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Among amino acid codons that require a third-position pyrimidine, there is a significant bias favoring the use of cytidine over uracil in MS2 phage RNA. This could arise from selection against wobble pairing in the interaction of transfer RNA and messenger RNA. Among amino acid codons with fourfold degeneracy, there is a bias favoring pyrimidines over purines.
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Walter M. Fitch (1976) studied this question.
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