chap 8

T/F: A nucleoside analog is a short nucleotide sequence that is complementary to a viral promoter sequence.

False

T/F: Most enveloped viruses exit the host cell through cell lysis.

False

T/F: The main function of a host cell receptor is to specifically bind to the viral attachment protein.

False

AZT is a nucleotide analog used to treat people infected with a) herpes virus. b) HIV. c) papilloma virus. d) influenza virus. e) polio virus.

HIV.

Several antiviral drugs that are produced today prevent attachment of the virus to the host cell. What information do you need to know to design the drugs? Briefly describe possible mechanisms of the drugs.

In order to design these drugs, scientists need to have a thorough knowledge of the viral attachment protein and host cell receptor molecule and how these molecules interact with one another. The antiviral drug would be designed to interfere with the attachment process either by binding to the viral attachment protein or the host cell receptor molecule. The binding of the drug to one of these two molecules will prevent them from interacting with one another and thus prevent attachment.

Most non-enveloped viruses exit the cell by the following process a) exocytosis. b) budding. c) Golgi transport. d) cell lysis. e) cytokinesis.

cell lysis.

The second step in viral replication is entry. For bacteriophage, entry usually involves a) phagocytosis. b) receptor-mediated endocytosis. c) direct entry of the nucleic acid into the cell. d) membrane fusion mechanism of entry. e) digestion of the peptidoglycan.

direct entry of the nucleic acid into the cell.

A viral fusion peptide contains a short hydrophobic amino acid sequence that helps facilitate membrane fusion of the viral envelope to the cell membrane for entry into the host cell. The protein _____________ functions as a fusion peptide for HIV entry into the host cell.

gp41

Phage lambda (λ) is referred to as a temperate phage. What is a temperate phage? a) A phage that always causes a lytic infection. b) The same thing as a lysogen. c) A phage that only lyses a cell at a specific temperature. d) A phage that can undergo either a lytic or lysogenic phase of replication. e) A bacteriophage that contains single-stranded RNA for its genome.

A phage that can undergo either a lytic or lysogenic phase of replication.

The capsid for many mammalian viruses must be removed after it enters the cytoplasm. Why is this important? Give an example of a virus where capsid removal occurs in the cell membrane

After the viral nucleocapsid enters the cytoplasm the capsid must be removed to expose the viral nucleic acid. Viral nucleic acid exposure is required for nucleic acid replication and transcription/translation of the viral genome. An example of a virus that uses this strategy is the poliovirus.

Describe the series of events that usually occur for an enveloped virus to exit the cell with an intact envelope containing the outer envelope viral proteins.

All enveloped viruses acquire their envelope from a host cell membrane (usually the plasma membrane). The membrane that makes up the envelope contains viral envelope proteins. These proteins are first synthesized in the cell and a signal sequence will direct the protein for insertion into the cell membrane. Next the assembled nucleocapsid must migrate to the area of the membrane that contains the viral envelope proteins. The nucleocapsid buds through the membrane in this area, acquiring its intact envelope.

The viral attachment protein specifically binds to a host cell receptor. All of the following are examples of known host cell receptors except a) DNA. b) proteins. c) glycoproteins. d) lipopolysaccharides. e) sialic acids.

DNA

E. coli strain K12 is susceptible to infection by phage T2. T2 binds to K12 via two attachment proteins. These two proteins specifically bind to the following two host cell receptor proteins found on the surface of strain K12. a) lipopolysaccharide and cytochrome c b) flagellin and cytochrome c c) OmpF and lipopolysaccharide d) OmpF and flagellin e) flagellin and lipopolysaccharide

OmpF and lipopolysaccharide

_________ __________ _________ is initiated by the binding of a virus to a host cell receptor for the entry of the virus into the host cell via an endosome.

Receptor-mediated endocytosis

what are the functions of the retrovirus enzymes reverse transcriptase and integrase for the replication of the virus?

