in two identical “daughter”
cells. Many prokaryotes can form endospores when conditions are unfavorable in order to protect
their DNA. They can also exchange genetic information by conjugation.
The Importance of Prokaryotes Prokaryotes are vital to maintaining the ecological
balance of the living world.
Some are decomposers that break down dead matter.
Others are producers that carry out photosynthesis.
Some soil bacteria convert natural nitrogen gas into a form plants can use through a process
called nitrogen fixation.
Humans use bacteria in industry, food production, and other ways.
Classifying Prokaryotes
For Questions 1–5, complete each statement by writing the correct word or words.
1. Unicellular organisms that lack a nucleus are called
2. The two different domains of prokaryotes are
3. A cell wall made of
protects some bacteria from damage.
4. Archaea are more closely related to
5. Some bacteria have a second
outside the cell wall.
6. THINK VISUALLY Use the box to draw and label a diagram of a typical bacterium.
Structure and Function
Write the letter of the correct answer on the line at the left.
7. What are rod-shaped bacteria called?
A. cocci
B. bacilli
C. spirilla
D. endospores
8. What are spherical bacteria called?
A. cocci
B. bacilli
C. spirilla
D. endospores
9. Whiplike structures on a bacterium that produce movement are called
A. pilli.
C. flagella.
B. capsids.
D. endospores.
10. Complete the table about the different ways prokaryotes obtain energy.
Energy Capture by Prokaryotes
Organism that carries out photosynthesis in a manner similar to that of plants
Organism that takes in organic molecules and then breaks them down
11. What occurs in the process of binary fission?
12. What occurs during conjugation?
The Importance of Prokaryotes
13. How do decomposers help the ecosystem recycle nutrients when a tree dies?
14. What would happen to plants and animals if decomposers did not recycle nutrients?
15. Why do all organisms need nitrogen?
16. Why is the process of nitrogen fixation important?
17. What kind of relationship do many plants have with nitrogen-fixing bacteria?
18. Describe three different ways that humans use bacteria.
Apply the Big idea
19. Suppose you were studying an infectious unicellular organism with a cell wall under a
microscope. How could you confirm that the organism was a prokaryote? How could
scientists determine whether it should be classified in domain Bacteria or domain Archaea?
20.3 Diseases Caused by Bacteria and
Lesson Objectives
Explain how bacteria cause disease.
Explain how viruses cause disease.
Define emerging disease and explain why emerging diseases are a threat to human health.
Lesson Summary
Bacterial Diseases Microorganisms that cause diseases are known as pathogens. Bacterial
pathogens can produce many diseases that affect humans and other animals. They do so in one of
two general ways:
They destroy living cells and tissues directly or by causing an immune response that destroys
They damage the cells and tissues of the infected organism directly by breaking down the
cells for host.
They release toxins (poisons) that travel throughout the body, interfering with the normal
activity of the host.
Many bacterial pathogens can be controlled by washing, using disinfectants, preparing and
storing food safely, or sterilizing exposed items. Bacterial diseases can be prevented and treated
through the following methods:
A vaccine is a preparation of weakened or killed pathogens or inactivated toxins. A vaccine
can prompt the body to produce immunity to the disease. Immunity is the body's natural way
of killing pathogens.
When a bacterial infection does occur, antibiotics can be used to fight the disease. Antibiotics
are compounds that block the growth and reproduction of bacteria.
Viral Diseases Viruses produce disease by directly destroying living cells or by affecting
cellular processes in ways that disrupt homeostasis. In many viral infections, viruses attack and
destroy certain body cells, causing the symptoms of the disease. Viral diseases in humans include
the common cold, influenza, AIDS, chicken pox, and measles. Viruses produce other serious
diseases in other animals and in plants. Protection against viruses, either by hygiene or
vaccination, is the best way to avoid viral illness. A handful of antiviral drugs have been
developed that help reduce the symptoms of specific viruses.
