Chapter 19 Viruses

10) Based on the above table, which virus meets the requirements for a bacteriophage?

A

Class V

*Single-stranded RNA*. Serves as a template for *mRNA sythesis*. Has its own viral enzymes that carry out the viral RNA -> more RNA process. Includes filovirus (ebola virus), orthomyxovirus (influenza virus), paramyxovirus (measles, mumps), rhabdovirus (rabies)

Class VI

*Single-stranded RNA*. Serves as a template for DNA synthesis. Example: retrovirus (HIV, AIDS, leukemia)

29) What are prions? A) misfolded versions of normal brain protein B) tiny molecules of RNA that infect plants C) viral DNA that has had to attach itself to the host genome D) viruses that invade bacteria E) a mobile segment of DNA

A) misfolded versions of normal brain protein

prion

According to the leading hypothesis, a _____ is a misfolded form of a normal brain protein. When it gets into a cell with the normal form of the protein, it can convert the normal protein into its misfolded version, creating a chain reaction that increases their numbers.

11) A linear piece of viral DNA of 8 kb can be cut with either of two restriction enzymes (X and Y). These are subjected to electrophoresis and produce the following bands: Cutting the same 8 kb piece with both enzymes together results in bands at 4.0, 2.5, 1.0, and 0.5. Of the possible arrangements of the sites given below, which one is most likely?

B

after

Because viruses depend on cells for their own propagation, it is reasonable to assume that they evolved _______ the first cells appeared. Most molecular biologists favor the hypothesis that viruses originated from fragments of cellular nucleic acids that could move from one cell to another.

1) What characteristics of electron microscopes make them most useful for studying viruses? A) high energy electrons with high penetrance B) requirement that specimens be viewed in a vacuum C) necessity for specimens to be dry and fixed qE) use of magnetic fields to focus electrons

D) shorter wavelengths providing higher resolution

Class III

Double-stranded RNA; Includes Reovirus.

Class I/II

Double-stranded and single-stranded DNA viruses, respectively. Includes Adenovrius, papovavirus, herpesvrius, poxvirus, and parvovirus.

influenza virus

Its outer viral envelope is studded with glycoprotein spikes. Its genome consists of eight different RNA molecules, each wrapped in a helical capsid

mimivirus

Largest virus yet discovered. Has double-stranded DNA in an icosahedral capsid about the size of a small bacteria. What is significant about this virus is that some of its genes code for proteins involved in translation, DNA repair, protein folding, and polysaccharide synthesis.

horizontal transmission

In _____________, a plant is infected with the virus by an external source. Plants are more susceptible if their protective epidermis is damaged, perhaps by wind, chilling, injury, or insects. Insects are often carriers of viruses, transmitting disease from plant to plant.

virulent phage

a phage that only reproduces by the lytic cycle. The T4 phage is an example.

(Bacterio)phage T4

infect viruses. Has a complex capsid that consists of an icosahedral head in which DNA resides, a tail sheath "body" and tail fiber "legs".

viral infection

A __________ begins when the genome of the virus enters the host cell (via endocytosis by the cell, fusing of viral to cell membrane, or injection by tail). Once inside, the viral genome commandeers its host, reprogramming the cell to copy viral nucleic acid and manufacture proteins from the viral genome. The host provides nucleotides, ribosomes, tRNAs, amino acids, ATP, and other components for making the viral components dictated by viral genes. Most DNA viruses use the *DNA polymerases* of the host cell to synthesize new genomes along the templates provided by the viral DNA. RNA viruses use special *virus-encoded polymerases* that can use RNA as a template. The nucleic acid molecules and capsomeres then self-assemble into viral particles and exit the cell.

HIV

A retrovirus that causes AIDs, double-stranded viral DNA is synthesized from the viral RNA genome, and then integrated into the host cell's DNA, forming a *provirus*, where it is a permanent resident of the cell. mRNA is synthesized from the provirus, which may be used to put in new viruses or for synthesizing viral proteins.

host

A viral genome usually has more in common with the genome of its _______ than with those of viruses infecting other hosts. However, some viruses have genetic sequences that are quite similar to seemingly distantly related viruses (plant virus vs. animal virus). This genetic similarity may reflect the persistence of groups of viral genes that were *evolutionarily successful* during the early evolution of viruses and their eukaryotic host cells.