The viral enzyme reverse transcriptase (RT) is an RNA-dependent DNA polymerase. When the viral nucleic acid enters the cytoplasm, RT uses the single-stranded RNA as a template and synthesizes a complementary DNA molecule. The enzyme also possesses RNase H activity and it will degrade the RNA of the DNA/RNA heteroduplex and synthesize the complementary DNA strand. Thus, RT forms a double-stranded DNA molecule from a single-stranded RNA template. The DNA molecule enters into the nucleus and is integrated into the chromosome using the viral integrase enzyme. Once the viral DNA is integrated into the chromosome it is referred to as a prophage. The genes on the prophage can be transcribed into mRNA for translation into viral proteins and the genome can be transcribed into genomic RNA using the host cell DNA-dependent RNA polymerase II. The viral proteins and genome can assemble into new virus particles.

Protease inhibitors are routinely used in the treatment of an HIV infection to slow down the progression of the disease. How do these inhibitors work? a) They inhibit the process of translation for the production of viral proteins. b) They inhibit the process of entry by interacting with the fusion peptide. c) They block the attachment process by interacting with the viral attachment protein. d) They inhibit the proteolytic modification of the viral capsid to form an infectious virion. e) They inhibit the activity of the viral enzyme reverse transcriptase.

They inhibit the proteolytic modification of the viral capsid to form an infectious virion.

Which of the following viral replication processes would not be a good target for an antiviral drug? a) Attachment b) Viral entry c) Translation of viral proteins d) Uncoating of viral capsid e) Assembly of virus capsid

Translation of viral proteins

T/F: Reverse transcriptase is an RNA-dependent DNA polymerase.

True

T/F: The second step in viral replication, "entry", is prevented by some antiviral drugs.

True

T/F: The single-stranded RNA of positive-sense RNA viruses can act directly as mRNA for translation in the cytoplasm of the host cell.

True

T/F: Translation of mRNA for the production of viral proteins always takes place in the cytoplasm of eukaryotic cells.

True

Drugs that inhibit the acidification of the endosome block the replication of many viruses that enter the cell through receptor-mediated endocytosis. Why are these drugs effective?

Viruses that enter into the cell by receptor-mediated endocytosis need to escape from the endosome. The acidification of the endosome causes a conformational change in some viral capsid proteins. Some of these altered proteins function to aid in the release of the virus by forming pores in the endosome membrane. This will facilitate the release of the capsid or the viral nucleic acids. If the acidification did not occur, these pore-forming capsid proteins would not assume the correct configuration for pore formation in the endosome membrane.

The influenza virus gains entry into a host cell by an endocytotic process. The viral nucleocapsid leaves the endosome and enters the cytoplasm through a membrane fusion mechanism. Fusion of the viral envelope with the endosomal membrane is facilitated by a) a conformation change in the hemagglutinin to expose a fusion protein. b) the production of a lipase by the influenza virus. c) sialic acid residues of the receptor proteins. d) specific viral protein recognition of pores in the endosome vesicle. e) the production of proteases by the influenza virus.

a conformation change in the hemagglutinin to expose a fusion protein.

A bacteriophage genome that is integrated into the bacterial chromosome is called a) a prophage. b) a virulent phage. c) a transforming phage. d) a genome phage.

a prophage.

Most RNA viruses utilize a) an RNA-dependent RNA polymerase. b) a DNA-dependent RNA polymerase. c) an RNA-dependent DNA polymerase. d) a DNA-dependent DNA polymerase. e) reverse transcriptase.

an RNA-dependent RNA polymerase.

A chemical that is structurally similar to a normal nucleoside is called a nucleoside _______.

analog

Antibodies produced in response to a viral infection may specifically bind to the virus particle to effectively block _______________ to the host cell.

attachment

Antibody produced in response to a viral infection is able to neutralize viral attachment by a) binding nucleic acid polymerases and preventing viral nucleic acid replication. b) binding ribosomes and preventing viral protein translation. c) activating macrophages to phagocytize the virus. d) binding to the viral attachment proteins to prevent attachment to the host cell receptor. e) binding to the host cell receptor molecule to prevent the attachment of the virus.

binding to the viral attachment proteins to prevent attachment to the host cell receptor.

