DNA Can Form a Regular Helix
DNA molecules usually have regular helical configurations. This is because most DNA molecules contain two antiparallel polynucleotide strands that have complementary structures (see Chapter 6 for more details). Both internal and external noncovalent bonds stabilize the structure. The two strands am held together by hydrogen bonds between pairs of complementary purines and pyrimidines (Figure 5-1). Adenine is always hydrogen-bonded to thymine, whereas guanine is
70 Weak ont! Strong Hands OH «nui ne Macromoleenlar Structure
FIGURE 5-1 The hydrogen-bortded base pain of DNA. The figure shews the position ¿nd length of the hydrogen bondi hfiween the base pyirE. The codaient bonds bciween the atoms withtn each base are shown, but double and single bonds are not distinguished (see Figure 6-6 in the next diepter).
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