Refer to the treatments listed below to answer the following questions. You isolate an infectious substance that is capable of causing disease in plants, but you do not know whether the infectious agent is a bacterium, virus, viroid, or prion. You have four methods at your disposal that you can use to analyze the substance in order to determine the nature of the infectious agent. I. treating the substance with nucleases that destroy all nucleic acids and then determining whether it is still infectious II. filtering the substance to remove all elements smaller than what can be easily seen under a light microscope III. culturing the substance by itself on nutritive medium, away from any plant cells IV. treating the sample with proteases that digest all proteins and then determining whether it is still infectious 23) Which treatment could definitively determine whether or not the component is a viroid? A) I B) II C) III D) IV E) first II and then III

A) I

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

A) They cannot reproduce outside of a host cell

30) Which of the following is the best predictor of how much damage a virus causes? A) ability of the infected cell to undergo normal cell division B) ability of the infected cell to carry on translation C) whether the infected cell produces viral protein D) whether the viral mRNA can be transcribed E) how much toxin the virus produces

A) ability of the infected cell to undergo normal cell division

22) Which of the following can be effective in preventing viral infection in humans? A) getting vaccinated B) taking nucleoside analogs that inhibit transcription C) taking antibiotics D) applying antiseptics E) taking vitamins

A) getting vaccinated

6) Which of the following molecules make up the viral envelope? A) glycoproteins B) proteosugars C) carbopeptides D) peptidocarbs E) carboproteins

A) glycoproteins

7) Most human-infecting viruses are maintained in the human population only. However, a zoonosis is a disease that is transmitted from other vertebrates to humans, at least sporadically, without requiring viral mutation. Which of the following is the best example of a zoonosis? A) rabies B) herpesvirus C) smallpox D) HIV E) hepatitis virus

A) rabies

20) What is the name given to viruses that are single-stranded RNA that acts as a template for DNA synthesis? A) retroviruses B) proviruses C) viroids D) bacteriophages E) lytic phages

A) retroviruses

4) The host range of a virus is determined by A) the proteins on its surface and that of the host. B) whether its nucleic acid is DNA or RNA. C) the proteins in the host's cytoplasm. D) the enzymes produced by the virus before it infects the cell. E) the enzymes carried by the virus.

A) the proteins on its surface and that of the host.

28) The difference between vertical and horizontal transmission of plant viruses is that A) vertical transmission is transmission of a virus from a parent plant to its progeny, and horizontal transmission is one plant spreading the virus to another plant. B) vertical transmission is the spread of viruses from upper leaves to lower leaves of the plant, and horizontal transmission is the spread of a virus among leaves at the same general level.

A) vertical transmission is transmission of a virus from a parent plant to its progeny, and horizontal transmission is one plant spreading the virus to another plant.

21) What is the function of reverse transcriptase in retroviruses? A) It hydrolyzes the host cell's DNA. B) It uses viral RNA as a template for DNA synthesis. C) It converts host cell RNA into viral DNA. D) It translates viral RNA into proteins. E) It uses viral RNA as a template for making complementary RNA strands.

B) It uses viral RNA as a template for DNA synthesis.

17) Why do RNA viruses appear to have higher rates of mutation? A) RNA nucleotides are more unstable than DNA nucleotides. B) Replication of their genomes does not involve the proofreading steps of DNA replication. C) RNA viruses replicate faster. D) RNA viruses can incorporate a variety of nonstandard bases. E) RNA viruses are more sensitive to mutagens.

B) Replication of their genomes does not involve the proofreading steps of DNA replication.

26) Which of the following describes plant virus infections? A) They can be controlled by the use of antibiotics. B) They are spread throughout a plant by passing through the plasmodesmata. C) They have little effect on plant growth. D) They are seldom spread by insects. E) They can never be inherited from a parent.

B) They are spread throughout a plant by passing through the plasmodesmata.

27) Which of the following represents a difference between viruses and viroids? A) Viruses infect many types of cells, whereas viroids infect only prokaryotic cells. B) Viruses have capsids composed of protein, whereas viroids have no capsids. C) Viruses contain introns; viroids have only exons. D) Viruses always have genomes composed of DNA, whereas viroids always have genomes composed of RNA. E) Viruses cannot pass through plasmodesmata; viroids can.