Most enveloped viruses exit their host cell through a process called __________.

budding

Most enveloped viruses exit the cell by the following process a) exocytosis. b) budding. c) Golgi transport. d) cell lysis. e) cytokinesis.

budding.

Class I viruses, double-stranded DNA viruses, usually utilize the following polymerases for mRNA synthesis and DNA replication a) host cell DNA-dependent RNA polymerase and host cell DNA-dependent DNA polymerase. b) viral DNA-dependent RNA polymerase and viral DNA-dependent DNA polymerase. c) viral RNA-dependent RNA polymerase and host cell DNA-dependent DNA polymerase. d) host cell RNA-dependent RNA polymerase and host cell DNA-dependent DNA polymerase. e) viral RNA-dependent RNA polymerase and viral DNA-dependent DNA polymerase.

host cell DNA-dependent RNA polymerase and host cell DNA-dependent DNA polymerase.

Class VII viruses, double-stranded DNA viruses that utilize reverse transcriptase, replicate their genome using the following polymerase a) host cell DNA-dependent DNA polymerase. b) viral DNA-dependent DNA polymerase. c) viral DNA-dependent RNA polymerase. d) viral RNA-dependent DNA polymerase. e) host cell DNA-dependent RNA polymerase.

host cell DNA-dependent RNA polymerase.

The Baltimore classification scheme for viruses divides all viruses into seven groups based on a) their shape and host. b) host and presence or absence of an envelope. c) how they produce mRNA and replicate their genome. d) their shape and presence or absence of an envelope. e) their shape, host, and presence or absence of an envelope.

how they produce mRNA and replicate their genome.

Plant viruses often gain entry into host cells by a) receptor mediated endocytosis. b) injury caused by insects to host cell. c) membrane fusion mechanism. d) pinocytosis. e) direct injection of nucleic acid into the host cell.

injury caused by insects to host cell.

Most likely, "host specificity" of a virus is determined by a) the type of nucleic acid it contains. b) the presence or absence of an envelope. c) viral enzymes. d) interactions between viral attachment proteins and host cell receptors. e) viral nucleic acid replication strategies.

interactions between viral attachment proteins and host cell receptors.

A bacterial cell that contains the integrated genome of a temperate bacteriophage is called a ______.

lysogen

Bacteriophage T4 lyses the bacterial host cell for release of newly formed virions. The cell wall of the bacterium is broken down by this virally encoded enzyme a) lysozyme. b) protease. c) β-lactamase. d) nuclease. e) peptidase.

lysozyme.

HIV enters the cell through a/an _________ mechanism. a) phagocytic b) receptor-mediated endocytotic c) membrane fusion d) acid-dependent endosomal e) receptor-independent endocytotic

membrane fusion

For most double-stranded DNA eukaryal viruses, DNA replication occurs in the _________ and translation occurs in the __________. a) cytoplasm, cytoplasm b) cytoplasm, nucleus c) nucleus, nucleus d) nucleus, cytoplasm e) mitochondria, cytoplasm

nucleus, cytoplasm

Antiviral drugs that act at the level of host recognition are designed to a) prevent endocytotic vesicle formation. b) prevent virus binding to the host cell receptor. c) enhance antibody production. d) inactivate viral nucleic acid polymerases. e) bind to viral metabolic enzymes.

prevent virus binding to the host cell receptor.

Host __________ is determined through the specific binding of the viral attachment protein to the host cell receptor.

range or specificity

Most non-enveloped animal viruses enter the host cell by a) membrane fusion. b) pinocytosis. c) injection of nucleic acid into host cell. d) lysis of host cell membrane. e) receptor mediated endocytosis.

receptor mediated endocytosis.

The influenza virus gains entry into a host cell by a) direct injection of its nucleic acid. b) receptor-independent endocytosis. c) lysis of the cell membrane. d) receptor-mediated endocytosis. e) pinocytosis.

receptor-mediated endocytosis.

