Mastering Concepts
15.1
1. How are viruses similar to and different from bacteria and eukaryotic cells?
Viruses are similar to bacteria and eukaryotic cells in that they have some kind of genetic
material and can evolve. But viruses are much smaller than both bacteria and eukaryotes, and
they cannot reproduce independently or respond to stimuli. Most biologists do not consider
viruses to be alive.
2. What features do all viruses share?
All viruses contain genetic information in the form of either DNA or RNA and are surrounded by
a protein coat.
3. What determines a virus’s host range?
A virus’s host range is the array of cells (hosts) that it will be able to infect. It is determined by
the presence of a specific receptor on the host cell.
4. Why is it important to know the reservoir species for a virus that causes disease in humans?
It is important to know the reservoir species for a virus that causes a disease in humans because
this knowledge can be used to control an outbreak of the viral disease.
5. How do viruses evolve?
Viruses evolve as their DNA or RNA mutate, and some variants are more successful that others
at infecting cells and leaving descendants.
6. What criteria do biologists use to classify viruses?
Viruses are classified based on several criteria including: whether they have DNA or RNA, their
structure, their method of replication, and the type of disease they cause.
15.2
1. What are the five steps in viral replication?
1) Attachment -- virus adheres to host cell receptor. 2) Penetration -- virus enters the cell. 3)
Synthesis -- multiple copies of the viral genome and proteins are produced by the host cell. 4)
Assembly -- the genetic information is packaged in a protein coat. 5) Release -- new viruses
leave cell.
2. What is the source of energy and raw materials for the synthesis of viruses in a host cell?
The source of energy and raw materials is the host cell’s ATP and its stores of ribosomes,
nucleotides, amino acids, and enzymes.
15.3
1. What is a lytic viral infection?
In a lytic viral infection the virus immediately replicates after infecting a host cell and then bursts
out of the host cell, killing it.
2. How is a lysogenic viral infection similar to and different from a lytic cycle?
A lytic viral infection is similar to a lysogenic viral cycle in that both allow viruses to reproduce.
Whereas a lytic infection destroys a cell immediately, a lysogenic virus inserts its DNA into the
host chromosome, where it is carried on into daughter cells. Eventually an environmental change
triggers reproduction of viruses and the lysogenic virus follows a lytic pathway, destroying the
host cell.
15.4
1. How can a person acquire and transmit a viral infection?
Viral infections can be acquired by inhalation of respiratory droplets or ingestion of
contaminated food and water. Some viruses are also acquired through the bloodstream and
transmitted by blood transfusion, sexual contact, or intravenous needles.
2. How do symptoms of a viral infection develop?
Different viral infections lead to different symptoms because the viruses destroy different kinds
of host cells.
3. What is a latent animal virus?
In a latent virus the host cells are infected but the virus is dormant and not replicating.
4. Describe how HIV replicates in host cells.
HIV uses the enzyme reverse transcriptase to convert its RNA into DNA and then insert itself
into the host cell's DNA. As new HIV are made they are released by budding which continues in
small numbers in infected cells of the lymph nodes.
5. How are some latent viral infections linked to cancer?
Latent viruses like the human papilloma virus signal host cells to divide in order to maintain
copies of the virus, and this can cause cancer.
6. How are viral infections treated and prevented?
Viral infections are difficult to treat because the viruses infect host cells and because viruses are
genetically variable. The best weapon is the vaccine; however, there are some antiviral drugs on
the market that focus on prevention of viral replication.
15.5
1. How do viruses enter plant cells and spread within a plant?
Because viruses have to penetrate thick waxy leaves and cell walls to infect the plant, the most
frequent cause of infection is through insect transmission as the insects feed from plant to plant.
2. What are some symptoms of a viral infection in plants?
Some symptoms include small dead spots on leaves, blotchy mottled leaves, abnormal growth, or
even streaking in some flowers.
15.6
1. How are viroids and prions different from viruses?
Viroids contain RNA only and no protein coat, while prions are only abnormally shaped proteins
with no genetic information.
