Micro 2421 Ch 8
What are the minimal components needed for a virion?
capsid and genome
The virus repressor protein
controls both the lytic genes on the prophage and prevents an incoming virus of the same type.
Mutations contribute to the vast diversity of viruses. What viral genetic type has the most stable genome and the lowest mutation rate?
double-stranded DNA virus
Regarding the viral membrane of an enveloped virus, the lipids are derived from the ________, and the proteins are encoded by ________.
host's cell membrane / viral genes
Viruses go through phases during which they are actively replicating and destructive to the host cell, or dormant within the host cell. During which part of the bacteriophage life cycle will the viral genome be replicated in synchrony with the host cell?
lysogenic phase
Virions infecting some bacteria possess the enzyme ________ that makes a small hole in the bacterial cell wall, allowing the viral nucleic acid to enter.
lysozyme
A virus that kills its host is said to be
lytic or virulent.
The consequence of an infection by a temperate bacteriophage is that the bacterial cell
may lyse before it divides or may continue to divide and replicate both the virus and the cell.
Viral structural proteins and proteins involved in the release of new viral particles that are synthesized after genomic replication begins would be considered
middle and late proteins
Viral size is generally measured in
nanometers
A cell that allows the complete replication cycle of a virus to take place is said to be a
permissive host.
Viruses infecting ________ are typically the easiest to grow in the laboratory.
prokaryotes
Which of the following enzymes would you expect to find in the virion of a retrovirus, but NOT in a bacteriophage?
reverse transcriptase
A retrovirus has a genome that consists of __________.
ssRNA
The concentration of infectious plaque forming units (pfu) per volume of fluid is known as the
titer
Place the steps in the bacteriophage replication cycle into the correct order.
1. Attachment 2. Penetration of viral nucleic acid 3. Synthesis of viral nucleic acid and proteins 4. Assembly and packaging of new viruses 5. cell lysis and release of new virons.
What is the benefit, for a virus, to be a temperate or lysogenic virus?
A single infection event can produce millions of new viral particles instead of hundreds of viral particles.
When describing the viral genome, which terms could be used?
All of the listed responses are correct.
The introductory passage suggests that selection on mutation rate—from rapidly mutating ssRNA viruses to slower dsDNA viruses—may have been a critical step in the evolution of more stable DNA-based genomes, and subsequently first cells. However, if viruses were able to infect the vast majority of host cells, evolution would have stalled. According to the chapter, what is one way that cells have evolved to avoid viral infection?
An attachment receptor on the host cell surface that is recognized by the virus is mutated, making the host resistant to infection.
In which stage does formation of mature viruses occur?
Assembly
How do bacteriophage influence bacterial evolution?
Bacteriophage transfer genetic information between bacterial cells through transduction, thus increasing the genetic diversity of bacterial populations.
When determining the number of bacterial virus particles in a suspension, why is the number obtained always less than the actual viral titer?
Because all of the virus particles do not infect with 100% efficiency.
The host DNA is usually degraded during which stage?
Biosynthesis
Some viruses, especially animal viruses, have an envelope surrounding their nucleocapsid. This envelope may have viral proteins embedded in it. Why are the viral proteins more readily recognized and targeted by the immune system than the envelope?
Enveloped viruses are often coated with animal cell membrane as they leave the cell. This membrane is a major component of the viral envelope. The viral proteins are more readily recognized by the immune system, as they are more distinct from the materials normally found in and on the cells. However, the envelope can contain several types of viral components as well.
Which of the following events might trigger induction of a temperate bacteriophage?
Exposure to UV light
The introductory passage mentions that mutations conferred an evolutionary advantage to certain viruses. Mutations arise when viruses replicate themselves inside a host cell. This requires a precise sequence of events where the virus hijacks the host's machinery to use for itself. The general mechanism of cell infection by a virus always involves the same five general steps. What are the steps of the viral replication cycle?
First step: The virus attaches to the host cell, which can be mediated by cell surface receptors Second Step: Nucleic acids penetrates the host cell and the capsid is discarded Third Step: Host cells enzymes and used to synthesize new viral genetic and capsid material Fourth Step: Newly synthesized viral genomes and capsids are assembled into complete viral particles Fifth Step: The host cell lyses,releasing the newly assembled virus particles
Differentiate between persistent and latent animal virus infections.