An HIV mutant has been identified that is resistant to the drug AZT. Most likely the mutation occurred in the viral gene that encodes for a) the DNA integrase. b) the viral attachment protein. c) reverse transcriptase. d) gp41 membrane fusion protein. e) viral mRNA polymerase.

reverse transcriptase.

The Baltimore classification scheme classifies viruses based on their mechanism of mRNA synthesis. Using this classification scheme, all viruses are placed into ______ classes. a) seven b) five c) four d) ten e) twenty five

seven

The intact virion of the tobacco mosaic virus is assembled by this method a) the capsid is first assembled and the nucleic acid is packaged into the capsid. b) part of the capsid is assembled and the nucleic acid is packaged into the capsid. c) the capsid proteins assemble around the viral nucleic acid. d) the nucleic acid is inserted into the cell membrane and the capsid acquires the nucleic acid as it buds through the membrane, e) the capsid is assembled in the cytoplasm and the nucleic acid is packaged in the nucleus.

the capsid proteins assemble around the viral nucleic acid.

The ability of a virus or bacteriophage to specifically attach to a host cell occurs through the interactions of the viral attachment protein with the host cell receptor. This binding determines a) the rate of viral replication. b) the host range. c) the mechanism of viral entry. d) whether or not the virus will contain an envelope. e) the rate of viral RNA replication.

the host range.

Most non-enveloped viruses enter the host cell by receptor-mediated endocytosis. The virus or its nucleic acid is able to leave the endosome and enter the cytoplasm a) through the production of a lipase to lyse the endosome membrane. b) as a result of digestive enzymes found in the endosome. c) through the fusion of the capsid proteins with endosome proteins by digesting the endosomal membrane. d) through pores formed from viral capsid proteins.

through pores formed from viral capsid proteins.

Fuzeon is an antiviral drug that prevents membrane fusion for entry into the cell by HIV. The mode of action for this drug is a) to prevent formation of a viral lipase that lyses the cell membrane. b) to prevent attachment of the virus to the host cell receptor. c) to interrupt receptor mediated endocytosis. d) to bind to gp41 membrane fusion protein of HIV. e) to digest the envelope of HIV, thereby preventing membrane fusion.

to bind to gp41 membrane fusion protein of HIV.

Class III viruses, double-stranded RNA viruses, utilize the following polymerase for genome synthesis a) host cell RNA-dependent RNA polymerase. b) host cell DNA-dependent RNA polymerase. c) viral RNA-dependent RNA polymerase. d) viral DNA-dependent RNA polymerase. e) viral DNA-dependent DNA polymerase.

viral RNA-dependent RNA polymerase.

Class V viruses, negative sense single-stranded RNA viruses, utilize the following polymerase for mRNA synthesis a) viral DNA-dependent RNA polymerase. b) host cell RNA-dependent RNA polymerase. c) host cell DNA-dependent RNA polymerase. d) viral RNA-dependent RNA polymerase. e) don't need polymerase, nucleic acid can act directly as mRNA.

viral RNA-dependent RNA polymerase.

Many antiviral drugs are nucleoside analogs. The nucleoside analogs primarily target these enzymes a) host cell DNA polymerases. b) host cell RNA polymerases. c) host cell nucleases. d) viral nucleic acid polymerases. e) viral nucleases.

viral nucleic acid polymerases.

The reason nucleoside analogs are effective against viral nucleic acid polymerases is because a) viral polymerases have a very high rates of polymerization. b) viral polymerases have a low rate of polymerization. c) viral polymerases have a high affinity for incorporation of the analogs. d) viral polymerase activity is inhibited by the analogs. e) viral polymerase activity is enhanced by the analogs.

viral polymerases have a high affinity for incorporation of the analogs.

The enzyme _______ has a high affinity for AZT and will incorporate it into a newly synthesized DNA strand. This will effectively terminate DNA replication because the incoming nucleotide cannot be bound to AZT. a) host cell DNA polymerase b) viral reverse transcriptase c) host cell RNA polymerase d) viral RNA polymerase e) viral DNA-dependent DNA polymerase

viral reverse transcriptase