2. How do viroids and prions cause disease?
Viroids interfere with the production of important proteins, but one abnormal protein prion
causes others to refold in a chain reaction that results in cell death.
3. What is the best way to avoid prion diseases?
The best way to avoid prion disease is to keep animal brains and spinal cords out of the human
food chain.
15.7
1. What hypothesis was Hahn’s team investigating, and how did they test their hypothesis?
Hahn’s team used epidemiological criteria to test the hypothesis that the source of the HIV-1 was
a form of SIV found in central chimpanzees. They tested the hypothesis by investigating DNA
sequences of HIV-1 and SIV in four chimpanzees that tested positive for SIV but had not
previously been used in AIDS testing. The researchers then looked for an overlap in the habitat
of wild central chimpanzees in the occurrence of HIV-1. The final part of their study tested for
the presence of SIV in wild chimpanzees.
2. How could researchers use a similar strategy to study the origin of a new influenza virus?
Researchers could use the epidemiological approach to investigate the origin of a new influenza
virus by comparing the genetic similarities between the infectious strain and strains in suspected
animal hosts. When the closest match was found the researchers could then investigate how
often individuals came in contact with the animal host and how many other individuals of the
species tested positive for the disease.
Write It Out
1. Your biology lab instructor gives you a Petri dish of agar covered with visible colonies. Your
lab partner says the colonies are viruses, but you disagree. How do you know the colonies are
bacteria?
Viruses do not produce colonies on agar; they require living host cells to reproduce. Many
bacteria, on the other hand, can live and reproduce using the nutrients present in agar.
2. Why is it inaccurate to refer to the “growth” of viruses?
Because viruses are not cells, they are not capable of growth either by adding more cells or by
enlarging pre-existing cells.
3. With a diameter of about 600 nm, mimiviruses are enormous compared with other viruses.
The mimivirus genome consists of about 1.2 million base pairs and encodes more than 1000
genes—more than some bacteria. If you encountered a mimivirus-like object in your research,
what sorts of studies could you carry out to determine whether the object was a virus or a
bacterium?
Using an electron microscope, you could search for structures (such as ribosomes) that occur in
bacteria but not viruses. You could also use physiological tests to search for evidence of
metabolism (which would occur only in bacteria), you could try to inactivate the object using
antibiotics that kill bacteria, or you could attempt to culture the object on agar.
4. Raccoons account for the most reported rabies cases in the eastern United States, whereas
most cases of rabies in the central and western United States involve bats, foxes, and skunks.
Develop a hypothesis that explains this observation. How would you test your hypothesis? Can
you think of a way to reduce the incidence of rabies in wild animals?
[Answers will vary].
5. Rhinoviruses replicate in the mucus-producing cells in a person’s nose, throat, and lungs,
causing the common cold. Papillomaviruses, which infect skin cells, cause growths called warts.
HIV infects T cells and causes AIDS. How do these three types of viruses “know” which human
cells to infect?
The virus “knows” which cells are suitable for infection based on the receptors found on the
surface of the host cells.
6. Imagine a hybrid virus with the capsid of virus X and the DNA of virus Y. Will a host cell
infected with this hybrid virus produce virus X, virus Y, a mix of virus X and Y, or hybrid
viruses? Explain your answer.
The host cell would produce virus Y since the capsid of virus X would contain no genetic
instructions.
7. Chapter 7 compared chromosomes to cookbooks and genes to recipes. How could you
incorporate viruses into this analogy?
Viruses would be very short cookbooks containing just enough recipes for one day of meals.
Alternatively, you can think of viruses as containing new recipes that are slipped into a preexisting cookbook.
8. Describe how a virus’s envelope can include lipid molecules that are also part of host cell
membranes.
The viral envelope is derived from the host cell membrane as viruses emerge from the host cell.
9. Why are lytic viruses better suited as agents of “phage therapy” than are lysogenic viruses?
How would you test whether such a treatment would be effective? Would you be willing to take
a “viral antibiotic”?
