IMAT 2013 Q40 [DNA Nucleotides]

A sample of DNA contains 32 % guanine.

Which answer shows the percentage of thymine in the same sample?

A. 32
B. 18
C. 23
D. 34
E. 24

The answer to this question is choice B, the percentage of thymine is 18%

Let’s try to approach this in a step-by-step manner to be able to arrive at the answer:

You have to be aware of the concept of base pairing: Within DNA, there is a constant trend of nitrogenous base pairing. Adenine pairs with Thymine, while Guanine pairs with Cytosine. For the sake of answering this question, we will be denoting them as A, T, G, and C, respectively. To begin answering the question, you need to remember that in terms of proportion, it’s safe to assume that A = T, G = C since these base pairs are constant.

To solve the problem given:

  1. Total guanine = 32%
  2. G = 32% (and we know that G =C), therefore;
  3. C = 32%
  4. The total nucleotides therefore of G and C combined is = (32 x 2) = 64%
  5. So it would make sense that, (A and T combined) = Total Nucleotides MINUS (G and C combined)
  6. 100% - 64% = 36, which means that there are 36% of nucleotides that are A and T (Note that, 100% is used as the value of ‘Total nucleotides’ since the values are expressed in terms of percentages.)
  7. Since we know that A = T, we can then divide that number by 2 = (36/2) = 18%
  8. This means that there are 32% G, 32% C, 18% A, and 18% T
  9. Therefore, the answer to this question is that the percentage of thymine in the sample is 18%.