Exam 1: Viruses Flashcards
A virus is a ___ surrounded by a _____. Some viruses are also surrounded by a ___
nucleic acid
protein coat
Membranous coat
A viruses genome can consist of ____. The genome is usually organized as ____
double/single D/RNA
a single linear or circular molecule of nucleic acid, may consist of multiple molecules of nucleic acid
A capsid is the protein shell enclosing the viral genome. They are built of protein subunits called
capsomeres.
Viral envelopes are ___ they contain
membranous structures that help viruses infect hosts. They contain contain host cell phospholipids and membrane proteins. They also contain proteins and
glycoproteins of viral origin
Bacterio/phages are
Viruses that infect bacteria, and contain some of the most complex capsids.
The host range of the virus is
the number of host species a virus can infect
Many DNA viruses use the DNA
polymerases of the host cell to synthesize new genomes along
the templates provided by the viral DNA. In contrast, to replicate their genomes, RNA viruses
use virally encoded RNA
polymerases that can use RNA as a template.
After the viral nucleic acid molecules and capsomeres are
produced, they
they self-assemble into new viruses
The simplest type of viral replicative cycle ends with
the exit of
hundreds or thousands of viruses from the infected host cell,
a process that often damages or destroys the cell
Describe the lytic cycle of a phage
A phage replicative cycle that culminates in death of the host
cell
A phage that replicates only by
a lytic cycle is a
virulent phage
They lysogenic cycle
allows replication of phage without destroying the host
Phages capable of both the lytic and lysogenic cycles are known as
temperate phages
One prophage gene codes for a protein that
prevents transciprtion of most of the other prophage genes, making the genome silent within the bacterium
Prophages are capable of generating active phages that lyse their host cells. This occurs when
the alpha genome is induced to exit the bacterial chromosome and initiate a lytic cycle
give a general overview of the CRISPR Cas system
if a phage infects a bacterial cell, its DNA is integrated into the genome between two repeat sequences
RNA is processed into short strands, which binds to a cas protein, forming a complex.
Complementary RNA binds to DNA from the invading phage. Cas protein then cuts phage DNA.
The DNA molecule is degraded
If the cell survives infection, any further attempt by the phage to infect the cell triggers CRASPR region transcription
Natural selection favors phage mutants that can
bind to altered receptors or are resistant to enzymes
There are only a few bacteriophages that have an envelope or RNA genome, Animal viruses have___
an envelope or RNA genome
How does an animal virus enter a host cell?
The envelope/outer layer
viral glycoproteins bind to surface cell receptors
Animal viruses make viral glycoproteins by
Ribosomes bound to
the endoplasmic reticulum (ER) of the host cell make the pro-
tein parts of the envelope glycoproteins; cellular enzymes in the
ER and Golgi apparatus then add the sugars.
These viral glycoproteins are then transported to the cell surface
New viral capsids are wrapped in membrane as they bud from the cell in animal viruses in a process much like
exocytosis
The RNA animal viruses with the most complicated replica-
tive cycles are the____ These viruses have
an enzyme called_____
retroviruses
reverse transcriptase
Reverse transcriptase transcribes
an RNA template into a DNA copy, an RNA—>DNA flow
Give a general overview of the HIV replicative cycle. Note that it is a retrovirus
Viral proteins and RNA are released into the cytoplasm and catalyze reverse transcriptase synthesis of viral DNA.
The new viral DNA enters
the cell’s nucleus and integrates into a chromosome.
The integrated viral DNA, called a provirus, never leaves the host’s genome.
viral proteins synthesized in the cytosol envelope glycoproteins made in the ER and Gogli and vesicles transport them to the surface to cover the membranous coat.
The RNA polymerase of the host transcribes the proviral DNA into RNA molecules.
The RNA molecules produced by reverse transcriptase can
can function both as mRNA for the synthesis of viral proteins and as genomes for the new viruses that will be assembled and released from the cell.
A provirus never leaves the hosts genome. In contrast,
a prophage leaves the hosts genome at the start of a lytic cycle
What is a provirus
Integrated viral DNA within a hosts genome produced during retrovirus replicative cycle
Most molecular
biologists favor the hypothesis that viruses originated from
naked bits of cellular nucleic acids that moved from one cell to
another, perhaps via injured cell surfaces.
Compare the effect on the host cell of a lytic (virulent)
phage and a lysogenic (temperate) phage.
