Chapter 19: Viruses

Some viruses have membranous envelopes. Where do viral envelopes typically originate from? The virus produces the envelope itself. They come from other viruses. They come from the cell walls of bacteria. They develop from the capsid. Membranes from the host cell Capsomeres produce them.

Membranes from the host cell

Viral DNA incorporated into a bacterial chromosome is known as a(n) __________. adenovirus virulent phage bacteriophage prophage retrovirus capsid

prophage

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

- An infected plant produces seeds that contain the virus, giving rise to infected progeny.

How does a retrovirus like HIV enter a host cell? - HIV binds to the host cell and injects its RNA. - HIV passes through a channel protein in the lipid bilayer of the plasma membrane. - HIV passes through the lipid bilayer of the plasma membrane. - Glycoproteins on the viral envelope bind to receptors on the host cell. The viral envelope fuses with the host cell's plasma membrane, facilitating uptake of the virus by the host cell. - HIV binds to the host cell and injects its DNA.

- Glycoproteins on the viral envelope bind to receptors on the host cell. The viral envelope fuses with the host cell's plasma membrane, facilitating uptake of the virus by the host cell.

How do prions propagate and replicate themselves? - Prions make copies of themselves using ribosomes. - Prions convert normal proteins into the misfolded prion version. - Retroviruses produce them during their own replication. - Bacteriophages produce them during their own replication. - Prions take over a host cell's DNA in order to replicate.

- Prions convert normal proteins into the misfolded prion version.

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

lytic; virulent

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

- Herpes virus can leave its DNA behind as minichromosomes in nerve cell nuclei. Stress can trigger another round of virus production, producing characteristic blisters and sores.

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

- RNA viruses, because no proofreading is done on RNA molecules

How are retroviruses different from other types of viruses? - Retrovirus RNA is incorporated into the host cell's DNA in order to be translated. - Retroviruses use the enzyme reverse transcriptase to transcribe a copy of DNA from their own RNA. - Retrovirus DNA is used directly by the infected cell to transcribe and translate viral proteins. - Retrovirus DNA is incorporated into the host cell's DNA in order to be translated. - Retrovirus RNA is used directly by the infected cell to produce viral proteins.

- Retroviruses use the enzyme reverse transcriptase to transcribe a copy of DNA from their own RNA.

Which of the following events occurs during the lytic life cycle of phages? - The host cell usually dies, releasing many new copies of the virus. - The host cell lives, with the viral genome becoming incorporated into the host cell's own genome. - The phage capsid fuses with the host cell membrane, thus allowing the viral genome to be inserted into the host cell. - The host cell usually lives and secretes the copies of the virus. - The phage enters the host cell intact during infection.

- The host cell usually dies, releasing many new copies of the virus.

Which of the following statements correctly describes one difference between virulent phages and temperate phages? - None of the listed responses is correct. - Virulent phage DNA can be integrated into the bacterial chromosome and become a prophage, but temperate phages cannot do this. - Temperate phages replicate through the lytic cycle only, and virulent phages replicate using both the lytic and the lysogenic cycles. - Virulent phages replicate through the lytic cycle only, and temperate phages replicate using both the lytic and the lysogenic cycles. - Temperate phage DNA is methylated to prevent digestion by bacterial restriction enzymes, but virulent phage DNA is not. - Virulent phages infect only animal cells, and temperate phages infect only bacterial cells.

- Virulent phages replicate through the lytic cycle only, and temperate phages replicate using both the lytic and the lysogenic cycles.

Why are viruses often considered to be nonliving? - Viruses carry genetic information. - Viruses can replicate outside of a host cell. - Viruses have plasma membranes similar to those of other types of cells. - Viruses do not carry out metabolic processes, a primary characteristic of living organisms. - Viruses can reproduce.

- Viruses do not carry out metabolic processes, a primary characteristic of living organisms.

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

- abnormally shaped proteins; inducing similar but normally shaped proteins in the brain to adopt the abnormal form

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

- cutting viral DNA once it has entered the cell

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

- harmless derivatives of pathogenic viruses; stimulating the immune system to mount a defense against the actual pathogen

Class IV viruses are characterized by which of the following types of genomes? - single-stranded RNA that serves as a template for DNA synthesis - single-stranded RNA that serves as a template for mRNA synthesis - single-stranded RNA that serves as mRNA - single-stranded DNA - double-stranded DNA - double-stranded RNA

- single-stranded RNA that serves as mRNA

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

- the virus binds to specific receptors that are only present on certain immune cells

Genomes of viruses may consist of which of the following types of nucleic acids? double-stranded DNA single-stranded DNA None of the listed responses is correct. double-stranded RNA All of the listed responses are correct. single-stranded RNA

All of the listed responses are correct.

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

RNA information is copied into DNA

Viruses that infect bacteria are called __________. proviruses bacteriophages capsomeres bacterioviruses retroviruses

bacteriophages

A widespread outbreak of a viral disease is called a(n) __________, and a global outbreak is called a(n) __________. vaccine; emerging virus pandemic; epidemic epidemic; pandemic retrovirus; phage viroid; prion

epidemic; pandemic

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

it has a significantly higher mortality rate

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

lysogenic

Animals that harbor and can transmit a particular virus but are generally unaffected by it are said to act as a __________ for that virus. natural reservoir retrovirus nontransmitter vaccine viroid

natural reservoir

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

only the nucleic acid

Which of the following is not a class I virus? polyomavirus papillomavirus adenovirus herpesvirus poxvirus parvovirus

parvovirus

All of the following types of viruses have envelopes except __________. retrovirus rhabdovirus poxvirus flavivirus herpesvirus picornovirus

picornovirus

Candidates for the original source of viral genomes include __________. plasmids transposons plants archaea bacteria plasmids and transposons

plasmids and transposons

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

the presence or absence of metabolic machinery

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

viroids