Chapter 5
Describe the structure of a virus
Capsid: protein coat surrounding viral nucleic acid genome (helical capsids: rod shaped; polyhedral capsids are multiple sided geometric figures with regular surfaces) Head: the genome is located Tail fibers: attachment to surface sheath:
Hydrolase is an example of _________ because it is one of the first viral gene products made and its purpose is tooo
Early gene hydrolytic enzyme that degrades the host genome
The two shapes of a virus capsid are and composed of _______
Helical: rod shaped Polyhedral: multiple sided geometric figures They are made of protein, while the genome is composed of nucleic acid
The evolutionary disadvantage to the lytic cycle:
The host cells are destroyed
The process ________The prophage removes itself from the host genome and the enters the lytic cycle
excision.
Endocytosis
process by which a cell takes material into the cell by infolding of the cell membrane
Upon infection, the phage genome is incorporated into the bacterial genomes termed _______ and the host is called ________
prophage and lysogen
in the animal lysogenic cycle, the dormant form of the viral genome is referred to as ______
provirus Provirus (animal)= prophage (bacteria)
An Enzyme called ________ destroys the bacterial cell wall to release the phage
lysozyme
The two life cycles of a phage
lytic cycle & lysogenic cycle
A piece of single stranded viral RNA serving as mRNA is called
+RNA
Describe a viral genome
- DNA or RNA - Single stranded (ss) or double stranded (ds) - linear or circular
Describe the adaptation of the viral genome
- compact - rely on host encoded proteins for transcription, translation, replication - encode more than one protein in a given lenght of genome
What are the 6 steps to the lytic cycle
1. attachment of phage to E.coli/injection of phage chromosome 2. breakdown of bacterial chromosome by phage specific enzyme 3. replication of phage using bacterial materials and phage enzymes 4. expression of phage genes to produce proteins 5. assembly 6. release progeny by lysis of bacterial wall
What are the three options for an animal virus once inside the cell
1. lytic cycle 2. productive cycle: lytic like cycle 3. Lysogenic cycle
The 6 steps to the lysogenic Cycle
1. the phage binds to the bacterium 2. the phage DNA enters the host cell 3. The phage DNA integrates and becomes a noninfective prophage 4. Chromosome with integrated prophage replicates 5. In rare cases, the prophage may separate and the cell be induced to lyse
purpose of viral envelope
A membrane on the exterior of the virus derived from the membrane of the host cell. Protection
A retrovirus must encode for________ while a double stranded DNA Viruses must encode for_______
A retrovirus must encode for reverse transcriptase while a double stranded DNA Viruses must encode for enzymes required for dNTP synthesis and DNA replication
A viruses relies on _____ to reproduce
A viruses relies on host cell machinery to reproduce they are unable to perform any of the chemical reactions characteristic of life
What steps are necessary to replicate a (-)RNA virus? A. Create a (+)RNA copy of the genome using RNA-dependent RNA polymerase then replicate (-)RNA copies from that template using the same enzyme. B. Create a (+)RNA template using RNA-dependent DNA polymerase then replicate (-)RNA copies from that template using DNA-dependent RNA polymerase. C. Use RNA-dependent DNA polymerase to synthesize a DNA version of the genome and then replicate more RNA copies using the host cell's transcriptional machinery. D. Make more (-)RNA copies directly from the genome using RNA-dependent RNA polymerase.
A. Create a (+)RNA copy of the genome using RNA-dependent RNA polymerase then replicate (-)RNA copies from that template using the same enzyme. Since nucleotide synthesis is complementary and not identical, the genome itself cannot act as template. A host cell's transcriptional machinery cannot be used to make viral genomes and an RNA-dependent DNA polymerase would produce DNA, not RNA.
Treatment of an enveloped animal virus with a mild detergent solubilizes several proteins from the virus, although the genome does not become accessible. Which of the following is consistent with this scenario? A) Some of the proteins that are released by detergent may be encoded by the genome of the infected cell. B) The infectivity of the virus is not affected by detergent treatment. C) The proteins released by detergent are capsid proteins. D) All the proteins released by the detergent are encoded by the viral genome.
A) Some of the proteins that are released by detergent may be encoded by the genome of the infected cell. Some envelope proteins are encoded by the virus and some are derived from the hosts membranes during budding. Removal of envelope proteins will impair viral adsorption impair viral adsorption and reduce infectivity
Which of the following viruses should be able to reproduce successfully if they carry RNA-dependent RNA polymerase into their host cell? I.(+) RNA virus II.(-) RNA virus III.ds DNA virus
ALL of them item III is true: while a ds DNA virus would not require RNA-dependent RNA polymerase to make mRNA or replicate its genome, it could still reproduce if the enzyme was present. It would simply ignore the enzyme.
Cyanide (an inhibitor of the electron transport chain) is added to a culture of virus infected mammalian cells. Which one of the following best describes what occurs? A) Mammalian cells will die, and all viruses will be destroyed as well, regardless of their stage of development. B) Cells are killed and viral replication halted, but the culture remains infectious C) Cells stop growing, and viral replication is unaffected D) mammalian cells continue to grow, but viral replication is halted
B) Cells are killed and viral replication halted, but the culture remains infectious Any viruses that had already completed the replication process when cyanide was added will not be affected, and will remain infectious
Which of the following describes a mechanism by which a viroid can cause disease? A. The capsid of the viroid binds to the exterior of the cell and the DNA is injected into the cell. The viroid enters the lysogenic cycle, and is replicated along with the host genome. B. Viroids can serve as siRNAs that silence expression of genes that are necessary for cell function. C. Viroid RNA codes for abnormally folded proteins that cause host proteins to misfold as well. D. The capsid of the viroid binds to the exterior of the cell and the DNA is injected into the cell. The viroid enters the lytic cycle, and the cell bursts, releasing new viroids.