Lytic phages can only carry out lysis of the host cell, whereas lysogenic phages may either lyse the host cell or integrate into the host chromosome. In the latter
case, the viral DNA (prophage) is simply replicated along with the host chromosome. Under certain conditions, a prophage may exit the host chromosome and initiate a lytic cycle
The RNA virus in Figure 19.8 has a
viral RNA polymerase that functions in step 3 of the virus’s
replicative cycle. Compare this with a cellular RNA polymerase
in terms of template and overall function (see Figure 17.10).
Both the viral RNA polymerase and the RNA polymerase in Figure 17.10
synthesize an RNA molecule complementary to a template strand. However,
the RNA polymerase in Figure 17.10 uses one of the strands of the DNA double
helix as a template, whereas the viral RNA polymerase uses the RNA of the viral
genome as a template
How much damage
a virus causes depends partly on
the ability of the infected tissue to regenerate by cell division.
People usually recover completely from colds because the epithelium ____. In contrast, damage inflicted by poliovirus to mature nerve cells
is permanent because
of the respiratory
tract, which the viruses infect, can efficiently repair itself.
these cells do not divide and usually cannot be replaced.
Most antiviral drugs resemble ___and thus interfere with ___
nucleosides
viral nucleic acid synthesis.
RNA viruses have a high rate of mutation because
viral RNA
polymerases do not proofread and correct errors in replicating
their RNA genomes
Name three causes of new viral disease in humans
spread of existing viruses from other animals
spread of viral diseases from small, isolated human population
Viruses mutate more easily
True or False Plant viruses have the same basic structure and mode
of replication as animal viruses
true
In horizontal transmission of plant viral diseases,
an external source infects
the plant.
In vertical transmission of plant viral diseases,
a plant inherits a viral infection from a parent
Prions are
proteins that appear to cause degenerative brain diseases
how can prions be transmitted?
food
Prions are alarming for two reasons, which are
long incubation period
they are not deactivated by heating, no known cure
How do prions become transmissible pathogens?
When the prion gets into a cell containing the normal form of the protein,it converts normal protein molecules to the misfolded prion versions.
Several prions then aggregate into a complex that can con-
vert other normal proteins to prions, which join the chain
Contrast horizontal and vertical transmission of viruses in plants.
n horizontal transmission,
a plant is infected from an external source of virus, which enters through a break
in the plant’s epidermis due to damage by herbivores or other agents. In vertical
transmission, a plant inherits viruses from its parent either via infected seeds
(sexual reproduction) or via an infected cutting (asexual reproduction).
Which of the following characteristics, structures, or processes
is common to both bacteria and viruses?
(A) metabolism
(B) ribosomes
(C) genetic material composed of nucleic acid
(D) cell division
C
Emerging viruses arise by
(A) mutation of existing viruses.
(B) the spread of existing viruses to new host species.
(C) the spread of existing viruses more widely within their
host species.
(D) all of the above.
D
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
(A) T2 protein and T4 DNA.
(B) T4 protein and T2 DNA.
(C) T2 protein and T2 DNA.
(D) T4 protein and T4 DNA
D
RNA viruses require their own supply of certain enzymes because
(A) host cells rapidly destroy the viruses.
(B) host cells lack enzymes that can replicate the viral genome.
(C) these enzymes translate viral mRNA into proteins.
(D) these enzymes penetrate host cell membranes
B
Give a general overview of how viruses make other viruses
Virus releases genome
Genome replicated and mRNA transcribed by host enzymes
Translated by host ribosomes
Self assemble
Leave,either destroy cell or leave it
The tobacco mosaic virus structure…
A rod shape with a single strand of RNA
helical capsid
The adenovirus viral structure…
2x DNA
Isocahedral capsid with protein spike at each corner
The influenza virus structure…
Membranous envelope with glycoprotein spikes
Capsid with eight 2xRNA protein complexes
The bacteriophage T4 structure…
Isocahedral head with tail apparatus
2xDNA
Give an overview of the lytic cycle, using bacteriophage entering E. Coli as an example
- Attachent: t4 uses tail fibers to bind to surface proteins
- Entry and degradation: Sheath of tail contracts, injecting phage DNA into cell. Cell DNA is hydrolyzed
3.synthesis of viral genomes and proteins: phage directs DNA production of phage proteins and copies of phage genome by host and viral enzymes, using components within the cell
- self-assembly: phage genome packaged inside capsid, head forms
- Release: cell swells and bursts as fluid enters broken wall
Some characteristics that are unique to retroviruses are
reverse
transcriptase +
integrase enzymes, &
integrating the
synthesized viral DNA
into the host’s genome
Coronaviruses are ….
enveloped,
single-stranded RNA viruses that
appear to have a crown (corona)
like structure when observed
under an electron microscope.
