1. 18.1 Section Objectives – page 475
Identify the different kinds of viruses and their structures.
Compare and contrast the replication cycles of viruses.
18.1 Section Objectives – page 475

2. A vaccine is an injection of particles of viruses or bacteria that provide the human body with a defense against disease.

3. Section 18.1 Summary – pages 475-483
What is a Virus?
You’ve probably had the flu—influenza—at some time during your life.
Nonliving particles called viruses cause influenza.
Viruses are composed of nucleic acids enclosed in a protein coat and are smaller than the smallest bacterium.
Section 18.1 Summary – pages

4. Section 18.1 Summary – pages 475-483
What is a Virus?
Most biologists consider viruses to be nonliving because:
They don’t carry out respiration, grow, or develop.
All viruses can do is replicate—make copies of themselves—and they can’t even do that without the help of living cells.
Section 18.1 Summary – pages

5. A cell in which a virus replicates inside of is called the host cell.
What is a Host Cell?
A cell in which a virus replicates inside of is called the host cell.

6. Section 18.1 Summary – pages 475-483
What are viruses named after?
Viruses, such as rabies viruses and polioviruses, were named after the diseases they cause.
Other viruses were named for the organ or tissue they infect.
Section 18.1 Summary – pages

7. What is a bacteriophage?
A virus that infects a bacterium is called a bacteriophage or “phage” for short.

8. Section 18.1 Summary – pages 475-483
Viral Structure
A virus has an inner core of nucleic acid, either RNA or DNA
and an outer protein coat called a capsid.
Capsid
Nucleic acid
Section 18.1 Summary – pages

9. Section 18.1 Summary – pages 475-483
Viral Structure
Capsid
Some relatively large viruses, such as human flu viruses, may have an additional layer, called an envelope, surrounding their capsids.
Nucleic
acid
Envelope
Envelopes are composed primarily of the same materials found in the plasma membranes of all cells.
Section 18.1 Summary – pages

10. VIRUS SHAPES Polyhedral virus shape Human Papilloma Virus
Envelope studded with projections (flu and AIDS)

11. Long narrow helical shape (Tobacco Mosaic Virus)
VIRUS SHAPES
Long narrow helical shape (Tobacco Mosaic Virus)
Polyhedral-shaped head attached to a cylindrical tail with leg-like fibers

12. Section 18.1 Summary – pages 475-483
Before a virus can replicate, it must enter a host cell.
A virus recognizes and attaches to a host cell when one of its proteins interlocks with a molecular shape that is the receptor site on the host cell’s plasma membrane.
Virus
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13. Section 18.1 Summary – pages 475-483
Viral Attachment to Host
Each virus has a specifically shaped attachment protein. Therefore, each virus can usually attach to only a few kinds of cells.
In general, viruses are species specific, and some also are cell-type specific. For example, polio viruses normally infect only intestinal and nerve cells.
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14. Section 18.1 Summary – pages 475-483
The species specific characteristic of viruses is significant for controlling the spread of viral diseases.
For example, smallpox was easier to eradicate because it only affects humans (unlike the flu and West Nile that affect several types of animals.)
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15. Section 18.1 Summary – pages 475-483
Viral Replication
Once attached to the plasma membrane of the host cell, the virus enters the cell and takes over its metabolism.
Only then can the virus replicate.
Viruses have two ways of getting into host cells.
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16. Section 18.1 Summary – pages 475-483
Viral Replication
The virus may inject its nucleic acid into the host cell like a syringe injects a vaccine into your arm.
The capsid of the virus stays attached to the outside of the host cell.
Section 18.1 Summary – pages

17. Viral Replication
An enveloped virus (larger viruses) enter a host cell in a different way.
After attachment, the plasma membrane of the host cell surrounds the virus and produces a virus-filled vacuole inside the host cell’s cytoplasm.
Then, the virus bursts out of the vacuole and releases its nucleic acid into the cell.

