[W2] practical virology

Question 1

What are three methods used to quantify viruses?

Answer 1

Plaque assays, quantal dose response (LD50), hemagglutination.

Question 2

What techniques are used to study virus structure?

Answer 2

Electron microscopy, X-ray crystallography, density gradient centrifugation.

Question 3

How can viruses be detected in a sample?

Answer 3

By detecting viral proteins (serological methods) or viral nucleic acids (qPCR, sequencing).

Question 4

What is a plaque assay used for?

Answer 4

To measure virus quantity in PFU/mL by counting plaques formed on a bacterial lawn.

Question 5

What does LD50 represent?

Answer 5

The dose at which 50% of infected organisms die.

Question 6

What virus model was used in many foundational experiments?

Answer 6

Bacteriophages.

Question 7

What was the significance of the Hershey and Chase (1952) experiment?

Answer 7

It demonstrated that DNA, not protein, is the genetic material.

Question 8

What isotopes were used in the Hershey-Chase experiment?

Answer 8

³²P (for DNA) and ³⁵S (for protein).

Question 9

What did Luria and Delbrück’s fluctuation test prove?

Answer 9

That mutations arise spontaneously, not in response to selective pressure.

Question 10

What is the key concept of the one-step growth curve by Ellis and Delbrück (1939)?

Answer 10

Virus replication occurs in stages: infection, replication, assembly, and release.

Question 11

What is the eclipse period in the one-step growth curve?

Answer 11

A phase where no infectious particles are detected as viruses have disassembled and are replicating inside cells.

Question 12

What defines the rise period in viral replication?

Answer 12

Release of mature virions from the host cell.

Question 13

How is MOI (Multiplicity of Infection) calculated?

Answer 13

MOI = (total virus particles) / (total host cells).

Question 14

Does MOI = 1 mean all cells are infected?

Answer 14

No, only ~63% are infected due to the Poisson distribution.

Question 15

What is the formula for Poisson distribution used in virus infection?

Answer 15

P(n) = (mⁿ × e^(–m)) / n!, where m is the MOI.

Question 16

How do you calculate the percentage of infected cells?

Answer 16

P(n > 0) = 1 – P(0).

Question 17

What is burst size?

Answer 17

The number of phage particles produced per infected bacterium, calculated as phage yield (PFU/mL) divided by infected bacteria titre (CFU/mL).

Question 18

How can the eclipse period be experimentally measured?

Answer 18

By artificially lysing infected cells with chloroform to release intracellular viruses, which are then counted via plaque assay.

Question 19

What happens during the eclipse period?

Answer 19

Virus particles disassemble after entering the host, viral genome is released, host DNA is degraded, and viral mRNAs/proteins begin synthesis.

Question 20

What does the intracellular accumulation phase refer to?

Answer 20

The period after the eclipse, when new phages are assembled inside the cell but not yet released.

Question 21

When do the curves of total and extracellular virus counts merge in a one-step growth curve?

Answer 21

About 40 minutes post-infection, when all cells have lysed.

Question 22

What does burst size measure and how is it calculated?

Answer 22

It measures phage particles per infected cell: Burst size = (phage yield in PFU/mL) ÷ (infected bacterial titre in CFU/mL).

Question 23

Why is synchronizing infection important in the one-step growth curve?

Answer 23

It ensures that all host cells are infected simultaneously, allowing replication stages to be observed uniformly.

Question 24

In MOI calculation, why can’t you use just the titre of the virus stock?

Answer 24

Because MOI depends on the actual number of particles used in the volume added, not just the stock concentration.

Question 25

What is the probability of a cell being uninfected at MOI = 1?

Answer 25

P(0) = e^–1 ≈ 0.37 → 37% remain uninfected.

Question 26

What is the formula for probability that a cell is infected by at least one virus?

Answer 26

P(n > 0) = 1 – P(0).

Question 27

In a culture where MOI = 1, what percentage of cells are infected by 2 phages?

Answer 27

P(2) = (1^2 × e^–1) / 2! = 0.18 → 18%.

Question 28

What’s the Poisson distribution formula for viral infection?

Answer 28

P(n) = (mⁿ × e^(–m)) / n!, where m = MOI.

Question 29

Example: 1x10⁸ phage mixed with 1x10⁸ bacteria. What is MOI?

Answer 29

MOI = 1.

Question 30

Example: At MOI = 1, what percent of cells are infected?

Answer 30

63% → P(n > 0) = 1 – 0.37.

Question 31

Example: Add 100 µL of phage (1×10¹⁰ PFU/mL) to 100 mL culture. Final volume 50 mL. Bacteria = 1×10⁷ CFU/mL. What is MOI?

Answer 31

MOI = 2.

Question 32

For MOI = 2, what is P(n > 0)?

Answer 32

P(0) ≈ 0.14 → P(n > 0) = 0.86 → 86% of cells infected.