MLT GlobeVirology: General virology & lab methods – page 4
76 Virology MCQs on General virology & lab methods with answers and explanations.
Which nucleoside analogue has broad activity against RNA viruses, used for example in severe RSV infection and hepatitis C combination therapy?
Ribavirin, a guanosine analogue, inhibits many RNA viruses. Zidovudine targets HIV reverse transcriptase, ritonavir HIV protease, and enfuvirtide HIV fusion.
Negative-sense RNA viruses such as influenza must carry which enzyme inside the virion to start infection?
A negative-sense genome cannot be translated directly, so the virion brings its own RNA-dependent RNA polymerase to make mRNA. Reverse transcriptase is carried by retroviruses.
Potato spindle tuber disease is caused by a small circular RNA with no protein coat at all. What is this type of agent called?
Viroids are naked, circular, non-coding RNAs that infect plants. Prions are the reverse: infectious protein without nucleic acid.
Immune globulin is used for post-exposure prophylaxis of several viral infections. For which condition is it NOT used?
No immune globulin is used for EBV mononucleosis. Normal immune globulin protects against hepatitis A, and specific HBIG and rabies IG are part of post-exposure prophylaxis.
Bacterial identification often uses 16S rRNA sequencing. Why is no equivalent universal gene available for classifying all viruses?
Viruses lack ribosomes and rRNA and have arisen from diverse lineages, so no gene is common to all. Viral genomes are small, and a virion contains either DNA or RNA.
Which virus has a simple helical nucleocapsid with no envelope?
TMV is the classic naked helical virus. T4 and poxviruses have complex symmetry, and herpesviruses have an icosahedral capsid inside an envelope.
The envelope of an enveloped virus is best described as:
The envelope is obtained by budding through a host membrane, so its lipids are host-derived, while the glycoprotein spikes are encoded by the virus. This is why lipid solvents inactivate it.
Which of these is a giant virus with a genome of more than one megabase?
Megavirus chilensis, a mimivirus relative that infects amoebae, has a genome of about 1.26 Mb. Herpesviruses are about 150 kb, and RNA viruses are far smaller.
Why should calcium alginate or wooden-shaft swabs be avoided for viral specimens?
Calcium alginate and substances in wood can be toxic to viruses and inhibit amplification. Synthetic swabs such as flocked nylon or polyester with plastic shafts are used.
A viral specimen in transport medium cannot be tested for 5 days. How should it be stored?
Specimens can be held at 2–8 °C for up to about 72 hours. For longer delays, freezing at −70 °C preserves viruses; −20 °C and freeze–thaw cycles reduce recovery.
In real-time PCR, sample A has a cycle threshold (Ct) of 18 and sample B has a Ct of 32 for the same target. What does this mean?
Ct is the cycle at which fluorescence crosses the threshold; more starting target gives an earlier (lower) Ct. Each cycle difference is about a twofold difference in target.
What is the purpose of an internal control in a viral PCR assay?
The internal control is amplified in every sample. If it fails, a negative result is invalid because inhibitors or poor extraction may have caused it.
For rapid electron microscopy of viruses in a stool or fluid sample, particles are commonly 'negatively stained' with:
Heavy-metal salts such as phosphotungstic acid surround the particles, so they stand out light against a dark background.
Which virus has a lipid membrane located inside its capsid rather than as an outer envelope?
Iridoviruses have an internal lipid membrane beneath the icosahedral capsid. Rabies, influenza and Ebola viruses have conventional outer envelopes from host membranes.
In an icosahedral capsid such as adenovirus, the capsomeres at the 12 vertices are:
Each of the 12 vertices has a five-fold axis occupied by a penton. Hexons make up the faces and edges.
PCR mixes containing dUTP and uracil-N-glycosylase (UNG) are used to:
Amplicons made with dUTP contain uracil. UNG removes uracil from any carried-over product before the new run, so it cannot be amplified, while natural DNA is not affected.