1. Chapter 19
Viruses

2. Which of the following is a property of life shared by prokaryotic cells, eukaryotic cells, but not viruses?
nucleic acids used to store hereditary information
order and complexity in arrangement of biological molecules
the ability to process energy through metabolic reactions
the capacity to evolve
Answer: c
This question covers information in Concept 19.2 and can be used to start the lecture.

3. A Borrowed Life
Viruses called bacteriophages can infect and set in motion a genetic takeover of bacteria, such as Escherichia coli
Viruses lead “a kind of borrowed life” between life-forms and chemicals
The origins of molecular biology lie in early studies of viruses that infect bacteria

4. Why are viruses referred to as obligate parasites?
They cannot reproduce outside of a host cell.
Viral DNA always inserts itself into host DNA.
They invariably kill any cell they infect.
They can incorporate nucleic acids from other viruses.
They must use enzymes encoded by the virus itself.
Answer a

5. The Discovery of Viruses
Tobacco mosaic disease stunts growth of tobacco plants and gives their leaves a mosaic coloration
In the late 1800s, researchers hypothesized that a particle smaller than bacteria caused the disease
In 1935, Wendell Stanley confirmed this hypothesis by crystallizing the infectious particle, now known as tobacco mosaic virus (TMV)

6. Extracted sap from tobacco plant with tobacco mosaic disease 2
Figure 19.2
RESULTS 1
Extracted sap from tobacco plant with tobacco mosaic disease 2
Passed sap through a porcelain filter known to trap bacteria 3
Rubbed filtered sap on healthy tobacco plants
Figure 19.2 Inquiry: What causes tobacco mosaic disease? 4
Healthy plants became infected

7. Structure of Viruses Viruses are not cells
A virus is a very small infectious particle consisting of nucleic acid enclosed in a protein coat and, in some cases, a membranous envelope
Viral genomes are either
Double- or single-stranded DNA or RNA
The protein shell enclosing the viral genome is called the capsid, made up of individual units called capsomeres
Some viruses like the influenza virus have an envelope surrounding their capsid, consisting of host and viral protein particles

8. Which of the following molecules make up the viral envelope?
glycoproteins
proteosugars
carbopeptides
peptidocarbs
carboproteins
Answer: a

9. Membranous envelope Capsomere RNA RNA DNA Capsid DNA Head Tail sheath
Figure 19.3
Membranous envelope
Capsomere
RNA
RNA
DNA
Capsid
DNA
Head
Tail sheath
Capsomere of capsid
Tail fiber
Glycoprotein
Glycoproteins
18  250 nm
70–90 nm (diameter)
80–200 nm (diameter)
80  225 nm
Figure 19.3 Viral structure.
20 nm
50 nm
50 nm
50 nm
(a)
Tobacco mosaic virus
(b) Adenoviruses
(c) Influenza viruses
(d) Bacteriophage T4

10. What characteristics of electron microscopes make them most useful for studying viruses?
high energy electrons with high penetrance
requirement that specimens be viewed in a vacuum
necessity for specimens to be dry and fixed
shorter wavelengths providing higher resolution
use of magnetic fields to focus electrons
Answer d

11. Bacteriophages, also called phages, are viruses that infect bacteria
They have the most complex capsids found among viruses
Phages have an elongated capsid head that encloses their DNA
A protein tail piece attaches the phage to the host and injects the phage DNA inside

12. Bacteriophages were grown in a medium containing radioactive sulfur (35S) and radioactive phosphorous (32P), which are incorporated into proteins and DNA, respectively. If these phages were used to infect a bacterial culture, which isotope would be detected within the infected bacteria?
35S
32P
both
neither
Answer: b
This question relates to the Hershey and Chase experiment. Students should be able to apply the results to viral reproduction.

13. General Features of Viral Replicative Cycles
Once a viral genome has entered a cell, the cell begins to manufacture viral proteins
The virus makes use of host enzymes, ribosomes, tRNAs, amino acids, ATP, and other molecules
Viral nucleic acid molecules and capsomeres spontaneously self-assemble into new viruses

14. You have isolated viral particles from a patient, but you are not sure whether they are rhinoviruses or influenza viruses. The presence of which class of biological molecules would allow you to distinguish between the two types of virus?
RNA
phospholipids
proteins
glycoproteins
DNA
Answer: b
Students will need to recognize that influenza viruses have an envelope, but rhinoviruses do not. Refer to Figure 19.3 and Table 19.1.

