Chapter 16 Viruses Masks: ©Yuriy Dyachyshyn/AFP/Getty Images Copyright © McGraw-Hill Education. All rights reserved. No reproduction or distribution without

Chapter 16 Viruses

Masks: ©Yuriy Dyachyshyn/AFP/Getty Images

Copyright © McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education.

Section 16.1

A virus is a small, infectious agent.It is genetic material wrapped in a protein coat.

Viruses Are Genes Wrapped in a Protein Coat


Section 16.1

Viruses are very small.

Figure 16.1


Section 16.1

Viruses come in many shapes and sizes.

Figure 16.2


Section 16.1

Note the genetic material and the protein

coat, or capsid.

Figure 16.2


Section 16.1

The host range of a virus is the kind of organisms or cells that it can infect.

The reservoir of a virus is the site where it exists in nature.

Mosquito: USDA


Section 16.1

The host range of a virus is the kind of organisms or cells that it can infect.

The reservoir of a virus is the site where it exists in nature.

The virus that causes Yellow Fever has a host range that includes humans and has a reservoir in mosquitoes.

Mosquito: USDA

Clicker Question #1

What organic molecules are found in all viruses?

A. Carbohydrates and lipidsB. Nucleic acids and carbohydratesC. Proteins and nucleic acidsD. Lipids and proteins

Flower: © Doug Sherman/Geofile/RF

Clicker Question #1

What organic molecules are found in all viruses?

A. Carbohydrates and lipidsB. Nucleic acids and carbohydratesC. Proteins and nucleic acidsD. Lipids and proteins


16.1 Mastering Concepts

What features do all viruses share?


Viral Replication Occurs in Five Stages

Section 16.2 Figure 16.3

2

1

3

Protein coatViral DNA

Virus

Receptor

Transcription

RNA

Translation

Coat proteinsand otherproteins

Viral genes contain instructions for making viruses.

Host cell

Viral DNA

Viral DNA

Replication

3

4

5

Viral genes contain instructions for making viruses. Viral replication:

1. Virus attaches to host.

2

1

3


Virus

Receptor

Transcription

RNA

Translation


Host cell

Viral DNA

Viral DNA

Replication

3

4

5




1. Virus attaches to host. 2. Virus penetrates cell and releases its DNA.

2

1

3


Virus

Receptor

Transcription

RNA

Translation


Host cell

Viral DNA

Viral DNA

Replication

3

4

5




1. Virus attaches to host. 2. Virus penetrates cell and releases its DNA.3. Host transcribes and translates viral DNA as if it were its own.

2

1

3


Virus

Receptor

Transcription

RNA

Translation


Host cell

Viral DNA

Viral DNA

Replication

3

4

5




1. Virus attaches to host. 2. Virus penetrates cell and releases its DNA.3. Host transcribes and translates viral DNA as if it were its own. 4. New viruses assemble.

2

1

3


Virus

Receptor

Transcription

RNA

Translation


Host cell

Viral DNA

Viral DNA

Replication

3

4

5




1. Virus attaches to host. 2. Virus penetrates cell and releases its DNA.3. Host transcribes and translates viral DNA as if it were its own. 4. New viruses assemble.5. Viruses leave the cell.

2

1

3


Virus

Receptor

Transcription

RNA

Translation


Host cell

Viral DNA

Viral DNA

Replication

3

4

5



Clicker Question #2

The five steps of viral replication are listed below in no particular order. If you were to arrange them in order, what step would come fourth?

A. AssemblyB. PenetrationC. ReleaseD. AttachmentE. Synthesis


Clicker Question #2

The five steps of viral replication are listed below in no particular order. If you were to arrange them in order, what step would come fourth?

A. AssemblyB. PenetrationC. ReleaseD. AttachmentE. Synthesis



What is the source of energy and raw materials for the synthesis of viruses in a host cell?


Viruses Might or Might Not Kill Host Cells


Viruses following the lytic pathway burst from their host cells.



Viruses following the lysogenic pathway “hide” as they replicate.



A prophage is the DNA of a lysogenic bacteriophage that is inserted into the host chromosome.



The prophage replicates as the cell divides.



At some point, viruses shift from a lysogenic infection to the lytic pathway.

