The AT/GC rule indicates that purines (A and G) always bond with pyrimidines (T and C). This keeps the width of the double helix relatively constant. Three hydrogen bonds occur between G and C but only two between A and T.
•The AT/GC rule implies that we can predict the sequence in one DNA strand if the sequence in the opposite strand is known.
•For example, let’s consider a DNA strand with the sequence of 5ʹ–ATGGCGGATTT–3ʹ. The opposite strand would have to be 3ʹ–TACCGCCTAAA–5ʹ.
In genetic terms, we would say that these two sequences are complementary to each other or that the two sequences exhibit complementarity.
•This opposite orientation of the two DNA strands is referred to as an antiparallel arrangement