5' PO4 -------- A T T C C CG T A T C T G-----------OH 3'

3' OH -------- T A A G G GC A T A G A C----------PO4 5'

This diagram represents a portion of a DNA molecule. The organic bases that lie in the center of the molecule are represented by the letters A (adenine), T (thymine), C (cytosine), and G (guanine). Note the presence of a single CG sequence in the upper strand and its complement GC in the lower strand. For simplicity the hydrogen bonds joining the complementary base pairs are not shown. The ----- represent an unspecified number of base pairs.