Chapter 19 Questions Flashcards
Viruses can vary with respect to all of the following characteristics except _____.
the presence or absence of metabolic machinery
-Viruses are not classified by the presence or absence of metabolic machinery, because viruses do not contain metabolic machinery.
Viruses can vary with respect to all of the following characteristics
- single- or double-stranded nucleic acids
- the presence or absence of a membranous envelope
- DNA or RNA as the genetic material
- the type of host cell it can infect
A microbiologist analyzes chemicals obtained from an enveloped RNA virus that infects monkeys. He finds that the viral envelope contains a protein characteristic of monkey cells. Which of the following is the most likely explanation?
The viral envelope forms as the virus leaves the host cell.
Which of the following, if any, may be a component of a virus?
- double-stranded (ds) DNA
- single-stranded (ss) RNA
- protein
- phospholipid bilayer
Viruses that infect bacteria are called _____.
bacteriophages
HIV, the virus that causes AIDS, only infects certain cells within the immune system. This is because _____.
the virus binds to specific receptors that are only present on certain immune cells
Cancer cells often have protein receptor molecules on their surfaces that differ from those on normal body cells. Given this fact, how might viruses be used to treat cancer?
Viruses could be engineered to infect only cancer cells by altering viral surface proteins to recognize only the receptors on cancer cells.
-The host specificity of viruses could be used to make cancer cells “sick” whereas normal body cells would not be infected. This approach would reduce the collateral damage seen in chemotherapy.
Why are phages useful in treating bacterial infections in humans?
- Because of their host specificity, they only attack bacteria. They do not affect eukaryotic cells.
- Because of their specificity, a phage can be used that will only infect the pathogenic bacteria, leaving helpful bacteria alone.
- Because phages are free to evolve, bacteria are less likely to develop a lasting resistance to them.
When a virus infects an E. coli cell, what part of the virus enters the bacterial cytoplasm?
only the nucleic acid
-After attaching to receptors on the surface of the bacterial cell, the virus injects its DNA into the cell.
The phage reproductive cycle that kills the bacterial host cell is a _____ cycle, and a phage that always reproduces this way is a _____ phage.
lytic … virulent
-A lytic cycle ends with the lysis of the bacterial host cell.
In the lytic life cycle of phages _____.
the cell typically dies, releasing many copies of the virus
-After attaching to receptors on the surface of the bacterial cell, the phage injects its nucleic acid into the cell. The phage then hijacks the cellular machinery to manufacture many copies of itself.
Restriction enzymes help defend bacteria against viral infections by _____.
cutting viral DNA once it has entered the cell
A phage that inserts itself into the host DNA is called _____.
lysogenic
A prophage is a(n) _____.
viral genome that has been incorporated into a bacterial cell’s chromosome
In the lysogenic cycle of phages _____.
- only a small number of the viral genes are expressed
- the nucleic acid core of the phage is all that enters the host cell
- the viral nucleic acid inserts itself into the host chromosome
- the viral nucleic acid is replicated along with the host DNA
What is the origin of the phospholipid membrane that envelops many animal viruses?
It is “stolen” from the host cell, but it contains some proteins encoded by the viral genome.
Why can flare-ups of herpesvirus infection recur throughout a person’s life?
Herpesvirus can leave its DNA behind as minichromosomes in nerve cell nuclei. Stress can trigger another round of virus production, producing characteristic blisters and sores.
How do retroviruses, such as HIV, differ from other viruses?
They can transcribe a DNA copy from a RNA template.
Reverse transcription, carried out by retroviruses, is the process by which _____.
RNA information is copied into DNA
Which statement below is a correct comparison of a “regular” RNA virus and an RNA retrovirus?
Both produce protein coats via translation of mRNA.
When comparing DNA and RNA viruses, which mutate more quickly, and why?
RNA viruses, because no proofreading is done on RNA molecules
-There are no proofreading enzymes for RNA, so mutations in the RNA genome of a virus are not repaired, and mutations can persist.