In a persistent infection, the host cell is continually releasing new viral particles slowly. In a latent infection, there are periods of time where the virus is not replicating and creating new viral particles.
What is the fate of the prophage during the lysogenic stage?
It is copied every time the host DNA replicates.
In which stage is the viral DNA introduced into the cell?
Penetration
Viruses rely on the host cell machinery to make new viruses, but they sometimes provide their own enzymes (such as reverse transcriptase, which is an RNA-dependent DNA polymerase). Which of the following types of enzymes would a virus need to provide in order to replicate within a cell? In other words, which of these enzymes would not normally be found in a cell unless a virus provided it?
RNA replicases (RNA-dependent RNA polymerases) for RNA viruses
Which of the following groups contains some representatives with genomes that are so small that they may have as few as two genes?
RNA viruses
Reverse transcriptase is a(n)
RNA-dependent DNA polymerase.
Retroviruses mutate relatively rapidly. What is one of the reasons?
Reverse transcriptase lacks the proofreading function found in many DNA polymerases, so mutations accumulate more easily.
The replication and synthesis of new viral genetic information inside a host cell of infection is not simple due to the variety of ways that viral genetic information can be encoded: single-stranded DNA, double-stranded DNA, single-stranded RNA, double-stranded RNA, or reverse transcribing. Match the viral genetic category to the correct description.
Single-stranded DNA virus: replication involves generating a complementary DNA strand. Single-stranded DNA virus and Double-stranded DNA virus: replicate using DNA polymerases, either its own or from the host Double-stranded DNA virus: replicates, transcribes, and translates similar to prokaryotic or eukaryotic cells Single-stranded RNA virus and Double-stranded RNA virus: carry their own nucleic acid polymerases to replicate the viral genome Single-stranded RNA virus: genome configurations are either plus-sense or minus-sense. Retrovirus: contain an RNA-dependent DNA polymerase called reverse transcriptase to generate a DNA intermediate Retrovirus: contain an RNA-dependent DNA polymerase called reverse transcriptase to generate a DNA intermediate
How is penetration different in animal viruses as compared to bacterial viruses?
The entire viral particle penetrates an animal cell, while only the viral genome penetrates a bacterial cell.
How is the lytic cycle different from the lysogenic cycle with respect to the infected host cell?
The host cell dies during the lytic stage.
What would be the fate of a lytic bacteriophage if the host cell died prior to the assembly stage?
The virus would not be able to infect new hosts.
How are viruses different from cells?
They require a host in order to reproduce.
What is the function of the structural elements of a virus?
To package and protect the viral genome
Which of the following examples is an example of lysogenic conversion?
Vibrio cholerae bacteria produce cholera toxin when infected with a phage.
Which of the following is NOT a way in which viruses are different from living organisms?
Viruses are obligate parasites and no living organisms are obligate parasites.
Viruses are non-cellular infectious agents. The genetic information of a virus is surrounded by a protective coat made up of many protein monomers, called the capsid. The genetic component of a virus plus its capsid layer are collectively called the nucleocapsid. Which of the following accurately describe viruses?
Viruses do not belong to any of the three domains: Bacteria, Archaea, and Eukarya. , Viruses are as old as Bacteria and Archaea., Viruses are smaller than Bacteria and Archaea., Viruses cannot replicate on their own., Viruses are unable to perform replication on their own., The genetic information of a virus can be in the form of RNA or DNA.
Restriction is
a general host mechanism to prevent the invasion of foreign nucleic acid.
Lysogenic viral DNA integrating into the host genome is referred to as
a prophage.
Some viruses have plus sense RNA genomes. Which of the following could describe one of those viruses?
a virus that contains RNA with the same nucleotide sequence as the functional mRNA that its host cell will translate to produce viral products
A prophage replicates
along with its host while the lytic genes are not expressed.
Which of the following are the hosts for most enveloped viruses?
animals
For bacteriophages and animal viruses ________ is the step in the viral life cycle that determines host cell or tissue specificity.
attachment
Which of the following are viable methods of release of newly assembled viral particles?
budding or lysis of host cell