A lysogenic virus remains latent in the host cell, allowing the host cell to continue to survive for
a period of time. On the other hand, a lytic virus immediately kills the host cell, and the newly
released viruses keep killing until all the bacteria are dead. To test whether phage therapy would
work, you would have to find a population of patients with the same type of bacterial infection.
One group of patients would receive the phages, while the control group would receive a
placebo. The cure rate would be the dependent variable in the experiment. [Opinions may vary
on whether or not to take a “viral antibiotic”].
10. Search the Internet for information about the flu vaccine. Why is the flu shot administered
annually when many other vaccines last for years? Is it possible for the flu vaccine to give a
person influenza?
Because the influenza virus mutates so rapidly and often next year's virus will not be recognized
by this year’s vaccine. Because the vaccine contains just the significant protein portions of the
virus necessary for immune system recognition and not the genetic information, it is not possible
for the vaccine to transmit influenza.
11. Why do antibiotics such as penicillin kill bacteria but leave viruses unharmed?
Viruses lack the cell walls, ribosomes, and enzymes that antibiotic drugs target.
12. Several anti-HIV drugs are already on the market. List some reasons that we might need even
more new drugs to fight HIV in the future.
The main reason we may need more drugs to fight HIV is because when the virus transcribes its
RNA to DNA it doesn't have a proofreading step . This produces a lot of mutations that could
lead to drug resistance. Because the patient’s best chance of destroying the virus lies in a cocktail
of drugs, having more drugs could be an advantage.
13. Gene therapy aims to replace faulty or disease-causing genes with healthy DNA sequences.
How might you use a virus to deliver these new genes into a cell?
You could use a virus that inserts its genetic information into the host DNA. The virus would be
modified to include the DNA sequence desired. If key genes for viral replication were first
knocked out of the virus, it wouldn't be able to cause illness in the patient.
14. No one knows how or when viruses originated. Some scientists suggest that viruses were
once pieces of cellular DNA that eventually became independent. Why is this hypothesis hard to
test?
There would be nothing left behind in the fossil record of such an evolutionary event.
Furthermore the cellular DNA originating the fragments would be so ancient that it would be
difficult to find similar unchanged DNA to use in a comparison with the viral DNA.
15. How is a biological virus similar to and different from a computer virus?
Like a computer virus a biological virus can severely disrupt the functions of the host, and it
passes from human to human like a computer virus passes from computer to computer. It also
needs a human host to replicate like a computer virus needs a computer program. However,
biological viruses mutate quite frequently as they evolve; in addition, when their code is read a
physical molecule (DNA/RNA and proteins) is constructed.
16. The National Center for Biotechnology Information maintains an online list of viruses for
which genome sequence data are available.
Choose one and describe some discoveries that have come from research on this virus.
[Answers will vary].
Pull It Together
1. What are the four types and configurations of nucleic acids in viruses?
A viral genome can be made of double-stranded DNA, single-stranded DNA, double-stranded
RNA or single-stranded RNA.
2. How is a virus similar to and different from a bacterium, a viroid, and a prion?
A table is a useful way of setting up comparisons.
Virus
Composed of cells?
No
Plasma membrane?
No
Replicates without host cell?
No
Contains protein?
Yes
Contains genetic material?
Yes
Bacterium
Yes
Yes
Yes
Yes
Yes
Viroid
No
No
No
No
Yes
Prion
No
No
No
Yes
No
3. What events occur in each of the five stages of viral replication?
Attachment – the virus meets the cell and binds to its surface.
Penetration – the viral genetic material enters the cell.
Synthesis – the host makes the viral genetic material and proteins
Assembly – new viruses are assembled inside the cell
Release –new viruses are released from the infected cell
4. Distinguish between lytic and lysogenic infections
In a lytic infections the virus is produced and released by the host cell immediately after
infection, thus destroying the cell. Lysogenic infections occur when the viral genetic material
integrates into the host’s genome and is replicated with each cell division. A lysogenic infection
and can become lytic if the environmental cues are right.