The symptoms of a viral infection in a person can be caused by _____.
- toxic viral components, such as envelope proteins
- the death of infected cells
- the reaction of the individual’s immune system to the infection
- the production of toxins by infected cells
Vaccines for viral diseases are _____ and help prevent infection by _____.
harmless derivatives of pathogenic viruses … stimulating the immune system to mount a defense against the actual pathogen
Emerging viruses can originate from which of the following sources?
- animal viruses
- the mutation of existing human viruses
- viruses previously confined to small, isolated populations that can now spread due to technological or social changes such as the development of affordable international travel
What is the function of hemagglutinin in the influenza virus?
Hemagglutinin is the protein that helps the influenza virus attach to host cells.
-There are 16 types of hemagglutinin in various strains of influenza, but they all function in recognition and attachment.
Birds act as a natural _____ for the influenza _____ virus.
reservoir … A
-Birds are a major reservoir for influenza A, and their ability to migrate all over the world is a major factor in the spread of avian flu.
Which of the following is an example of vertical transmission of a virus in plants?
An infected plant produces seeds that contain the virus, giving rise to infected progeny.
Plant viruses spread throughout the plant by way of _____.
plasmodesmata
-Plasmodesmata connect the cytoplasm of different cells and allow nutrients and fluids to move throughout the plant. Viruses can spread through these connections.
Circular RNA molecules that function like a virus in plants are termed _____.
viroid
Prions are _____ that are thought to cause disease by _____.
abnormally shaped proteins … inducing similar but normally shaped proteins in the brain to adopt the abnormal form
A new pathogenic form of influenza A can emerge when _____.
- All of the listed factors likely contribute to the emergence of a new pathogenic strain of influenza A.
- there is a concurrent infection of a pig or bird by two or more different strains of influenza A
- a mutation occurs in a virus as it passes from one host to another
- the genomes of two or more influenza A strains mix and match during viral assembly within a host
- a virus with a novel genetic makeup recombines with viruses that circulate widely among humans
What is the prevailing hypothesis for the surprisingly low infection and mortality rate among people over 64 years of age during the 2009 H1N1 pandemic?
Older people were likely exposed to earlier H1N1 viruses that primed their immune systems for the virus of the 2009 pandemic.
The avian flu virus H5N1 is considered a greater long-term threat than the swine flu virus H1N1 because _____.
it has a significantly higher mortality rate
Which of the following characteristics, structures, or processes is common to both bacteria and viruses?
genetic material composed of nucleic acid
Emerging viruses arise by
- mutation of existing viruses
- the spread of existing viruses to new host species
- the spread of existing viruses more widely within their host species
To cause a human pandemic, the H5N1 avian flu virus would have to
become capable of human-to-human transmission
A bacterium is infected with an experimentally constructed bacteriophage composed of the T2 phage protein coat and T4 phage DNA. The new phages produced would have
T4 protein and T4 DNA
RNA viruses require their own supply of certain enzymes because
host cells lack enzymes that can replicate the viral genome
The nucleic acid of a virus particle is enclosed in a protein coat. What is it called?
capsid
-The protein shell that encloses a viral genome is a capsid.
What do we call a virus that attacks a bacterium?
phage
-Viruses that infect bacteria are called bacteriophages or, simply, phages
Why are viruses called obligate intracellular parasites?
- They must use a host cell’s metabolic enzymes and pathways to obtain energy.
- They must use a host cell’s nucleotides for transcription and replication.
- They must use a host cell’s amino acids to synthesize proteins.
- They must use a host cell’s ribosomes to synthesize proteins.
–A virus needs a host cell to perform all of these functions (and more).
Why are retroviruses considered a special class of viruses?
They transcribe RNA to DNA using reverse transcriptase.
-Retroviruses reverse the normal flow of information, forming a DNA provirus from an RNA genome.
HIV uses which of the following processes to synthesize a DNA strand using its RNA genome as a template?
reverse transcription
-Retroviruses are equipped with an enzyme called reverse transcriptase, which makes a DNA copy of an RNA template, providing information flow from RNA to DNA.