18. Section 18.1 Summary – pages 475-483
The Lytic Cycle
The lytic cycle is a viral replication cycle in which a virus:
Click image to play movie
1. takes over a host cell’s genetic material
2. uses the host cell’s structures and energy to replicate the virus
3. then the host cell bursts, dies, and releases the replicated viruses.
Section 18.1 Summary – pages

19. Section 18.1 Summary – pages 475-483
The Lytic Cycle
Bacteriophage
Bacterial DNA
Nucleic
acid
Bacterial
host cell
B. Entry
A. Attachment
The bacteriophage
injects its nucleic acid
into the bacterial cell.
E. Lysis and Release
The host cell breaks open and
releases new virus particles.
C. Replication
D. Assembly
The host’s metabolic
machinery makes viral
nucleic acid and proteins.
New virus particles
are assembled.
Section 18.1 Summary – pages

20. Section 18.1 Summary – pages 475-483
The Lysogenic Cycle
A lysongenic cycle begins in the same way as a lytic cycle.
However, in a lysogenic cycle, instead of immediately taking over the host’s genetic material, the viral DNA is integrated into the host cell’s chromosome.
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21. Section 18.1 Summary – pages 475-483
The Lysogenic Cycle
B. Provirus Formation
A. Attachment and Entry
Provirus
Although
the provirus
is inactive,
it replicates
along with
the host cell’s
chromosome.
C. Cell Division
Bacterial host
chromosome
A lysogenic virus
injects its nucleic
acid into a bacterium.
The viral nucleic acid is called
a provirus when it becomes
part of the host’s chromosome.
LYSOGENIC CYCLE
LYTIC CYCLE
The provirus leaves
the chromosome.
The cell breaks
open releasing
viruses.
Viral nucleic acid and
proteins are made.
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22. Section 18.1 Summary – pages 475-483
The Lysogenic Cycle
Viral DNA that is integrated into the host cell’s chromosomes is called a provirus.
A provirus may not affect the functioning of its host cell, which continues to carry out its own metabolic activity.
However, every time the host cell reproduces, the provirus is replicated along with the host cell’s chromosome.
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23. Section 18.1 Summary – pages 475-483
The Lysogenic Cycle
Therefore, every cell that originates from an infected host cell has a copy of the provirus.
The lysogenic phase can continue for many years. However, at any time, the provirus can be activated and enter a lytic cycle.
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24. Section 18.1 Summary – pages 475-483
Viruses with Lysogenic Cycles
Many disease-causing viruses have lysogenic cycles.
Three examples of these viruses are herpes simplex I, herpes simplex II that causes genital herpes, and the hepatitis B virus that causes hepatitis B.
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25. Section 18.1 Summary – pages 475-483
Another lysogenic virus is the one that causes chicken pox.
Having chicken pox, which usually occurs before age ten, gives lifelong protection from another infection by the virus.
However, some chicken pox viruses may remain as proviruses in some of your body’s nerve cells.
Later in your life, these proviruses may enter a lytic cycle and cause a disease called shingles—a painful infection of some nerve cells.
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26. Section 18.1 Summary – pages 475-483
Releasing new viruses from host cells
Either lysis, the bursting of a cell, or exocytosis, the active transport process by which materials are expelled from a cell, release new viruses from the host cell.
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27. Section 18.1 Summary – pages 475-483
What is a retrovirus?
Many viruses have only RNA as their nucleic acid.
The RNA virus with the most complex replication cycle is the retrovirus
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28. Section 18.1 Summary – pages 475-483
What is a retrovirus?
Once inside a host cell, the retrovirus makes DNA from its RNA.
To do this, it uses reverse transcriptase, an enzyme it carries inside its capsid.
This enzyme helps produce double-stranded DNA from the viral RNA.
Then the double-stranded viral DNA is integrated into the host cell’s chromosome and becomes a provirus.
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29. Section 18.1 Summary – pages 475-483
Retrovirus
Retrovirus
RNA
DNA is made from
the viral RNA.
RNA
DNA
Reverse
transcriptase
Entering
cell
Provirus in
host chromosome
mRNA
Retrovirus Cycle
New virus parts
Exiting
cell
New virus
forming
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30. Section 18.1 Summary – pages 475-483
HIV
Once inside a human host, HIV infects white blood cells.
Normal white blood cell
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31. HIV
Newly made viruses are released into the blood stream by exocytosis and infect other white blood cells.