15. Transcription and manufacture of capsid proteins Capsid
Figure 19.4 1
Entry and uncoating
VIRUS
DNA 3
Transcription and manufacture of capsid proteins
Capsid
HOST CELL 2
Replication
Viral DNA
mRNA
Viral DNA
Capsid proteins
Figure 19.4 A simplified viral replicative cycle. 4
Self-assembly of new virus particles and their exit from the cell

16. Replicative Cycles of Phages

17. The lytic cycle is a phage replicative cycle that culminates in the death of the host cell
A phage that reproduces only by the lytic cycle is called a virulent phage
Bacteria have defenses against phages, including restriction enzymes that recognize and cut up certain phage DNA
The lysogenic cycle replicates the phage genome without destroying the host
This integrated viral DNA is known as a prophage
An environmental signal can trigger the virus genome to exit the bacterial chromosome and switch to the lytic mode
Phages that use both the lytic and lysogenic cycles are called temperate phages

18. Which of the following is characteristic of the lytic cycle?
Viral DNA is incorporated into the host genome.
The virus-host relationship usually lasts for generations.
A large number of phages is released at a time.
Many bacterial cells containing viral DNA are produced.
The viral genome replicates without destroying the host.
Answer: c

19. Table 19.1a
Table 19.1 Classes of Animal Viruses (part 1)

20. Table 19.1b
Table 19.1 Classes of Animal Viruses (part 2)

21. What is the function of reverse transcriptase in retroviruses?
It converts host cell RNA into viral DNA.
It hydrolyzes the host cell's DNA.
It uses viral RNA as a template for making complementary RNA strands.
It translates viral RNA into proteins.
It uses viral RNA as a template for DNA synthesis.
Answer: e

22. Viral Envelopes
Many viruses that infect animals have a membranous envelope
Viral glycoproteins on the envelope bind to specific receptor molecules on the surface of a host cell
Some viral envelopes are formed from the host cell’s plasma membrane as the viral capsids exit
Other viral membranes form from the host’s nuclear envelope and are then replaced by an envelope made from Golgi apparatus membrane

23. Capsid and viral genome enter the cell RNA
Figure 19.7
Capsid
Capsid and viral genome enter the cell
RNA
Envelope (with glycoproteins)
HOST CELL
Viral genome (RNA)
Template
mRNA
Capsid proteins ER
Copy of genome (RNA)
Figure 19.7 The replicative cycle of an enveloped RNA virus.
Glyco- proteins
New virus

24. RNA as Viral Genetic Material
The broadest variety of RNA genomes is found in viruses that infect animals
Retroviruses use reverse transcriptase to copy their RNA genome into DNA
HIV (human immunodeficiency virus) is the retrovirus that causes AIDS (acquired immunodeficiency syndrome)
The viral DNA that is integrated into the host genome is called a provirus
Unlike a prophage, a provirus remains a permanent resident of the host cell
The host’s RNA polymerase transcribes the proviral DNA into RNA molecules
The RNA molecules function both as mRNA for synthesis of viral proteins and as genomes for new virus particles released from the cell

25. People with nonfunctional chemokine membrane receptors on their helper T cells as a result of a genetic mutation are immune to HIV infection. Why?
HIV cannot enter the host cell.
Reverse transcriptase cannot transcribe RNA to DNA.
Viral mRNA cannot be transcribed from the integrated provirus.
Viruses cannot bud from the host cell.
Answer: a
This question examines the concept that viruses are able to recognize and attach to host cells via receptor proteins and recognition molecules on the plasma membrane.

