Exemplar Problems

Question:

If a double-stranded DNA has 20 percent of cytosine, calculate the percent of adenine in the DNA.

Answer:

To calculate the percent of adenine, we use Chargaff’s rules, which state that in DNA, the amount of adenine (A) is equal to the amount of thymine (T), and the amount of cytosine (C) is equal to the amount of guanine (G). Therefore, if there is 20 percent cytosine (C), there must also be 20 percent guanine (G).

The remaining percentage not accounted for by C-G pairs is 60 percent (100% - 20% - 20%).

Since A-T pairs are complementary, the percent of adenine (A) is also 60 percent.