The nucleotides are shown with standard numbering convention. The aromatic base atoms are numbered 1 through 9 for purines and 1 through 6 for pyrimidines. The ribose sugar is numbered 1' through 5'. Atoms or groups attached to the base or sugar rings atoms have the same number as the ring atom to which they are bonded. For example, the carbonyl O double-bonded to C6 of G is called O6. All of the nucleotides are shown in the anti conformation, which is typical of nucleotides in standard Watson-Crick base pairs. In more unusual cases, the base is oriented 180° opposite from that shown, in the syn conformation.
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