MLT GlobeVirology: General virology & lab methods – page 3
76 Virology MCQs on General virology & lab methods with answers and explanations.
During PCR, the step in which the reaction is cooled to about 50-65 °C so primers bind the template is:
At annealing, primers hybridise to complementary template sequences. Denaturation (~95 °C) separates strands, and extension (~72 °C) lets the polymerase add nucleotides.
A phage that can either lyse its host or integrate and persist with it as a prophage for generations is a:
Temperate phages such as lambda can enter lysogeny. Virulent (lytic) phages always kill their host.
Why must RNA viruses such as HCV and HIV undergo an extra step before PCR amplification?
Taq DNA polymerase needs a DNA template. Reverse transcription produces complementary DNA, which is then amplified (RT-PCR).
Paired sera tested 2 weeks apart show a rise in antibody titer from 1:8 to 1:64. This indicates:
A fourfold or greater rise in titer between acute and convalescent sera shows active infection. A rise from 1:8 to 1:64 is eightfold.
Viral transport medium usually contains protein (such as albumin or gelatin) and antimicrobial agents. The antimicrobials are added to:
Antibiotics and antifungals stop contaminating flora from damaging the specimen or cell cultures, while the protein stabilises the virus. They do not inactivate the virus.
Enveloped viruses are more easily inactivated than non-enveloped viruses by:
The envelope is a lipid membrane, so ether, alcohol and detergents destroy it and make the virus non-infectious.
Cytopathic effect in a cell culture monolayer grown in a tube or flask is usually examined with:
An inverted microscope views cells through the bottom of the vessel, so living monolayers can be checked daily for CPE.
Why does a molecular laboratory keep reagent preparation, sample extraction and amplified-product analysis in separate rooms with one-way workflow?
Amplified DNA is present in huge copy numbers; if it reaches pre-PCR areas it causes false positives. Physical separation and unidirectional flow prevent this carryover.
When designing primers for a routine diagnostic PCR, which primer length is typically chosen?
Primers of about 18-25 nt give adequate specificity and a workable annealing temperature. Very short primers bind non-specifically; very long ones anneal poorly and form secondary structures.
Which statement about the host response to viral infection is INCORRECT?
Cell-mediated immunity (cytotoxic T cells, NK cells) is essential to clear virus-infected cells; antibodies mainly neutralize free virus. The other statements are true.
Some bacteriophages carry a lysozyme-like enzyme in their tail. What is its role during infection?
Phage tail lysozyme punches a hole in the peptidoglycan so the DNA can be injected; later, endolysins lyse the cell for release. Animal viruses do not need this enzyme.
In 1898 a Dutch microbiologist showed the tobacco mosaic agent was a filterable "contagium vivum fluidum" and introduced the term virus. Who was he?
Beijerinck recognised the agent as a new kind of infectious entity and called it a virus. Ivanovsky first showed filterability (1892); Twort discovered bacteriophages.
In 1892, who showed that the agent of tobacco mosaic disease still infected plants after passing through bacteria-proof filters, the first evidence of viruses?
Ivanovsky's filtration experiment first showed a sub-bacterial infectious agent. Beijerinck later named it a virus (1898), and Twort discovered phages (1915).
A neonatal TORCH-style viral workup would NOT normally include which virus, since it does not cause congenital infection?
RSV infects infants after birth by the respiratory route and is not transmitted transplacentally. CMV, rubella, parvovirus B19 and VZV can all infect the fetus.
An icosahedral capsid is built with 5:3:2 rotational symmetry. How many triangular faces and vertices does it have?
An icosahedron has 20 equilateral triangular faces, 12 vertices and 30 edges; pentamers sit at the 12 vertices. A cube has 6 faces and 8 vertices.
Monkey kidney cells taken directly from tissue survive only one or two subcultures. What is this type of culture called?
Primary cultures come straight from tissue and have limited passage; they are sensitive for many viruses (e.g. influenza, enteroviruses). Diploid strains last about 50 passages; continuous lines are immortal.
Tobacco mosaic virus is typical of most plant viruses in genome type. What is that genome?
The majority of plant viruses, including TMV, have positive-sense ssRNA genomes. DNA plant viruses (geminiviruses, caulimoviruses) are the minority.
When an electron micrograph report describes a virus as "isometric", what capsid symmetry is usually meant?
Isometric (roughly spherical) viruses almost always have icosahedral symmetry. Helical viruses are rod-shaped or filamentous; complex/binal describes poxviruses and tailed phages.
A virus enters a cell and begins replication, but no infectious progeny are released. What is this type of infection called?
Abortive infection occurs in non-permissive cells or with defective virus: replication starts but produces no infectious progeny. Latent infection keeps the genome silently for later reactivation.
A vaccine is made by growing large quantities of wild-type virus and then treating it with formalin so it cannot replicate. What vaccine type is this?
Inactivated vaccines start from fully virulent virus that is chemically killed, e.g. IPV and hepatitis A vaccine. Live attenuated vaccines use weakened, replicating strains.