BIO 554 MIDTERM EXAM I (10/8/03)
1.
Which of the following statements regarding cellular receptors for viruses is
untrue?
a.
virus receptors correspond to normal cellular proteins
b.
viruses always use the same unique receptor no matter what cell type
c. binding of enveloped virus to receptors is often
followed by formation of an endocytotic vesicle
d.
receptor-binding proteins of viruses are not always glycoproteins
e.
some virus receptors are found on many cell types while others are found on
only a few cell types
2.
Which of the following forms the basis for the Baltimore scheme of RNA virus
classification?
a.
size of the virus genome
b.
pathogenic properties of the virus
c.
presence of a virus envelope
d.
how the virus produces mRNAs
e. presence of more than one genome segment
3.
Which of the following viruses is non-enveloped?
a.
Sindbis
b.
MS2
c.
SARS
d.
yellow fever
e.
West Nile
4.
Which of the following distinguishes protein translation in prokaryotes versus
eukaryotes?
a.
start codons are different
b.
stop codons are different
c.
binding of small ribosomal subunit to mRNA prior to assembly of full ribosome
d.
requirement for several initiation factors
e. recognition of the mRNA cap structure
5.
Which of the following enzymatic activities is always encoded by RNA viruses
other than retroviruses?
a.
RNA helicase
b.
protease
c.
RNA-dependent RNA polymerase
d.
membrane fusion activity
e.
capping activity
6.
Splicing of precursor mRNAs takes place with many viruses. Which of the
following features is common to all such viruses?
a.
genome is DNA
b.
genome is RNA
c.
genome encodes virus-specific splicing enzymes
d.
spliced mRNAs encode structural proteins only
e.
virus requires cell nucleus
7.
Which of the following is commonly required for icosahedral virus capsid
assembly?
a.
processing of viral glycoprotein through cellular Golgi apparatus
b.
sequential synthesis of each virus capsid protein
c.
insertion of viral proteins in cell membranes
d.
cleavage of precursor viral proteins by maturational proteases
e.
transport of capsid precursor from nucleus to cytoplasm
8.
Which of the following properties is used for titering transforming viruses in
cell culture?
a. inhibition of cell growth
b.
loss of contact inhibition of cell growth
c.
cytopathic effect of virus infection
d.
plaque formation
e.
change in cell shape
9.
Viruses are often purified by centrifugation techniques. Which of the following
applies to equilibrium as opposed to differential centrifugation?
a. speed of centifugation
b.
time of centrifugation
c.
virus is recovered in the pellet
d.
virus is recovered as a band in a solution gradient of salt or other small
molecular weight compound
e.
virus is separated from some cellular components
10.
Virus gene complementation assays differ from virus gene recombination assays
in which of the following ways?
a.
mutants must be capable of growing under permissive conditions
b.
double infection (both mutants infecting same cell) must take place under
permissive conditions
c.
one mutant must be a temperature sensitive mutant
d.
both mutants must be temperature sensitive
e.
both mutants must be growth restricted under non-permissive conditions
11.
Which of the following virus is not a picornavirus?
a.
hepatitis B
b.
Coxsackie
c.
rhinovirus
d.
poliovirus
e.
hepatitis A
12.
What is the function of the IRES element in the poliovirus genome?
a.
encodes the virus polymerase
b.
internal start site for the virus polymerase
c.
capsid protein binding site
d.
translation start site
e.
ribosome entry site for internal intiation of translation
13.
Which of the following distinguishes flaviviruses from togaviruses?
a.
cap structure at the 5Õ end of the genome
b.
positive-stranded RNA genome
c.
all members are enveloped viruses
d.
capsid proteins translated from a subgenomic RNA
e.
some viral proteins are processed from precursors
14.
Which of the following viruses require suppression of a translation termination
codon for multiplication?
a.
poliovirus
b.
coronavirus
c.
Qb
d.
Sindbis
e. yellow fever
15.
Which of the following correctly describes the structure of coronavirus
subgenomic RNAs?
a.
all contain a common 5Õ end and a common 3Õ end sequence
b.
all have a different 5Õ end and a common 3Õ end sequence
c.
all contain a common 5Õ end and a different 3Õ end sequence
d.
all are the result of splicing by cellular enzymes
e.
all but one are synthesized by the viral polymerase initiating internally on a
viral minus strand
16.
Which of the following virus properties is found uniquely in plant viruses?
a.
genome-linked Vpg protein
b.
lack of poly(A) tail at the 3Õ end
c.
dsRNA genome
d.
helical capsid
e.
multiple genome segments separately encapsidated
SHORT ESSAY QUESTIONS (10
points each)
1. Your innocent technician has just performed a plaque assay on a sample of SARS virus and reports the following results to you. He/she claims that at a dilution of 106, 41 and 46 plaques appeared on duplicate wells of a six-well plate. At 107 dilution, 52 and 43 plaques were visible while at at 108 dilution 3 and 6 plaques were observed (0.2 ml of virus solution is plated on each well in all cases). When the experiment is repeated the values were Òtoo many to countÓ at 106, 43 and 54 at 107, and 4 and 5 at 108. What is your best estimate of the titer of the virus solution and what is your best guess explanation for the discrepant results?
Results appear to be
reproducible at 107 dilution and 108 dilution. Plaque
numbers at 108 dilution are too low for an accurate estimate. The plaque counts at 107 are
41, 46, 43, and 54 with an average of 46/0.2 ml or 230/ml. The best estimate of
titer is therefore 230 X 107 or 2.3 X 109
pfu/ml.
The most likely
explanation for the discrepant result is mistakenly putting 0.2 ml of the 107
dilution instead of the 106 dilution in the wells labeled 106
dilution for the first experiment.
2. Explain temporal gene regulation and coupling to replication in Qb virus multiplication.
The genome of Qb bacteriophage is positive sense
and encodes three genes in the
following 5Õ to 3Õ order: A protein, capsid, and replicase. When the viral genome is released from
the virion into the cytoplasm, it
displays extensive secondary structure
that masks translation initiation sites
for the A protein and the capsid protein. The early phase of virus multiplication thus involves synthesis
of replicase only. Once the replicase copies
the virion RNA into negative sense strands, the latter serve as template for
more positive strand synthesis. Nascent positive sense strands lack the
secondary structure associated with completed
strands and host ribosomes gain access to translation initiation sites for A protein and capsid protein. This late phase of virus
multiplication also involves inhibition of replicase translation by capsid protein accumulation.