What happens first when a phage infects a bacterial cell and is going to enter a lysogenic cycle?
Linear DNA circularizes.
What is the most effective way to stop viral infections?
vaccines
-Vaccines stimulate the immune system to recognize and effectively fight off invading viruses. They have proven to be the most effective weapon against viruses.
A plant that has been raised in a sterile environment shows symptoms of a viral infection. How would you explain this?
The viral infection was acquired by vertical transmission.
-“Vertical transmission” refers to the inheritance of a viral infection from a parent.
The H1N1 2009 outbreak is considered to have been which of the following?
a pandemic
-The 2009 outbreak is considered a pandemic even though its death toll was much lower than that of 1918–1919 pandemic.
How do prions, which are misfolded proteins, infect organisms?
Prions enter brain cells and cause normal forms of the protein to refold into the prion form.
-Prions are a misfolded version of a protein normally found in the brain. When prions come into close proximity with the normal proteins, they cause them to refold into the prion form.
Viral DNA incorporated into host DNA is referred to as a “prophage.”
Integration of viral DNA into host DNA is an early stage of the lysogenic cycle.
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 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.
The lytic cycle concludes with rupture of the host cell.
The lysogenic cycle involves replication of viral DNA along with replication of the host.
The lytic cycle of bacteriophage infection ends with the _____.
rupture of the bacterium
The viral protein coat surrounds its
genome.
As a result of the lytic cycle, _____.
the host cell’s DNA is destroyed
The genetic material of HIV consists of _____.
single-stranded RNA
-The genetic material of HIV consists of two molecules of single-stranded RNA.
Glycoproteins on the viral envelope recognize and bind to receptors on
the host cell.
What is the function of reverse transcriptase?
It catalyzes the formation of DNA from an RNA template.
What is the source of a viral envelope?
host cell membrane
-The viral envelope is derived from host cell membrane.
HIV is an
RNA virus.
Reverse transcriptase catalyzes the formation of DNA from
an RNA template.
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.
Viral DNA makes mRNA by the process of _____.
transcription
-Viral DNA co-opts the cell’s reproductive machinery.
How does HIV cause disease?
HIV kills cells that defend the body against disease.
-HIV affects the immune system and kills cells that protect the body from foreign bacteria and viruses.
How do enveloped viruses differ from nonenveloped viruses?
They have a membrane-like outer covering.
-Enveloped viruses have a membrane-like outer covering (an envelope) surrounding their capsid.
Which replicative cycle describes a virus that can integrate its genome into the host cell’s genome?
Lysogenic
-A lysogenic virus can integrate its genome into the host cell’s genome and be duplicated by the host cell’s replication machinery.
Which enzyme inserts viral DNA into the host’s chromosomal DNA?
Integrase
-Integrase inserts viral DNA into the host’s chromosomal DNA.
How does HIV bind to a host cell?
The viral envelope proteins interact with CD4 and a co-receptor on the cell membrane.
-The viral envelope proteins gp120 and gp41 interact with CD4 and a co-receptor on the host-cell membrane.
Which of the following events stimulates the production of viral particles in a host cell?
Activation of the host cell by cytokines, growth factors, or antigens.
-Activation of the host immune cell by cytokines, growth factors, or antigens causes viral DNA to be transcribed at high rates in preparation for viral production.
True or false? The human immunodeficiency virus (HIV) uses reverse transcriptase to make double-stranded RNA copies of its DNA genome.
False
-A retrovirus such as HIV has an RNA genome and uses reverse transcriptase to make double-stranded DNA copies of the genome, which can then be integrated into the host cell’s genome.
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.
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.
Which statements about viruses are true?
- Enveloped viruses bud from the host cell.
- HIV contains reverse transcriptase.
- The capsid enters the host cell if the virus is enveloped.
- A retrovirus contains RNA.
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.
A disease that spreads rapidly around the world is called a(n) ___________.
pandemic