32. Section 18.1 Summary – pages 475-483
HIV
Infected host cells still function normally because the viral genetic material is a provirus that produces only a small number of new viruses at a time.
Because the infected cells are still able to function normally, an infected person may not appear sick, but they can still transmit the virus in their body fluids.
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33. Section 18.1 Summary – pages 475-483
Cancer and Viruses
Some viruses have been linked to certain cancers in humans and animals.
These viruses disrupt the normal growth and division of cells in a host, causing abnormal growth and creating tumors.
HPV
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34. Section 18.1 Summary – pages 475-483
Cancer and Viruses
Another example is Hepatitis B, which has been shown to play a role in developing liver cancer.
• this woman is not pregnant • she has hepatitis B and is suffering from liver cancer • photo taken in a Thailand refugee camp • this woman was a Cambodian refugee • she died 4 months after she arrived in the camp (average life   expectancy after diagnosis of liver cancer is 6 months)
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35. Section 18.1 Summary – pages 475-483
Prions
Prions are proteins that behave like viruses, but do not carry genetic information.
Prions are thought to act by causing other proteins to fold themselves incorrectly, resulting in improper functioning.
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36. Section 18.1 Summary – pages 475-483
Prions
Prions are responsible for many animal diseases, such as mad cow disease and its human equivalent, Creutzfeldt-Jakob disease.
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37. Section 18.1 Summary – pages 475-483
Viroids
Viroids are composed of a single circular strand of RNA with no protein coat.
The amount of viroid RNA is much less than the amount found in viruses.
Viroids have been shown to cause infectious diseases in several plants.
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38. Section 18.1 Summary – pages 475-483
The first virus to be identified was a plant virus, called tobacco mosaic virus, that causes disease in tobacco plants.
Tobacco mosaic virus causes yellow spots on tobacco leaves, making them unmarketable.
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39. Section 18.1 Summary – pages 475-483
Plant Viruses
Not all viral plant diseases are fatal or even harmful.
Some mosaic viruses cause striking patterns of color in the flowers of plants.
Rembrandt tulips
Section 18.1 Summary – pages

40. Question 1 A. Viruses don’t replicate. B. Viruses don’t respire.
Which of the following is NOT a reason that viruses are considered to be nonliving?
A. Viruses don’t replicate.
B. Viruses don’t respire.
C. Viruses don’t grow.
D. Viruses don’t develop.
The answer is A.
Section 1 Check

41. Question 2 A. RNA B. capsid C. DNA D. phage
Which is NOT a component of a virus?
A. RNA
B. capsid
C. DNA
D. phage
The answer is D.
Section 1 Check

42. Question 3
What two ways do viruses have of getting into host cells?
Answer
The virus can inject its nucleic acid into the host cell, or attach to the host cell’s membrane and become surrounded by the membrane and placed in a vacuole. The virus then bursts out of the vacuole and releases its nucleic acid into the cell.
Section 1 Check

43. Question 4 A. lysis and release B. replication C. assembly
In the lytic cycle, after the host’s metabolic machinery makes viral nucleic acid and proteins the next phase is _______.
A. lysis and release
B. replication
C. assembly
D. attachment
Section 1 Check

44. The answer is C. In the assembly phase, the new virus particles are assembled.
Section 1 Check

45. Question 5
With lysogenic viruses, what two phases of the lytic cycle are replaced by the lysogenic cycle?
A. entry and replication
B. replication and assemble
C. assembly and lysis and release
D. attachment and entry
Chapter Assessment

46. The answer is D. LYSOGENIC CYCLE LYTIC CYCLE Chapter Assessment
A. Attachment and Entry
LYSOGENIC CYCLE
LYTIC CYCLE
Chapter Assessment

47. Question 6
Explain why you can be infected with a virus but may have no symptoms of disease for years after the initial infection.
Answer
The virus enters a lysogenic phase remaining inactive but replicating along with the host cell’s chromosomes. Eventually, the virus enters a lytic phase where it destroys its host cells and causes symptoms of disease.
Chapter Assessment

48. Question 7
What is the difference between lysis and exocytosis with respect to host cells that contain viruses? (TX Obj 3; 4C)
Answer
Lysis, the bursting of the host cell, is caused when viruses break out of it. In exocytosis, the virus is enclosed in a vacuole that then fuses with the host cell’s plasma membrane. The virus is then released to the outside.
Chapter Assessment

49. Question 8
What is the importance of reverse transcriptase to a retrovirus? (TX Obj 3; 4C)
Answer
The enzyme reverse transcriptase allows the retrovirus to make DNA from its RNA so the DNA may attach to the chromosomes of the host cell and divide with the host cell.
Chapter Assessment

50. Question 9
Particles that are composed of proteins but have no nucleic acid to carry genetic information are _______.
A. proviruses
B. prions
C. viroids
D. retroviruses
The answer is B.
Chapter Assessment