MasteringBio: Viruses (Answers)
Which of these binds to receptor molecules on the host cell membrane?
A - Glycoproteins on the viral envelope recognize and bind to receptors on the host cell.
Which of these is reverse transcriptase?
C - This is reverse transcriptase.
Which of these is the viral genome?
E - HIV is an RNA virus.
Which statements about viruses are true? Select the four statements that are true.
Enveloped viruses bud from the host cell. The capsid enters the host cell if the virus is enveloped. A retrovirus contains RNA. HIV contains reverse transcriptase. - (Viruses are classified into two major groups: nonenveloped and enveloped. Nonenveloped viruses (a) typically exit the host cell by bursting through the plasma membrane. Enveloped viruses (b) generally escape from the host cell by budding. Because of this, the viral envelope is often derived from the host cell's plasma membrane. Viruses contain either single-stranded DNA, double-stranded DNA, single-stranded RNA, or double-stranded RNA. Human immunodeficiency virus (HIV) is a retrovirus that contains two copies of its RNA genome and two copies of reverse transcriptase. The HIV viral envelope facilitates binding to a white blood cell. After HIV enters the cell, the reverse transcriptase synthesizes viral DNA from viral RNA.)
What is the function of reverse transcriptase?
It catalyzes the formation of DNA from an RNA template. - This is the function of reverse transcriptase.
Classify each phrase as applying to the lytic cycle, the lysogenic cycle, or both types of reproductive cycles of phages.
Lytic cycle: the host is destroyed, new phages are assembled from viral DNA and proteins, the cell is lysed (broken open) Lysogenic cycle: the cell reproduces normally, the viral DNA inegrates into the chromosome of the host cell both: viral genes are replicated - (Bacteriophages, which contain double-stranded DNA, can reproduce by two alternative mechanisms: the lytic cycle and the lysogenic cycle. In the lytic cycle, virus parts are made, new viruses are assembled, and the cell is lysed, releasing the newly assembled viruses. In the lysogenic cycle, the viral DNA incorporates into the bacterial DNA and is passed on to daughter bacterial cells when the infected bacteria reproduce normally. Occasionally, an infected bacterium in the lysogenic cycle can enter the lytic cycle, in which viral particles are produced and the cell is eventually ruptured.)
Why do RNA viruses appear to have higher rates of mutation?
Replication of their genomes does not involve the proofreading steps of DNA replication.
Which of these is the best description of this animation?
The lysogenic cycle is making a transition to the lytic cycle. - Phage DNA has exited the bacterial chromosome as a prelude to taking over the host's metabolic machinery.
Why are viruses referred to as obligate parasites?
They cannot reproduce outside of a host cell.
Most molecular biologists think that viruses originated from fragments of cellular nucleic acid. Which of the following observations supports this theory?
Viral genomes are usually more similar to the genome of the host cell than to the genomes of viruses that infect other cell types.
Sort the items according to whether they may be found only in free virus particles, only in uninfected host cells, or in both viruses and host cells.
Viruses only: capsid, envelope with glycoproteins, capsomere (core protein) Host cell only: ribosome both: DNA, RNA, protein - (Viruses are infectious particles that contain genetic material (either DNA or RNA) surrounded by a capsid (protein shell). Some viruses also have a membranous envelope that contains glycoproteins.)
The arrow is indicating _____.
a prophage (blue line) - Viral DNA incorporated into host DNA is referred to as a "prophage."
In many ways, the regulation of the genes of a particular group of viruses will be similar to the regulation of the host genes. Therefore, which of the following would you expect of the genes of the bacteriophage?
control of more than one gene in an operon
The pointer is indicating the virus's _____.
genome (squiggle lines) - The pointer is indicating the virus's nucleic acid genome.
Which of the following can be effective in preventing viral infection in humans?
getting vaccinated
Which of the following molecules make up the viral envelope?
glycoproteins
What is the source of a viral envelope?
host cell membrane - The viral envelope is derived from host cell membrane.
Cycle A is the _____ cycle and cycle B is the _____ cycle.
lytic ... lysogenic - The lytic cycle concludes with rupture of the host cell. The lysogenic cycle involves replication of viral DNA along with replication of the host.
Double-stranded viral DNA is incorporated into a host cell as a _____.
provirus - "Provirus" is the name given to double-stranded viral DNA that has been incorporated into a host cell's genome.
What enzyme is responsible for the process seen here?
reverse transcriptase - Reverse transcriptase catalyzes the formation of DNA from an RNA template.
The lytic cycle of bacteriophage infection ends with the _____.
rupture of the bacterium - The bacterium ruptures and phages are released.
The genetic material of HIV consists of _____.
single-stranded RNA - The genetic material of HIV consists of two molecules of single-stranded RNA.
As a result of the lytic cycle, _____.
the host cell's DNA is destroyed - The host cell's DNA is destroyed, and ultimately, the host cell itself is destroyed in the lytic cycle.
This animation is illustrating an early stage of _____.
the lysogenic cycle - Integration of viral DNA into host DNA is an early stage of the lysogenic cycle.
The host range of a virus is determined by
the proteins on its surface and that of the host.
Viral DNA makes mRNA by the process of _____.
transcription - Viral DNA co-opts the cell's reproductive machinery.
In the lysogenic cycle _____.
viral DNA is replicated along with host DNA - Viral DNA, incorporated into host DNA as a prophage, is replicated along with host DNA.
The pointer is indicating the _____.
viral protein coat - The viral protein coat surrounds its genome.
