A2.3 Viruses questions for IB Biology

Which structural feature is found in all viruses?

The concentration of bacteriophage lambda in a stock suspension was estimated by a plaque assay. The stock was diluted in a tenfold series. For each dilution, 0.1 cm³ was mixed with a suspension of Escherichia coli in warm soft agar and poured over a nutrient agar plate, so that the bacteria grew as an even, cloudy layer (a lawn). Three plates were prepared for each dilution. After 18 hours at 37 °C, the clear circular areas (plaques) in the lawn were counted. A control plate received bacteria but no phage.

The control plate had no plaques.

In a clinical study, people infected with HIV received either a single antiretroviral drug that inhibits reverse transcriptase, or a combination of three antiretroviral drugs that act on different viral proteins. Every three months, blood samples were tested for HIV carrying mutations that make it resistant to the drugs being taken. The graph shows the percentage of people in each group in whom drug-resistant HIV was detected.

Which statements apply to all viruses?

I. The genetic material is enclosed in a capsid.
II. A cytoplasm surrounds the genetic material.
III. A virus particle does not grow after it has been assembled.

A culture of Escherichia coli was mixed with bacteriophage lambda for 5 minutes, using about one phage for every ten bacteria so that no cell was infected by more than one phage. Phages that had not attached were then removed and the infected cells were diluted into fresh medium at 37 °C (time 0). Samples were taken every 10 minutes and the number of plaque-forming units (each forming one plaque in a plaque assay) was measured in two ways:

• Untreated: the sample was plated directly onto a bacterial lawn.

• Chloroform-treated: chloroform was first added to the sample to break open all bacterial cells, releasing any complete phages inside them, then the sample was plated.

Influenza vaccines are prepared several months before each winter, using strains of virus that are expected to circulate. A health agency estimated the effectiveness of the vaccine in six winters (the percentage reduction in the risk of illness from influenza in vaccinated people compared with unvaccinated people). In each season it also recorded whether the main circulating strain was a good or poor match to the vaccine strain, judged by how well antibodies against the vaccine strain bound to the circulating virus.