6.3 The development of understanding of genetics and evolution

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Question 12
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Scientists investigated the effectiveness of three different antibiotics (labeled A, B, and C) against a strain of Escherichia coli (E. coli) bacteria.

They placed paper discs containing the antibiotics onto an agar plate inoculated with the bacteria. They also placed a control paper disc, labeled D, containing no antibiotic.

The plate was incubated and the results are shown in the diagram below:

A Petri dish containing nutrient agar inoculated with E. coli bacteria. There are four circular paper discs placed on the agar: Disc A has a medium-sized clear circular zone of inhibition around it, with a measured diameter of 24 mm. Disc B has a smaller clear circular zone of inhibition around it, with a measured diameter of 16 mm. Disc C has no clear zone around it. Disc D is labeled as a control disc with no antibiotic and has no clear zone around it. The background agar is shaded grey representing bacterial growth, while the zones of inhibition are white.

a.

What is the purpose of the control disc D in this investigation?

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b.

The diameter of the clear zone of inhibition around antibiotic disc A is 24 mm24\text{ mm}24 mm.

Calculate the area of this zone of inhibition.

Use the equation: Area=πr2\text{Area} = \pi r^2Area=πr2

Take π≈3.142\pi \approx 3.142π≈3.142. Give your answer to 3 significant figures.

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c.

A year later, a new outbreak occurred, and antibiotic A was no longer effective against the E. coli bacteria. Suggest why antibiotic A no longer had an effect.

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6.3 The development of understanding of genetics and evolution Questions

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