26. Figure 19.8
Membrane of white blood cell
Glycoprotein
Viral envelope
HIV
Capsid
RNA (two identical strands)
HOST CELL
Reverse transcriptase
HIV
Reverse transcriptase
Viral RNA
RNA-DNA hybrid
DNA
0.25 m
HIV entering a cell
NUCLEUS
Provirus
Chromosomal DNA
RNA genome for the next viral generation
Figure 19.8 The replicative cycle of HIV, the retrovirus that causes AIDS.
mRNA
New virus
New HIV leaving a cell

27. reverse transcription of DNA from RNA
AZT is a nucleoside analog used to treat HIV infections. It is a thymine (T) nucleoside with an azide group instead of the hydroxyl group found in typical thymine nucleosides. Which step does AZT hamper in the reproductive cycle of the HIV virus?
entry into the cell
reverse transcription of DNA from RNA
transcription of RNA from proviral DNA
viral assembly within the cell
Answer: b
Because AZT is a thymine nucleoside, it will specifically block reverse transcription from RNA to DNA.

28. Evolution of Viruses
Viruses do not fit our definition of living organisms
Since viruses can replicate only within cells, they probably evolved as bits of cellular nucleic acid
Candidates for the source of viral genomes are plasmids, circular DNA in bacteria and yeasts, and transposons, small mobile DNA segments
Plasmids, transposons, and viruses are all mobile genetic elements
Mimivirus, a double-stranded DNA virus, the largest virus yet discovered, is the size of a small bacterium
There is controversy about whether this virus evolved before or after cells

29. Viruses, viroids, and prions are formidable pathogens in animals and plants
Diseases caused by viral infections affect humans, agricultural crops, and livestock worldwide
Smaller, less complex entities called viroids and prions also cause disease in plants and animals, respectively
Viruses may damage or kill cells by causing the release of hydrolytic enzymes from lysosomes
Some viruses cause infected cells to produce toxins that lead to disease symptoms
Others have molecular components such as envelope proteins that are toxic

30. Vaccines are harmless derivatives of pathogenic microbes that stimulate the immune system to mount defenses against the harmful pathogen
Vaccines can prevent certain viral illnesses
Viral infections cannot be treated by antibiotics
Antiviral drugs can help to treat, though not cure, viral infections
Recently, a general outbreak (epidemic) of a flu-like illness appeared in Mexico and the United States, caused by an influenza virus named H1N1
Flu epidemics are caused by new strains of influenza virus to which people have little immunity

31. Viral diseases in a small isolated population can emerge and become global
New viral diseases can emerge when viruses spread from animals to humans
Viral strains that jump species can exchange genetic information with other viruses to which humans have no immunity
These strains can cause pandemics, global epidemics
The 2009 flu pandemic was likely passed to humans from pigs; for this reason it was originally called the “swine flu”

32. Viral Diseases in Plants
More than 2,000 types of viral diseases of plants are known and cause spots on leaves and fruits, stunted growth, and damaged flowers or roots
Most plant viruses have an RNA genome
Plant viruses spread disease in two major modes
Horizontal transmission, entering through damaged cell walls
Vertical transmission, inheriting the virus from a parent

33. Which of the following most likely describes the vertical transmission of a plant virus?
The plant shows symptoms of disease after being grazed on by herbivores.
Sap from one plant is rubbed on the leaves of a second plant; both plants eventually show disease symptoms.
Seeds are planted and reared under protected conditions, but mature plants show disease symptoms.
After a gardener prunes several plants with the same shears, they all show disease symptoms.
Answer: c
Students will need to distinguish between horizontal transmission of disease and vertical transmission.

34. Viroids and Prions: The Simplest Infectious Agents
Viroids are small circular RNA molecules that infect plants and disrupt their growth
Prions are slow-acting, virtually indestructible infectious proteins that cause brain diseases in mammals
Prions propagate by converting normal proteins into the prion version
Scrapie in sheep, mad cow disease, and Creutzfeldt-Jakob disease in humans are all caused by prions

35. You have isolated a sample from an animal suffering from an unknown disease. Which of the following tests would allow you to determine that the animal has a prion disease?
Pass the sample through a filter to remove bacteria and see if the filtrate is still infectious.
Treat the sample with nuclease and see if it is still infectious.
Treat the sample with protease and see if it is still infectious.
Heat the sample to normal cooking temperatures and see if it is still infectious.
Answer: c
You may need to explain what proteases and nucleases are before the students answer the question.

36. Prion Original prion Aggregates of prions New prion Normal protein
Figure 19.11
Prion
Original prion
Aggregates of prions
New prion
Normal protein
Figure Model for how prions propagate.