Emerging Diseases An unknown disease that appears in a population for the first time or a
well-known disease that suddenly becomes harder to control is called an emerging disease. The
increase of worldwide travel and food shipments is one reason new diseases are spreading.
Another is virus and bacteria evolution. Scientists are struggling to keep up with changes. They
recently discovered prions, which are disease-causing forms of proteins. Prions cause disease in
animals, including humans.
Bacterial Diseases
For Questions 1-5, complete each statement by writing the correct word or words.
1. One way bacteria can cause disease is by breaking down and damaging __________________
of the infected organism.
2. Bacteria can also cause disease by releasing ______________ that harm the body.
3. A(n) ______________ is a disease-causing agent.
4. One way to control bacterial growth is by subjecting the bacteria to high temperatures during a
process known as ____________________ .
5. A(n) ________________ is a preparation of weakened or killed pathogens or inactivated toxins
that can prompt the body to produce immunity to a disease.
6. What organs do the bacteria that cause tuberculosis typically damage?
7. What are antibiotics?
8. How are the causes of tuberculosis and diphtheria similar? How are they different?
9. Describe the similarities and differences of antibiotics and disinfectants.
10. Why should meat be cooked until it is well-done?
Match the bacterial control method with an example of the method
Bacterial Control Method
11. physical removal
A. Putting milk in a refrigerator
12. disinfectant
B. Using bleach to clean a countertop
13. safe food storage
C. Using boiling water to clean dishes
14. safe food processing
D. Washing hands
15. sterilization by heat
E. Boiling soup
Viral Diseases
16. What are some human diseases caused by viruses?
17. How do antiviral medications work? Why don’t they also kill host cells?
Write the letter of the correct answer on the line at the left.
_______ 18. A person has a low helper-T cell count. What viral disease does he or she most
likely have?
C. hepatitis B
D. chicken pox
19. A person has blister-like lesions on the skin. What viral disease does he or she
most likely have?
C. hepatitis B
D. chicken pox
Emerging Diseases
For Questions 20-24, write True if the statement is true. If the statement is false, change the
underlined word or words to make the statement true.
20. Pathogens are able to evolve over time.
21. A(n) noninfectious disease is an unknown disease that appears in a
population for the first time.
22. The widespread use of vaccines has led to the emergence of resistant
strains of bacteria.
23. Slight genetic changes would be needed for the bird flu virus to
become infectious to humans.
24. Scrapie is most likely caused by pathogens known as viroids.
Apply the Big idea
25. RNA viruses have shown an ability to evade antiviral drugs. How do you suppose this is
possible, when viruses are not alive? How may the reproductive methods of viruses help
the process?
Chapter Vocabulary Review
1. The picture shows three different bacteria shapes. Label each shape.
Match the term with its definition
2. lysogenic infection
3. prion
A. Compound that can block the growth and
reproduction of bacteria
4. bacteriophage
B. Misfolded protein that causes disease in animals
5. antibiotic
C. Bacteriophage DNA that is embedded in the host’s
6. virus
7. prokaryote
8. prophage
9. pathogen
10. lytic infection
11. endospore
12. binary fission
13. vaccine
D. Protective structure formed by a prokaryote when
growth conditions are unfavorable
E. A particle made of nucleic acid, protein, and in
some cases, lipids that can replicate only by
infecting living cells
F. Process in which viral DNA becomes part of a host
cell’s DNA
G. Disease-causing microorganism
H. Preparation of weakened or killed pathogens or
inactivated toxins used to produce immunity
I. Process in which a host cell bursts after being
invaded by a virus
J. Organism consisting of one cell that lacks a nucleus
K. Virus that infects bacteria
L. Process in which a bacterium replicates its DNA
and divides in half
Complete each statement by writing the correct word or words.
14. A protein coat surrounding a virus is a(n)
15. Viruses that have RNA as their genetic material are called
16. Some bacteria exchange genetic material through the process of
17. SARS, MRSA, Ebola, and bird flu are all examples of

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