B) Viruses have capsids composed of protein, whereas viroids have no capsids

3) Viral envelopes can best be analyzed with which of the following techniques? A) transmission electron microscopy B) antibodies against specific proteins not found in the host membranes C) staining and visualization with the light microscope D) use of plaque assays for quantitative measurement of viral titer E) immunofluorescent tagging of capsid proteins

B) antibodies against specific proteins not found in the host membranes

31) Antiviral drugs that have become useful are usually associated with which of the following properties? A) ability to remove all viruses from the infected host B) interference with the viral reproduction C) prevention of the host from becoming infected D) removal of viral proteins E) removal of viral mRNAs

B) interference with the viral reproduction

Use the following information to answer the following questions. In 1971, David Baltimore described a scheme for classifying viruses based on how the virus produces mRNA. The table below shows the results of testing five viruses for nuclease specificity, the ability of the virus to act as an mRNA, and presence (+) or absence (-) of its own viral polymerase. Virus Nuclease Sensitivity Genome as mRNA Polymerase A Dnase - - B Rnase + - C Dnase - + D Rnase - + E Rnase + - 8) Given Baltimore's scheme, a positive sense single-stranded RNA virus such as the polio virus would be most closely related to which of the following? A) T-series bacteriophages B) retroviruses that require a DNA intermediate C) single-stranded DNA viruses such as herpesviruses D) nonenveloped double-stranded RNA viruses E) linear double-stranded DNA viruses such as adenovirus

B) retroviruses that require a DNA intermediate

receptor sites, restriction enzymes

Bacteria have not been wiped out by phages because: 1. some bacterial mutants have ________ that are no longer recognized by a particular type of phage. 2. bacteria produce restriction endonucleases, or ____________, that recognize and cut up foreign DNA, including certain phage DNA. The bacteria's DNA is methylated in a way that prevents attack by its own rest. enzymes.

24) If you already knew that the infectious agent was either bacterial or viral, which treatment would allow you to distinguish between these two possibilities? A) I B) II C) III D) IV E) either II or IV

C) III

16) Which of the following statements describes the lysogenic cycle of lambda (_) phage? A) After infection, the viral genes immediately turn the host cell into a lambda-producing factory, and the host cell then lyses. B) Most of the prophage genes are activated by the product of a particular prophage gene. C) The phage genome replicates along with the host genome. D) Certain environmental triggers can cause the phage to exit the host genome, switching from the lytic to the lysogenic. E) The phage DNA is incorporated by crossing over into any nonspecific site on the host cell's DNA.

C) The phage genome replicates along with the host genome

19) A researcher lyses a cell that contains nucleic acid molecules and capsomeres of tobacco mosaic virus (TMV). The cell contents are left in a covered test tube overnight. The next day this mixture is sprayed on tobacco plants. Which of the following would be expected to occur? A) The plants would develop some but not all of the symptoms of the TMV infection. B) The plants would develop symptoms typically produced by viroids. C) The plants would develop the typical symptoms of TMV infection. D) The plants would not show any disease symptoms. E) The plants would become infected, but the sap from these plants would be unable to infect other plants.

C) The plants would develop the typical symptoms of TMV infection.

13) In many ways, the regulation of the genes of a particular group of viruses will be similar to the regulation of the host genes. Therefore, which of the following would you expect of the genes of the bacteriophage? A) regulation via acetylation of histones B) positive control mechanisms rather than negative C) control of more than one gene in an operon D) reliance on transcription activators E) utilization of eukaryotic polymerases

C) control of more than one gene in an operon

15) Which of the following terms describes bacteriophage DNA that has become integrated into the host cell chromosome? A) intemperate bacteriophages B) transposons C) prophages D) T-even phages E) plasmids

C) prophages

plasmids, transposons

Candidates for the original sources of viral genomes include: 1. __________: small, circular DNA molecules that are separate from chromosomes; found in bacteria and in eukaryote yeast, they can replicate independently of the rest of the cell and are occasionally transferred between cells.and transposable elements. 2. __________: DNA segments that can move from one location to another within a cell's genome. Both are mobile genetic elements, like viruses.

temperate phages

Capable of using both lytic and lysogenic cycles; lambda is an example. When the viral genome exits the bacterial chromosome, the lytic cycle is initiated. An environment signal, such as a certain chemical or high-energy radiation, usually triggers this switchover.

