What will be the composition of the second strand of DNA if the first contains 18% guanine, 30% adenine and 20% thymine?

1. Knowing that the chains of the DNA molecule are complementary to each other, we determine the content of nucleotides (in%) in the second chain:
since in the first chain G = 18%, so in the second chain C = 18%;
since in the first chain A = 30%, so in the second chain T = 30%;
since in the first chain T = 20%, so in the second chain A = 20%;

2. Determine the content of cytosine in the first chain (in%).
we summarize the content of three other types of nucleotides in the first DNA strand: 18% + 30% + 20% = 68% (G + A + T);
determine the proportion of cytosine in the first DNA strand: 100% – 68% = 32% (C);
if in the first chain C = 32%, then in the second chain G = 32%.

Answer: C = 18%; T = 30%; A = 20%; G = 32%