Clicker Question #3

Viruses following the lytic pathway kill their host cell as they replicate.

A. TrueB. False


Clicker Question #3

Viruses following the lytic pathway kill their host cell as they replicate.

A. TrueB. False



What is the difference between a lytic and a lysogenic viral infection?


Some Viruses Might Linger for Years


HIV often remains latent; it does not immediately induce disease symptoms.

Some Viruses Might Linger for YearsWhen HIV does replicate, it requires a few more steps than “typical” viral replication.


Some Viruses Might Linger for YearsHIV has an RNA genome.It must produce a DNA copy before incorporating genes into the host’s DNA.


Drugs and Vaccines Help Fight Viral Infections

Section 16.4

Vaccines “teach” your immune system to recognize a virus.

Vaccination: ©Peter Cade/Iconica/Getty Images



Antiviral drugs are difficult to develop because of viruses high mutation rate.

Section 16.4 Vaccination: ©Peter Cade/Iconica/Getty Images



Antiviral drugs are difficult to develop because of viruses high mutation rate.

Antibiotics do NOT work against viruses.

Section 16.4 Vaccination: ©Peter Cade/Iconica/Getty Images


A vaccine has the molecular components of a virus but does not expose a person to the disease.

Section 16.4 Figure 16.6Eggs: ©Stephen D. Cannerelli/The Image Works


Scientists inject flu viruses into fertilized chicken eggs. The viruses replicate in the eggs.



Harvesting viruses from the eggs provides the raw materials to produce a vaccine.


Clicker Question #4

Will taking left-over antibiotics help you fight off the flu?

A. No, antibiotics will not work against flu viruses.B. Yes, but only if you have enough antibiotics left to kill all of the viruses.C. Yes, the antibiotics kill the bacteria that produce flu viruses.


Clicker Question #4

Will taking left-over antibiotics help you fight off the flu?

A. No, antibiotics will not work against flu viruses.B. Yes, but only if you have enough antibiotics left to kill all of the viruses.C. Yes, the antibiotics kill the bacteria that produce flu viruses.



What is a latent virus?


Viruses Also Cause Disease In Plants

The color patterns on these plants might look interesting to us, but they’re caused by viruses.

Section 16.5 Figure 16.7Leaf: ©Nigel Cattlin/Science Source; Tulip: ©Science VU/Visuals Unlimited


Viral infections often spread on the mouths of plant-eating insects.



Although plants do not have an active immune system, they may prevent viral infections from spreading by degrading viral mRNA.



How do viruses enter plant cells?


Viroids and Prions Are Simpler Than Viruses


Viroids are circles of RNA.

Plants: USDA Plant Virology Laboratory/Theodore Diener


Viroids do not encode proteins, but they do use host cells to replicate.

Section 16.6 Figure 16.8Plants: USDA Plant Virology Laboratory/Theodore Diener


The viroid shown here apparently interferes with the plant’s ability to make essential proteins.

Section 16.6 Figure 16.8Plants: USDA Plant Virology Laboratory/Theodore Diener


A prion is a normal cellular protein that sometimes adopts an abnormal shape.

Section 16.6 Figure 16.9Cow: ©Pixtal/age fotostock RF; Tissue: ©Ralph Eagle Jr./Science Source


Strangely, upon contact with an abnormally formed prion, a normal prion switches to the abnormal shape.



Prions are responsible for mad cow disease.


Clicker Question #5

Viroids and prions are infectious agents, but they are not types of viruses.

A. TrueB. False


Clicker Question #5

Viroids and prions are infectious agents, but they are not types of viruses.

A. TrueB. False



How are viroids and prions different from viruses?


Investigating Life: Scientific Detectives Follow HIV’s Trail


HIV likely originated as a related virus, called SIV, that is common in chimps.



Amino acid sequences of viral proteins helped researchers to build the evolutionary tree of these related viruses.



Humans who hunt chimps for meat were likely infected with SIV. The virus mutated in humans, giving rise to multiple strains of HIV.

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Chapter 16 Viruses Masks: ©Yuriy Dyachyshyn/AFP/Getty Images Copyright © McGraw-Hill Education. All rights reserved. No reproduction or distribution without