How do these proteins, known as transcription factors, recognize their target sites on DNA? Th ey do so by interacting with the edges of the base pairs in double-helical DNA, which are exposed within the major and minor grooves. Th ere are four potential types of interactions with complementary base pairs, illustrated in Figure 2.13. Th ese involve, on the DNA, hydrogen-bond donors (for example, the amino group of adenine in Figure 2.13C and D), hydrogen-bond acceptors (for example, the N7 nitrogen atom of guanine in Figure 2.13A and B), hydrophobic groups (for exam- ple, the methyl group of thymine in Figure 2.13C and D), and nonpolar hydrogen atoms (for example, the ring proton of cytosine in Figure 2.13A and B).