19 Viruses

Reverse transcription, carried out by retroviruses, is the process by which __________. a. RNA information is "read" to form a protein molecule b. DNA information is copied into RNA c. RNA information is copied into DNA d. DNA is duplicated e. information is copied from a protein molecule into RNA

RNA information is copied into DNA Retroviruses are equipped with an enzyme called reverse transcriptase, which makes a DNA copy of an RNA template, providing information flow from RNA to DNA.

Prophage

a phage genome that has been inserted into a specific site on a bacterial chromosome

Viroid

a plant pathogen consisting of a molecule of naked, circular RNA a few hundred nucleotides long

Lysogenic Cycle

a type of phage replicative cycle in which the viral genome becomes incorporated into the bacterial host chromosome as a prophage, is replicated along with the chromosome, and does not kill the host

Lytic Cycle

a type of phage replicative cycle resulting in the release of new phages by lysis (and death) of the host cell

Provirus

a viral genome that is permanently inserted into a host genome

Bacteriophage(s)

a virus that infects bacteria; also called a phage

Epidemic

a widespread outbreak of a disease

Prion

an infectious agent that is a misfolded version of a normal cellular protein; prions appear to increase in number by converting correctly folded versions of the protein to more prions

Virus

an infectious particle incapable of replicating outside of a cell, consisting of an RNA or DNA genome surrounded by a protein coat (capsid) and, for some viruses, a membranous envelope

Viruses that infect bacteria are called __________. a. retroviruses b. proviruses c. bacteriophages d. capsomeres e. bacterioviruses

bacteriophages Viruses that infect bacteria are called bacteriophages or simply phages.

To cause a human pandemic, the H5N1 avian flu virus would have to a. arise independently in chickens in North and South America. b. spread to primates such as chimpanzees. c. become much more pathogenic. d. develop into a virus with a different host range. e. become capable of human-to-human transmission.

become capable of human-to-human transmission.

HIV (human immunodeficiency virus)

the infectious agent that causes AIDS. HIV is a retrovirus

Capsid

the protein shell that encloses a viral genome. It may be rod-shaped, polyhedral, or more complex in shape

Host Range

the limited number of species whose cells can be infected by a particular virus

Phage

a virus that infects bacteria; also called a bacteriophage

Human Immunodeficiency Virus (HIV)

the infectious agent that causes AIDS (acquired immunodeficiency syndrome); HIV is a retrovirus

Pandemic

a global epidemic

Vaccine

a harmless variant or derivative of a pathogen that stimulates a host's immune system to mount defenses against the pathogen

Viral Envelope

a membrane, derived from membranes of the host cell, that cloaks the capsid, which in turn encloses a viral genome

Retrovirus

an RNA virus that replicates by transcribing its RNA into DNA and then inserting the DNA into a cellular chromosome; an important class of cancer-causing viruses

Restriction Enzyme

an endonuclease (type of enzyme) that recognizes and cuts DNA molecules foreign to a bacterium (such as phage genomes); the enzyme cuts at specific nucleotide sequences (restriction sites)

Which of the following can a virus do without a host cell? a. synthesize proteins b. produce nucleotides for use in replication and transcription c. produce ATP for energy d. transcribe DNA e. None of the listed responses is correct.

None of the listed responses is correct. Viruses are obligate intracellular parasites. They require a host cell to carry out all of the above cellular processes (and more).

A bacterium is infected with an experimentally constructed bacteriophage composed of the T2 phage protein coat and T4 phage DNA. The new phages produced would have a. T4 protein and T4 DNA. b. T2 protein and T4 DNA. c. T2 protein and T2 DNA. d. a mixture of the DNA and proteins of both phages. e. T4 protein and T2 DNA.

T4 protein and T4 DNA.

Why are phages useful in treating bacterial infections in humans? a. Because of their host specificity, they only attack bacteria. They do not affect eukaryotic cells. b. Because of their specificity, a phage can be used that will only infect the pathogenic bacteria, leaving helpful bacteria alone. c. Because phages are free to evolve, bacteria are less likely to develop a lasting resistance to them. d. They are not useful, and the practice has been banned in all countries. e. The first three answers are correct.

The first three answers are correct. Phage therapy is useful because of its host specificity and ability to evolve to avoid the problems of resistance.

A microbiologist analyzes chemicals obtained from an enveloped RNA virus that infects monkeys. He finds that the viral envelope contains a protein characteristic of monkey cells. Which of the following is the most likely explanation? a. Its presence is a result of the monkey's immunological response. b. The virus fools its host by mimicking its proteins. c. The virus forced the monkey cell to make proteins for its envelope. d. The virus is a prophage. e. The viral envelope forms as the virus leaves the host cell.