9) Based on the above table, which virus meets the Baltimore requirements for a retrovirus?

D

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

D) A large number of phages is released at a time.

25) Which treatment would you use to determine if the agent is a prion? A) I only B) II only C) III only D) IV only E) either I or IV

D) IV only

12) Which of the following accounts for someone who has had a herpesvirus-mediated cold sore or genital sore getting flare-ups for the rest of life? A) re-infection by a closely related herpesvirus of a different strain B) re-infection by the same herpesvirus strain C) co-infection with an unrelated virus that causes the same symptoms D) copies of the herpesvirus genome permanently maintained in host nuclei E) copies of the herpesvirus genome permanently maintained in host cell cytoplasm

D) copies of the herpesvirus genome permanently maintained in host nuclei

2) Viral genomes vary greatly in size and may include from four genes to several hundred genes. Which of the following viral features is most apt to correlate with the size of the genome? A) size of the viral capsomeres B) RNA versus DNA genome C) double versus single strand genomes D) size and shape of the capsid E) glycoproteins of the envelope

D) size and shape of the capsid

32) Which of the following series best reflects what we know about how the flu virus moves between species? A) An avian flu virus undergoes several mutations and rearrangements such that it is able to be transmitted to other birds and then to humans. B) The flu virus in a pig is mutated and replicated in alternate arrangements so that humans who eat the pig products can be infected. C) A flu virus from a human epidemic or pandemic infects birds; the birds replicate the virus differently and then pass it back to humans. D) An influenza virus gains new sequences of DNA from another virus, such as a herpesvirus; this enables it to be transmitted to a human host. E) An animal such as a pig is infected with more than one virus, genetic recombination occurs, the new virus mutates and is passed to a new species such as a bird, the virus mutates and can be transmitted to humans.

E) An animal such as a pig is infected with more than one virus, genetic recombination occurs, the new virus mutates and is passed to a new species such as a bird, the virus mutates and can be transmitted to humans.

33) Which of the following is the most probable fate of a newly emerging virus that causes high mortality in its host? A) It is able to spread to a large number of new hosts quickly because the new hosts have no immunological memory of them. B) The new virus replicates quickly and undergoes rapid adaptation to a series of divergent hosts. C) A change in environmental conditions such as weather patterns quickly forces the new virus to invade new areas. D) Sporadic outbreaks will be followed almost immediately by a widespread pandemic. E) The newly emerging virus will die out rather quickly or will mutate to be far less lethal.

E) The newly emerging virus will die out rather quickly or will mutate to be far less lethal.

18) Most molecular biologists think that viruses originated from fragments of cellular nucleic acid. Which of the following observations supports this theory? A) Viruses contain either DNA or RNA. B) Viruses are enclosed in protein capsids rather than plasma membranes. C) Viruses can reproduce only inside host cells. D) Viruses can infect both prokaryotic and eukaryotic cells. E) Viral genomes are usually more similar to the genome of the host cell than to the genomes of viruses that infect other cell types.

E) Viral genomes are usually more similar to the genome of the host cell than to the genomes of viruses that infect other cell types.

host range

Each type of virus can infect cells of only a limited variety of hosts, called the _______________ of the virus. This host specificity depends on the evolution of recognition systems by the virus. Viruses identify host cells by a "lock and key" fit between proteins on the outside of the virus and specific receptor molecules on the host's surface (which evolved for functions that benefit the host). Range is also limited to particular tissues.

lysogenic cycle

In the ____________, the phage genome replicates without destroying the host cell. The circular viral and host DNAs are cut by viral proteins and then joined together, forming a *prophage*. One of the viral genes codes for a protein that represses most other prophage genes. As a result, the phage genome is largely silent, but every time the host divides, the prophage is passed to (the viral DNA is replicated along with the host DNA) daughter cells. A few other prophage genes may also be expressed during lysogenic cycles. Expression of these genes may alter the host's phenotype, which can have medical significance.

lytic cycle

In the lytic cycle, the phage reproductive cycle culminates in the death of the host. One of the first phage genes translated after the viral DNA enters the host cell codes for an enzyme that degrades the host cell's DNA. The phage DNA is protected because it contains a modified form of cytosine that is not recognized by the enzyme.

vertical transmission

In______________, a plant inherits a viral infection from a parent. This may occur by asexual propagation or in sexual reproduction via infected seeds.

adenovirus

Infects the respiratory tracts of animals, consist of 252 identical proteins arranged into a capsid with 20 triangular—an icosahedron. It and other similar viruses are referred to as *icosahedral viruses*. Glycoproteins extend from its capsid. DNA makes up its genome.

