15.
A single-stranded RNA virus was grown in cell culture, either in absence of or in
presence of chemicals X/Y. The intracellular genome copy number of the virus, and titer
in the cell culture supernatant (plaque forming units) were estimated over several hours.
Based on the data presented below, choose the most correct option.
a. X inhibits viral entry and Y inhibits viral genome replication.
b. X inhibits viral genome replication and Y inhibits viral entry.
c. X inhibits viral release and Y inhibits viral entry.
d. X inhibits viral entry and Y inhibits viral release.

The most correct option is (d) X inhibits viral entry and Y inhibits viral release.​

Understanding the question

The experiment measures two parameters over time for a single-stranded RNA virus:

• Intracellular viral genome copy number (left graph).

• Plaque-forming units (PFU) in the supernatant as a proxy for infectious virus released from cells (right graph).​

Three conditions are compared: mock (no drug), chemical X, and chemical Y.

• In the genome copy graph, the curve for chemical X is lower than mock, while chemical Y is similar to or slightly above mock.

• In the PFU graph, chemical Y is much lower than mock, whereas chemical X is only moderately reduced relative to mock.

From basic virology, a drug that blocks entry will reduce both intracellular genome copies and PFU, whereas a drug that blocks release will allow genome replication but reduce extracellular PFU.​

Why option (d) is correct

• Chemical X:

  • Intracellular genome copies are substantially reduced compared to mock, meaning fewer cells are successfully infected or viral replication is initiated in fewer cells.

  • PFU titers are also reduced, consistent with fewer initially infected cells, but the magnitude of drop in genome copies is the key signal.

  • This pattern matches an entry inhibitor, which prevents virions from entering cells and starting replication.​

• Chemical Y:

  • Intracellular viral genome copies are similar to or even higher than mock, indicating that once the virus is inside the cell it replicates efficiently.

  • However, PFU in the supernatant is sharply reduced, showing that infectious virions are not efficiently released or matured.

  • This is the hallmark of an inhibitor of viral release (or late assembly/maturation step), where genomes accumulate inside but infectious particles outside remain low.​

Therefore, the best interpretation is:
X inhibits viral entry and Y inhibits viral release → option (d).

Detailed analysis of each option

Option (a): X inhibits viral entry and Y inhibits viral genome replication

• If Y inhibited genome replication, intracellular viral RNA would remain low because the virus could not amplify its genome effectively.​

• The graph shows high intracellular genome copies with Y, contradicting genome replication inhibition.

• Thus, option (a) is inconsistent with the data.

Option (b): X inhibits viral genome replication and Y inhibits viral entry

• An entry inhibitor (Y, as stated in this option) would cause low intracellular genome copies (few infected cells) and low PFU.​

• The data show high intracellular genome copies for Y, ruling out an entry block.

• For X, an inhibitor of genome replication would allow normal initial entry but then reduce accumulation of viral RNA per infected cell; however, PFU often still rises initially because some virions can be made from input genomes.​

• The pattern of X fits better with reduced number of infected cells (entry) than with a pure replication block, so option (b) is not the best choice.

Option (c): X inhibits viral release and Y inhibits viral entry

• A release inhibitor (X, as stated here) would show normal or high intracellular genome copies but low PFU, which is the opposite of the observed pattern for X (low genomes, modest PFU reduction).​

• Again, Y behaves like a release inhibitor, not an entry inhibitor, because genomes accumulate but PFU stay low.

• Hence, option (c) clearly contradicts the graphs.

Option (d): X inhibits viral entry and Y inhibits viral release

• Matches the expected kinetic signatures:

  • Entry inhibition (X): fewer infected cells → lower intracellular genomes and lower PFU.​

  • Release inhibition (Y): genome replication intact → high intracellular genomes but reduced extracellular PFU.​

• This option aligns with both graphs and known virus life-cycle dynamics, so it is the most correct interpretation.

SEO-optimized introduction using the key phrase

Understanding how to read a viral growth curve with chemical X and Y for a single-stranded RNA virus is essential for CSIR NET Life Sciences and other competitive exams. Interpreting changes in intracellular genome copies and plaque-forming units allows precise identification of inhibitors targeting viral entry, replication, or release.​