The viral envelope forms as the virus leaves the host cell. An enveloped virus is wrapped in host plasma membrane or nuclear membrane (which contains host proteins).

Temperate Phage

a phage that is capable of replicating by either a lytic or lysogenic cycle

Virulent Phage

a phage that replicates only by a lytic cycle

Prions are __________ that are thought to cause disease by __________. a. abnormally shaped proteins ... inducing similar but normally shaped proteins in the brain to adopt the abnormal form b. DNA molecules ... jumping around the genome and mutating genes c. an abnormal type of capsid ... dramatically enhancing the rate of viral infection d. RNA molecules ... encoding toxic proteins e. mutant DNA molecules ... encoding toxic proteins

abnormally shaped proteins ... inducing similar but normally shaped proteins in the brain to adopt the abnormal form The abnormally shaped proteins are thought to clump together, causing microscopic holes in the brain that are ultimately fatal.

Emerging viruses arise by a. the spread of existing viruses to new host species. b. mutation of existing viruses. c. the spread of existing viruses more widely within their host species. d. none of the above e. all of the above

all of the above

Reverse Transcriptase

an enzyme encoded by certain viruses (retroviruses) that uses RNA as a template for DNA synthesis

Which of the following characteristics, structures, or processes is common to both bacteria and viruses? a. cell division b. ribosomes c. genetic material composed of nucleic acid d. metabolism e. independent existence

genetic material composed of nucleic acid

A phage that inserts itself into the host DNA is called __________. a. semipermeable b. a bud c. lysogenic d. a bacteriophage e. a capsomere

lysogenic In the lysogenic cycle, the nucleic acid enters the host cell and the phage "ghost" remains outside. The phage DNA forms a circle and is incorporated into the host chromosome as a prophage.

In the lytic life cycle of phages __________. a. DNA replication is not part of the life cycle b. the entire phage is taken into the bacterium c. the cell typically dies, releasing many copies of the virus d. phage DNA is incorporated into the host cell's genome e. the viral capsid is assembled according to the genetic information of the bacterium

the cell typically dies, releasing many copies of the virus After attaching to receptors on the surface of the bacterial cell, the phage injects its nucleic acid into the cell. The phage then hijacks the cellular machinery to manufacture many copies of itself.

Viruses can vary with respect to all of the following characteristics EXCEPT __________. a. the type of host cell it can infect b. the presence or absence of a membranous envelope c. DNA or RNA as the genetic material d. the presence or absence of metabolic machinery e. single- or double-stranded nucleic acids

the presence or absence of metabolic machinery Viruses are not classified by the presence or absence of metabolic machinery, because viruses do not contain metabolic machinery.

AIDS (acquired immunodeficiency syndrome)

the symptoms and signs present during the late stages of HIV infection, defined by a specified reduction in the number of T cells and the appearance of characteristic secondary infections

HIV, the virus that causes AIDS, only infects certain cells within the immune system. This is because __________. a. the virus binds to specific receptors that are only present on certain immune cells b. the virus is not very mobile within the body and only comes into contact with a limited number of immune cells c. infection requires the presence of a specific DNA sequence that is only present in the genome of certain immune system cells d. the virus gets into all cells, but the viral RNA is immediately destroyed in all but a small number of immune system cells e. other cells produce toxins that destroy the virus before infection can take place

the virus binds to specific receptors that are only present on certain immune cells The virus binds to receptors such as CD4 that are only present on certain cells such as helper T cells.

The symptoms of a viral infection in a person can be caused by __________. a. the reaction of the individual's immune system to the infection b. the production of toxins by infected cells c. toxic viral components, such as envelope proteins d. the death of infected cells e. All of the listed responses are correct.

All of the listed responses are correct. Each of these can contribute to the symptoms of a viral infection.

In the lysogenic cycle of phages __________. a. the nucleic acid core of the phage is all that enters the host cell b. only a small number of the viral genes are expressed c. the viral nucleic acid inserts itself into the host chromosome d. the viral nucleic acid is replicated along with the host DNA e. All of the listed responses are correct.

All of the listed responses are correct. In the lysogenic cycle, the phage DNA enters the host cell, forms a circle, and is then incorporated into the host chromosome as a prophage. The prophage replicates with each round of host replication. Under certain conditions, the prophage exits the host chromosome and enters the lytic cycle.

A new pathogenic form of influenza A can emerge when __________. a. there is a concurrent infection of a pig or bird by two or more different strains of influenza A b. a virus with a novel genetic makeup recombines with viruses that circulate widely among humans c. the genomes of two or more influenza A strains mix and match during viral assembly within a host d. a mutation occurs in a virus as it passes from one host to another e. All of the listed responses are correct.

