Chapter 16
Viruses
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Viruses are much smaller and simpler than cells
Section 16.1
Table 16.1
TABLE 16.1 Cells and Viruses Compared
Feature | Cells | Viruses |
Size | Typically 1-100 μm | Typically ~80 nm |
Genetic Material | DNA | DNA or RNA |
Protein coat (capsid) | No | Yes |
Cell membrane | Yes | No |
Viral envelope | No | Some viruses |
Nucleus/membrane-bounded organelles | Eukaryotes only | No |
Cytoplasm | Yes | No |
Ribosomes | Yes | No |
Enzymes | Yes | Some viruses |
Metabolism | Yes | No |
Independent replication | Yes | No |
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Viruses are genes wrapped in a protein coat
Section 16.1
A virus is a small, infectious agent made of nucleic acid and protein.
Figure 16.1
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Viruses come in many shapes and sizes
Section 16.1
Each virus has genetic material inside, surrounded by a protein coat, or capsid.
Figure 16.2
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Viruses infect specific host cells
Section 16.1
The proteins surrounding each virus are shaped to bind with proteins on host cells.
Each virus has specific target cells that it can match up with to infect.
This virus is a bacteriophage – it can only bind to and infect bacterial cells.
Figure 16.2
©Eye of Science/Science Source
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Each virus has a host range
Section 16.1
The host range is the types of cells or organisms a virus can infect.
Viruses can infect plants, bacteria, and human host cells.
Figure 16.2
(a): ©G. Wanner/ScienceFoto/Getty Images; (b): ©Eye of Science/Science Source; (c): CDC/Dr. Erskine Palmer & Byron Skinner; (d): ©G. Murti/Science Source; (e): ©James Cavallini/BSIP SA/Alamy
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Reservoir hosts help spread viruses to other species
Section 16.1
The Zika virus infects human cells and causes birth defects.
It is carried and spread by mosquitoes, without harming the mosquitoes.
The mosquito is a reservoir host, providing a continual source of Zika virus infection for humans.
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Viruses use cells to replicate
Section 16.2
Viral genes contain instructions for making viruses.
Viruses cannot replicate on their own, though. They can only replicate by infecting host cells.
Figure 16.3
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Viruses infect the host cell
Section 16.2
Viral replication:
Figure 16.3
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Virus nucleic acids take over the host cell
Section 16.2
Viral replication:
Figure 16.3
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The host cell produces new viruses
Section 16.2
Viral replication:
Figure 16.3
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New viruses are released
Section 16.2
Viral replication:
Figure 16.3
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Viruses can kill cells right away
Section 16.3
Viruses following the lytic pathway burst from their host cells soon after infection.
Figure 16.4
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Viruses can lurk inside cells for years
Section 16.3
Viruses following the lysogenic pathway “hide” as they replicate, without damaging the host cell.
Figure 16.4
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Virus DNA can combine with host DNA
Section 16.3
A prophage is the DNA of a lysogenic bacteriophage that is inserted into the host chromosome. The prophage replicates when cells divide.
Figure 16.4
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Viruses might or might not kill host cells
Section 16.3
Depending on the condition of the host cell, viruses can shift from a lysogenic infection to the lytic pathway.
Figure 16.4
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Animal viruses can cause illnesses
Section 16.4
When virally infected cells begin to die, symptoms will reflect they type of cells that are destroyed.
Figure 16.5
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Example: HIV infects human T cells
Section 16.4
T cells are needed in the immune system.
Patients infected with HIV show symptoms that reflect a defective immune response (AIDS).
Figure 16.5
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Some viruses might linger for years
Section 16.4
HIV often remains latent; it does not immediately induce disease symptoms.
Figure 16.5
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Replication of RNA viruses is complex
Section 16.4
HIV contains RNA as its genetic material.
When HIV does replicate, it requires a few more steps than DNA virus replication does.
Figure 16.5
Attachment and penetration
Synthesis
Assembly
Release
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Viral RNA is converted to DNA
Section 16.4
HIV must produce a DNA copy of its RNA before incorporating genes into the host’s DNA.
An enzyme called reverse transcriptase carries out the reaction.
Figure 16.5
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Vaccines help prevent viral infections
Section 16.4
A vaccine contains inactive virus or viral proteins.
