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Modern Genetic Analysis Chapter 2. The Structure of Genes and Genomes
Solved Problem

1. | A molecule of double-helical DNA was found to have a purine:pyrimidine ratio of 1:4 in one nucleotide chain. What must be the purine:pyrimidine ratio in the other strand? See answer

| Solution What facts about DNA are relevant here? We know from the general structure of DNA that a purine in one strand will always pair with a pyrimidine in the other strand (purine A pairs with pyrimidine T, and purine G pairs with pyrimidine C). In the strand in question the purines occupy 1/(1 + 4), or 1/5, of the sites in one strand, so in the other strand 1/5 of the sites must be pyrimidines. Likewise 4/5 of the sites in the other strand must be purines. Therefore, the purine:pyrimidine ratio in the other strand will be 4/5:1/5, which is 4:1. (Note that this is the inverse of the ratio in the other strand.) See question

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