All of the listed responses are correct. Mutations in viral genomes and recombination between different viral strains within a pig or bird host both contribute to the emergence of viral strains that are unfamiliar to the human immune system.

Emerging viruses can originate from which of the following sources? a. animal viruses b. the mutation of existing human viruses c. viruses previously confined to small, isolated populations that can now spread due to technological or social changes such as the development of affordable international travel d. All of the listed responses are correct. e. None of the listed responses is correct.

All of the listed responses are correct. These are all ways that previously unknown pathogenic viruses can emerge.

Which of the following, if any, may be a component of a virus? a. protein b. phospholipid bilayer c. single-stranded (ss) RNA d. double-stranded (ds) DNA e. All of the listed responses are correct.

All of the listed responses are correct. Viral genomes may consist of double-stranded or single-stranded DNA, double-stranded or single-stranded RNA, and many contain an envelope made up of a phospholipid bilayer.

Which of the following is an example of vertical transmission of a virus in plants? a. Viral particles are carried from one plant to another by a pair of pruning shears. b. Viral particles are carried by the wind from one plant to another. c. An infected plant produces seeds that contain the virus, giving rise to infected progeny. d. Two neighboring plants touch each other, allowing viruses present in one plant to infect the other plant. e. All of the listed responses are correct.

An infected plant produces seeds that contain the virus, giving rise to infected progeny. Vertical transmission refers to the spread of a virus from parent to offspring.

Which statement below is a correct comparison of a "regular" RNA virus and an RNA retrovirus? a. Only RNA retroviruses produce DNA using DNA replicase. b. Both produce protein coats via translation of mRNA. c. Only the RNA retrovirus performs translation. d. Only the regular RNA virus produces DNA from an RNA template. e. Only the regular RNA virus performs transcription.

Both produce protein coats via translation of mRNA. Both regular RNA viruses and retroviruses have genes encoding the protein coats in which they are encased.

What is the function of hemagglutinin in the influenza virus? a. Hemagglutinin is involved in assembling the membrane envelope that the virus uses as a cloak when it leaves an infected cell. b. Hemagglutinin is the protein that helps the influenza virus attach to host cells. c. Hemagglutinin helps release new viruses from infected cells. d. Hemagglutinin is the name of the reverse transcriptase enzyme in the influenza virus. e. Hemagglutinin is part of the protein capsid of the influenza virus.

Hemagglutinin is the protein that helps the influenza virus attach to host cells. There are 16 types of hemagglutinin in various strains of influenza, but they all function in recognition and attachment.

Why can flare-ups of herpesvirus infection recur throughout a person's life? a. Herpesvirus may cloak itself in a cell's nuclear envelope, making it very difficult for the immune system to recognize it. b. Herpesvirus is very common in the environment, so it is easy for a person to become re-infected. c. Herpesvirus can leave its DNA behind as minichromosomes in nerve cell nuclei. Stress can trigger another round of virus production, producing characteristic blisters and sores. d. The herpesvirus always kills its host cell when it leaves. Cycles of cell death cause the recurring flare-ups. e. The herpesvirus becomes a prophage during the lysogenic cycle of the infection, so the viral genome is recombined with the host DNA.

Herpesvirus can leave its DNA behind as minichromosomes in nerve cell nuclei. Stress can trigger another round of virus production, producing characteristic blisters and sores. Herpesvirus remains in a person's cells virtually for their whole lifetime, so flare-ups are always a possibility.

What is the origin of the phospholipid membrane that envelops many animal viruses? a. It is produced by viral enzymes and contains only viral proteins and phospholipids. b. It is assembled in the ER based on signals sent out by the viral genome. c. It is assembled from free phospholipids floating in the cell's cytoplasm. d. It is "stolen" from the host cell, but it contains some proteins encoded by the viral genome. e. It is "stolen" from the host cell and contains only host cell proteins and phospholipids.

It is "stolen" from the host cell, but it contains some proteins encoded by the viral genome. Newly formed viruses "cloak" themselves in phospholipid membrane derived from the host, but certain components encoded by the viral genome are also included in the envelope.

When comparing DNA and RNA viruses, which mutate more quickly, and why? a. RNA viruses, because RNA is single-stranded and thus more prone to mutations b. DNA viruses, because they usually have larger genomes and thus more loci for mutations to occur c. DNA viruses, because the host cell's proofreading enzymes do not work on viral DNA d. They all mutate at the same rate. e. RNA viruses, because no proofreading is done on RNA molecules

RNA viruses, because no proofreading is done on RNA molecules There are no proofreading enzymes for RNA, so mutations in the RNA genome of a virus are not repaired, and mutations can persist.