These molecular components of a virus produce an immune response without causing a disease.
Figure 16.6
©Stephen D. Cannerelli/The Image Works
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Vaccines jump-start the immune system
Section 16.4
Vaccines “teach” the immune system to recognize a virus.
If you are exposed to the real virus after being vaccinated, your body will be primed to destroy it before it can infect your cells.
Figure 16.6
©Stephen D. Cannerelli/The Image Works
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To make a vaccine, scientists inoculate host cells with the virus
Section 16.4
Scientists inject flu viruses into fertilized chicken eggs which are the host cells. The viruses replicate in the eggs.
Harvesting viruses from the eggs provides the raw materials to produce a vaccine.
Figure 16.6
©Stephen D. Cannerelli/The Image Works
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Human papilloma viruses (HPV) cause cancer
Section 16.4
Viruses like HPV cause cells to replicate too much, instead of killing the cells.
HPV is sexually transmitted, and can infect epithelial and cervical cells. It causes cervical, oral, and anal cancers.
HPV infection can be prevented by vaccine, condoms, and abstinence.
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Anti-HIV drugs slow viral replication
Section 16.4
For patients already infected with a virus, vaccines won’t be helpful.
Antiviral drugs prevent the virus from interacting with the host cell and using it for replication.
Figure 16.5
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Anti-HIV drugs block the replication steps
Section 16.4
Drugs can block viral binding to cells, reverse transcription, integration of viral DNA, or viral release.
Antiviral “cocktails” are combinations of different drugs that interfere with one virus.
Figure 16.5
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Antibiotics kill bacteria, not viruses
Section 16.4
Antibiotics are drugs that block bacterial proteins from functioning, which prevents bacterial cells from carrying out life’s processes.
Antibiotics are useful for treating bacterial infections.
Figure 17.6
(a): ©Michael Abbey/Science Source; (b): ©CNRI/Science Source; (c): ©Kwangshin Kim/Science Source
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Antiviral gene therapy is a potential treatment strategy
Section 16.4
Gene therapy would change the DNA in human cells, so they cannot make proteins the virus needs for infecting them.
The T cell shown here is missing one of the proteins that HIV binds to.
Figure 16.B
T cell with coreceptor: HIV can infect
T cell without coreceptor: no HIV infection
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Viruses cause disease in plants
Section 16.5
The color patterns on these plants might look interesting to us, but they’re caused by viruses. The pale areas are dead plant cells.
Viral infections often spread on the mouths of plant-eating insects.
Figure 16.7
(a): ©Nigel Cattlin/Science Source; (b): ©Photodisc/Getty Images/Getty Images RF
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Plants can fight viruses too
Section 16.5
Although plants do not have an active immune system, their cells can undergo apoptosis (suicide) when infected.
Some plants may prevent viral infections from spreading by degrading viral mRNA.
Figure 16.7
(a): ©Nigel Cattlin/Science Source; (b): ©Photodisc/Getty Images/Getty Images RF
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Viroids are infectious RNA molecules
Section 16.6
Viroids are circles of RNA that can infect cells.
Viroids do not encode proteins, but they do use host cells to replicate.
Figure 16.8
(healthy): ©SerAlexVi/Getty Images RF; (infected): ©Nigel Cattlin/Alamy
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Viroids can cause diseases
Section 16.6
The potato spindle viroid kills plant cells by preventing production of essential plant proteins.
Figure 16.8
(healthy): ©SerAlexVi/Getty Images RF; (infected): ©Nigel Cattlin/Alamy
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Prions are infectious protein molecules
Section 16.6
A prion is a normal cellular protein that sometimes adopts an abnormal shape.
Upon contact with an abnormally formed prion, a normal prion switches to the abnormal shape.
Figure 16.9
(a, cow): ©Pixtal/age fotostock RF; (a, tissue): ©Ralph Eagle Jr./Science Source
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Prions can cause diseases
Section 16.6
Prions are responsible for mad cow disease and some cases of Creutzfeldt-Jakob disease in people.
It is very difficult to destroy prions. They are unaffected by heat, radiation, and chemicals.
Figure 16.9
(a, cow): ©Pixtal/age fotostock RF; (a, tissue): ©Ralph Eagle Jr./Science Source
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