B. Viroids can serve as siRNAs that silence expression of genes that are necessary for cell function. Viroids are small fragments of RNA, and do not have capsids. While viroids are RNA, they do not cause disease by causing proteins to misfold.
A 1,000 base pair region of viral genome is found to encode two polypeptides (200 bp & 300 bp) unrelated in amino acid sequence during infection of eukaryotic cell. A) missense mutation B) viruses use a different genetic code than eukaryotes do C) overlapping reading frames D) The polypeptides are splicing variants
C) overlapping reading frames
An experiment with a previously unidentified pathogen is done using guinea pigs and various means of inoculation are attempted. Within a few days, the guinea pigs begin to demonstrate a loss of coordination along with other neurological symptoms. Could the organism be a prion? A. Yes; loss of coordination is specific to prion infections. B. No; prions do not infect guinea pigs. C. No; the onset of symptoms was too rapid. D. Yes; direct inoculation is required to contract a prion infection
C. Prion incubation times are long; in humans, it can take years or even decades.
HIV is a +RNA retrovirus whose genome can be directly translated to form a variety of viral proteins even before insertion into the host's genome. Which of the following enzymes would be needed AFTER insertion into the host genome to transcribe the mRNA for viral proteins? A. RNA dependent DNA polymerase B. DNA dependent DNA polymerase C. DNA dependent RNA polymerase D. RNA dependent RNA polymerase
C. DNA dependent RNA polymerase The first part of the enzyme name ("___ dependent") describes what type of nucleic acid the enzyme uses as a template, and the second part of the enzyme name ("___ polymerase") describes the type of nucleic acid the enzyme synthesizes.
Which of the following is NOT a means by which prion infection can typically occur? A. Inheritance B. Spontaneous mutation C. Consumption D. Aerosol contact
D. Aerosol contact
If antibodies to a viral capsid protein are ineffective in blocking infection, what might this indicate about the virus?
If suggest that the virus is enveloped; the antibody cannot reach its epitope on the capsid surface in infectious virus
If lysozymes were an early gene, would this be an advantage?
No, the host cell would lyse prior to phage reproduction
Will an infectious virus be produced if the genome of an enveloped (+) strand RNA virus is added to an extract prepared from the cytoplasm of eukaryotic cells that retains translational activity but lacks DNA replication or transcription of host genes?
No. The (+) strand RNA virus will be able to produce viral genome and proteins, but the progeny will not be able to acquire the envelope they need to be infectious
__________ is the termed used to describe injection of the viral genome into the host cell
Penetration or eclipse
Exocytosis
Process by which a cell releases large amounts of material
Bacteria cultured in the presence of 35-S-labeled cysteine and 32p labeled phosphates are infected with phage t4. When phage from this culture are used to infect a new nonradiolabled bacterial culture, which isotopes will be found in the interior of the newly infected bacteria?
Proteins do not contain P Nucleic acids do not contain S 35S cysteine will be incorporated into viral coat proteins 32P will be incorporated into the viral nucleic acid
Retrovirus enter the cell in _______ form encoding for an enzyme to revert back to _______
Retrovirus enter the cell in RNA form encoding for an enzyme to revert back to DNA
What is the purpose of the tail fibers and the sheath in a virus
Tail fibers attach to cell surface sheath contracts using energy stored, injecting the genome into the host
When phage are first added to a bacterial culture, the number of infective viruses initially decreases before it later increases. Why does this occur?
The initial decrease is due to the simple act that many phage have injected their genomes into hosts and are no longer infectious
Describe the process transduction
The presence of the ne DNA will become evident if it codes for a strait that the newly infected host did not previously possess
The productive cycle differs for the lytic cycle because it does not _____
The productive cycle differs for the lytic cycle because it does not destroy the host cell. The viruses exit the host cell by budding through the host cells membrane
lysogenic cycle
a viral reproductive cycle in which the viral DNA is added to the host cell's DNA and is copied along with the host cell's DNA
A bacteriophage with an important capsid gene deleted infects the same cell as another virus with a normal copy of the same gene. At the time of host cell lysis: a) all release viruses will be capable of infection new host but only some of these new infections will give rise to phage capable of infecting new host b) each individual virus that is released will produce a mixture of infective and non infective viruses in subsequent infections
a) all release viruses will be capable of infection new host but only some of these new infections will give rise to phage capable of infecting new host
an +RNA virus must encode ______ while an - RNA virus must carry __________
an +RNA virus must encode RNA dependent RNA pol while an - RNA virus must carry RNA dependent RNA pol
A virus binding to the exterior of a bacterial cell in a process is _____________
attachment or adsorption
The role of RNA- dependent RNA polymerase is to
copy the RNA genome for viral replication; the host never makes RNA from RNA
A viruses is a ________ __________ ______ because they are only able to reproduce within cells
obligate Intracellular parasite
one of the first gene products made is an enzyme called_________ which is a hydrolytic enzyme that degrades the host genome
one of the first gene products made is an enzyme called hydrolase which is a hydrolytic enzyme that degrades the host genome
If the ratio of adenine to thymine in a DNA virus is not one to one, what can be said about the genome of this virus?
single stranded DNA
If the new pathogen is a viroid, an analysis of the pathogen should show that: A. the pathogen has a complex capsid. B. the ratio of adenine to guanine in the genome is 1:1. C. the genome contains no thymine. D. the genome codes for numerous proteins.
the genome contains no thymine. Viroids are RNA, and thus, there would be no thymine in its genome.
The prophage is silent due to ___
the transcription of phage genes is blocked by phage encoded repressor protein that binds to operators.