Retrovirus

Make up class VI. Have the most complicated reproductive cycles. Have enzymes called *reverse transcriptase* which transcribes its RNA template into DNA, providing an RNA -> DNA template flow, which is usually the opposite.

nucleic acids

Perhaps the earliest viruses were naked bits of _____________ that passed between cells via injured cell surfaces. The evolution of capsid genes may have facilitated the infection of undamaged cells.

Class IV

Single-stranded RNA. Serves as mRNA. Includes picornavirus (common cold), cornoavirus, flavivirus (yellow fever, west nile virus), togavirus (rubella virus)

herpesvirus

Some viruses have envelopes that are not derived from plasma membrane. The envelope of the ____________ is derived from the nuclear envelope of the host. These double-stranded DNA viruses reproduce within the cell nucleus using viral and cellular enzymes to replicate and transcribe their DNA. In some cases, copies of the herpesvirus DNA remain behind as minichromosomes in the nuclei of certain nerve cells. There they remain for life until triggered by physical or emotional stress to leave the genome and initiate active viral production.

capsid, viral envelopes

The _______ is the protein shell enclosing the viral genome. They are built of a large number of protein subunits called capsomeres. The number of different kinds of proteins making up this is usually small. Some viruses have accessory structures to help them infect their hosts. A membranous envelope surrounds the capsids of flu viruses. These _________ are derived from the membrane of the host cell. They also have some host cell viral proteins and glycoproteins, as well as molecules of viral origin.

genome

The _________ of viruses may consist of double-stranded DNA, single-stranded DNA, double-stranded RNA, or single-stranded RNA, depending on the kind of virus. A virus is called a DNA virus or an RNA virus, according to the kind of nucleic acid that makes up its genome. The viral genome is usually organized as a single linear or circular molecule of nucleic acid.

mutation, species

The emergence of these new viral diseases is due to three processes: 1. _________: RNA viruses tend to have high mutation rates because replication of their nucleic acid lacks proofreading. Some mutations create new viral strains with sufficient genetic differences from earlier strains. 2. spread of existing viruses from one _________ to another 3. dissemination of a viral disease from a small, isolated population: Technological and social factors, including affordable international travel, blood transfusion technology, sexual promiscuity, and the abuse of intravenous drugs allowed a previously rare disease to become a global scourge.

influenza

There are three types of ________ virus: types B and C, which infect only humans and have never caused an epidemic, and type A, which infects a range of animals.

obligate intracellular

Viruses are __________________ parasites; they can only reproduce within a host cell.

Animal

________ viruses have both an envelope and RNA genome; in fact, nearly all animal viruses with RNA genomes have an envelope, as well as some DNA genomes.

Prions

_________ are infectious *proteins* that spread disease, and are likely transmitted in food. Their slow incubation period of at least ten years prevents sources of infection from being identified until long after the first cases appear. Prions are virtually indestructible. They are not destroyed or deactivated by heating to normal cooking temperatures--there is no known cure for them.

Viroids

__________ , smaller and simpler than even viruses, consist of tiny molecules of naked circular RNA that infect plants. Their several hundred nucleotides do not encode for proteins but can be replicated by the host's cellular enzymes. These small RNA molecules can disrupt plant metabolism and stunt plant growth, perhaps by causing errors in the regulatory systems that control plant growth. They show that *molecules* can act as infectious agents to spread disease.

Antibiotics, nucleosides

___________, which can kill bacteria by inhibiting enzymes or processes specific to bacteria, are powerless against viruses, which have few or no enzymes of their own. Most antiviral drugs resemble ___________ and interfere with viral nucleic acid synthesis. An example is acyclovir, which impedes herpesvirus reproduction by inhibiting the viral polymerase that synthesizes viral DNA. Azidothymidine (AZT) curbs HIV reproduction by interfering with DNA synthesis by reverse transcriptase.

TMV (tobacco mosaic virus)

rod-shaped with a capsid made up of one protein arranged in a helix. It and other similar viruses are referred to as *helical viruses*. RNA makes up its genome.