Question

When setting up a PCR experiment, you must prepare a positive control sample. What would the positive control help you discover?

A) You forgot to add Loading Dye to your samples

B) The DNA ladder is denatured

C) Your thermocycler is broken

D) Your reagents are contaminated with foreign DNA

Answer 1

Answer and Explanation:

(D) Your reagents are contaminated with foreign DNA

In PCR, we use positive control sample which will always give the postive result for the test. We can compare our test results with positive control to infer the positive result. We also use negative control which is used to infer the negative result for the test.

For example in case of gene testing for cancer, a positive control sample will always gives results for the presence of mutated gene and a negative control sample will give negative results for the test (absence of mutation). We can compare our samples with positive and negative control to infer the results.

Suppose if positive control sample is producing a band at 200 bp and our sample produces a band which is very different from the positive control and produces a band at 500 bp or if it gives two or three bands it can be inferred that our reagents are contaminated with foreign DNA.

For other options,

If we did not add loading dye to the samples, the samples will not settle in the gel and also the tracking of the samples is not possible. If the DNA ladder is denatured, it will affect the calculation of size of the PCR product. If a thermal cycler is broken, even the positive control will not work.