Coronaviruses are ____entry viruses, and that SARS-CoV 2 recognizes the ____receptor
cell fusion entry
ACE2
major difference between prions and viruses is that
viruses carry
genetic material and possibly some of its own proteins while prions are simply misfolded proteins
which can form aggregates.
A main similarity between viruses and prions is
Main similarity: Prions are infectious just as viruses are
The host range of a virus is determined by:
a. The proteins on its surface and that of the host.
b. Whether its nucleic acid is DNA or RNA.
c. The proteins in the hostʹs cytoplasm.
d. The enzymes produced by the virus before it infects the cell.
e. The enzymes carried by the virus.
a. The proteins on its surface and that of the host.
What molecule would you NOT expect to find in a
retrovirus?
a. Uracil.
b. Adenine.
c. Cytosine.
d. Thymine.
e. Guanine.
d. Thymine.
Which of the following is NOT a component of HIV (Human
Immunodeficiency Virus)?
a. Protein.
b. RNA.
c. Ribosomes.
d. Proteases.
e. Reverse transcriptase.
f. All of the above are components of HIV.
c. Ribosomes.
A SARS-CoV 2 virion is approaching a cell which does NOT have an ACE2 receptor on its surface. Based off of this information, which of the following is the most likely to occur?
a. The cell in question will become infected with SARS-CoV 2.
b. The cell in question will soon begin to synthesize viral proteins specific to
SARS-CoV 2.
c. The cell in question would not survive an encounter with the SARS-CoV 2
virion, if it were to occur.
d. The SARS-CoV 2 virion may infect a different cell inside of the host body it is
in.
e. All of the choices listed above are equally likely to occur.
d. The SARS-CoV 2 virion may infect a different cell inside of the host body it is
in.
A SARS-CoV 2 virion is approaching a cell which does NOT have an ACE2 receptor on its surface. Based off of this information, which of the following could NOT occur?
a. The cell in question will become infected with SARS-CoV 2.
b. The cell in question will soon begin to synthesize viral proteins specific to
SARS-CoV 2.
c. The cell in question would not survive an encounter with the SARS-CoV 2
virion, if it were to occur.
d. The SARS-CoV 2 virion may infect a different cell inside of the host body it is
in.
e. A & B
f. C & D
A and B
Antiviral drugs that have become useful are usually
associated with which of the following properties?
a. ability to remove all viruses from the infected host
b. interference with viral replication
c. prevention of the host from becoming infected
d. removal of viral proteins
e. removal of viral mRNAs
b. interference with viral replication
Which viruses have single-stranded RNA that acts as a
template for DNA synthesis?
a. Bacteriophages.
b. Any virus with an RNA genome.
c. Proviruses.
d. Retroviruses.
e. More than one of the above.
d. Retroviruses.
Which of the following examples of biological
matter CANNOT ever become infectious to
humans?
A. A bacterial strain that can only infect canines.
B. A virus that can only infect poultry.
C. A normally functioning human protein.
D. More than one of the above.
E. None of the above.
none of the above
All of the following are true about the CRISPR-Cas system EXCEPT …
a. In bacteria with a CRISPR-Cas system, the cas9 protein will cut and destroy the
DNA of any phage which infects that bacteria.
b. The Cas protein complex is made of a short RNA strand that is complementary to
the viral DNA strand and the Cas protein
c. CRISPR-Cas system is a defense mechanism used by bacteria to protect itself
from bacteriophage infections.
d. The CRISPR-Cas system can “remember” phages which have previously infected
it, by keeping a portion of the phage’s DNA in its genome.
e. More than one of the above
a. In bacteria with a CRISPR-Cas system, the cas9 protein will cut and destroy the
DNA of any phage which infects that bacteria.
The specific receptors on white blood cells
that HIV can bind to are the
CD4 receptor
and CCR5
co-receptors
Name some differences in the replicative cycle of an enveloped RNA virus vs a capsid one
enveloped: glycoproteins bind to receptors, may enter due to endocytosis
glycoproteins made in the ER and Golgi, vesicles transport to plasma membrane while a capsid assemebles around each viral genome
True or False: All retroviruses must have an RNA genome and must carry the reverse transcriptase enzyme
True
True or False: all retroviruses can attack mammalian CD4 cells
False, just HIV
