Question

If the percentage of cytosine is 18%, then the percentage of thymine will be
(a) 32%
(b) 64%
(c) 36%
(d) 23%

Answer 1

Hint: The number of purines is equal to the number of pyrimidines, following Chargraff 's law. Adenine and guanine are purine while cytosine and thymine are pyrimidines.

Complete answer:
Complementary bases are identical to one another. Therefore the amount of guanine should be equal to cytosine and thymine should be equal to the amount of adenine. As we all know, 100 percent of the bases are made up of adenine, guanine, cytosine, and thymine. To calculate the percentage of thymine, first, you need to decide the percentage of guanine, which is very simple since we know that guanine is equivalent to cytosine; hence, there is also 18 percent of guanine. Now we implement Chargaff’s Rule to find the thymine number. We can therefore deduce the proportion of guanine and cytosine as 36% (18% of G and 18% of C) , which we can then deduce from 100%, leaving us 64%. Since adenine and thymine are complementary base pairs, we now have a 32 percent of thymine in this DNA sequence.
So, the correct answer is, '32%'.

Note: Adenine (A) base pairs with thymine (T) while cytosine (C) base pairs with guanine (G) due to their ability to create hydrogen bonds between the respective bases (two bonds between A & T and three bonds between C & G). The capacity to form hydrogen bonds makes the base pairs structurally more stable. These simple pair relationships are also referred to as Chargaff 's DNA base pairing laws, named after scientists at Columbia University who have found that there is the equal molar concentration of A & T, as well as G & C in most DNA molecules.