How do retroviruses, such as HIV, differ from other viruses? a. They contain nucleic acids that code for proteins. b. They contain DNA that is used as a template to make RNA. c. They have much simpler reproductive cycles than other RNA viruses. d. They can reproduce only inside living cells. e. They can transcribe a DNA copy from a RNA template.

They can transcribe a DNA copy from a RNA template. Retroviruses contain the enzyme reverse transcriptase, which transcribes DNA from an RNA template, which is opposite of the usual direction of information flow.

Restriction enzymes help defend bacteria against viral infections by __________. a. preventing entry of the viral DNA into the cell b. preventing integration of the viral genome into the host chromosome c. cutting viral DNA once it has entered the cell d. preventing the synthesis of viral capsomeres in the cell e. preventing the binding of the virus to the cell surface

cutting viral DNA once it has entered the cell Restriction enzymes cut viral DNA, but bacterial DNA is modified in such a way as to protect it against the enzymes.

Vaccines for viral diseases are __________ and help prevent infection by __________. a. antibiotic formulations ... specifically killing infected cells b. nucleoside inhibitors ... inhibiting the replication of the viral genome c. protease inhibitors ... preventing synthesis of envelope proteins d. antibiotic formulations ... killing bacteria that assist viruses in infecting animal cells e. harmless derivatives of pathogenic viruses ... stimulating the immune system to mount a defense against the actual pathogen

harmless derivatives of pathogenic viruses ... stimulating the immune system to mount a defense against the actual pathogen Vaccines can be tremendously effective. For example, vaccination has eradicated smallpox.

RNA viruses require their own supply of certain enzymes because a. host cells lack enzymes that can replicate the viral genome. b. these enzymes cannot be made in host cells. c. these enzymes translate viral mRNA into proteins. d. host cells rapidly destroy the viruses. e. these enzymes penetrate host cell membranes.

host cells lack enzymes that can replicate the viral genome.

The avian flu virus H5N1 is considered a greater long-term threat than the swine flu virus H1N1 because __________. a. it is more readily transmitted between human hosts b. H5N1 has a greater tendency to become a prophage c. it has a significantly higher mortality rate d. H5N1 is a DNA virus and the similarity of genome chemistry can promote greater virulence and therefore the likelihood of death in humans. e. more people eat chicken meat than pork

it has a significantly higher mortality rate Although transmission between humans appears to be rare, the mortality rate among those who contract the virus from birds is close to 50%, whereas the rate for H1N1 is about 1%. A mutation and/or recombination event could transform H5N1 into a virus that could be transmitted between humans.

The phage reproductive cycle that kills the bacterial host cell is a __________ cycle, and a phage that always reproduces this way is a __________ phage. a. lytic ... lysogenic b. lysogenic ... virulent c. virulent ... lytic d. lytic ... virulent e. lysogenic ... temperate

lytic ... virulent A lytic cycle ends with the lysis of the bacterial host cell.

When a virus infects an E. coli cell, what part of the virus enters the bacterial cytoplasm? a. the tail fibers b. the protein capsid and enclosed nucleic acid c. the entire virus d. the protein capsid only e. only the nucleic acid

only the nucleic acid After attaching to receptors on the surface of the bacterial cell, the virus injects its DNA into the cell.

Plant viruses spread throughout the plant by way of __________. a. central vacuoles b. plasmodesmata c. vertical transmission d. the lymphatic system e. chloroplasts

plasmodesmata Plasmodesmata connect the cytoplasm of different cells and allow nutrients and fluids to move throughout the plant. Viruses can spread through these connections.

Birds act as a natural __________ for the influenza __________ virus. a. reservoir ... C b. buffer ... B c. defense ... A d. reservoir ... A e. pandemic ... B

reservoir ... A Birds are a major reservoir for influenza A, and their ability to migrate all over the world is a major factor in the spread of avian flu.

A prophage is a(n) __________. a. emerging virus b. viral genome that has been incorporated into a bacterial cell's chromosome c. prion that has been integrated into a bacterial cell's chromosome d. type of retrovirus e. virus that infects other viruses

viral genome that has been incorporated into a bacterial cell's chromosome This is one of the events of the lysogenic cycle.

Circular RNA molecules that function like a virus in plants are termed __________. a. retrovirus b. viroid c. prion d. rhabdovirus e. None of the listed responses is correct.

viroid Viroids are tiny molecules of circular RNA that infect plants.