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According to Chargaff purines and pyrimidines are in equal amounts. Purine (adenine) is equimolar with pyrimidine (thymine) and purine (guanine) is equimolar with pyrimidine (cytosine). Base ratio \( \frac{A + T}{C + G} \) is specific for species.

1. Base ratio is \( \frac{A + T}{C + G} \).
2. Base ratio is \( \frac{C + G}{A + T} \).
3. Base ratio is \( \frac{A}{T} \).
4. Base ratio is \( \frac{C}{G} \).

Correct answer: Base ratio is \( \frac{A + T}{C + G} \).

Solution

Chargaff’s observations show A pairs with T and G pairs with C, so A = T and G = C in double-stranded DNA. Therefore the species-specific base ratio is written as the total of A and T divided by the total of C and G.

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