Worked example 4 — base percentages

Given: a DNA sample contains 28% adenine. Find the percentages of T, G and C. A = T = 28%; A + T = 56%; so G + C = 44%, and G = C = 22%. Hydrogen bonds: a segment of 100 base pairs with 30 A–T pairs and 70 G–C pairs has (30×2) + (70×3) = 270 hydrogen bonds. Segments rich in G–C are more heat-stable.