Gödel, Escher, Bach An Eternal Golden Braid by Douglas R. Hofstadter

ADENINE

H-

THYMINE

H-

H

H

CYTOSINE

I

H

H

FIGURE 91. The four constituent bases of DNA: Adenine, Guanine. Thymine, Cytosine.
[From Hanawalt and Haynes, The Chemical Basis of Life (San Francisco: W. H. Freeman, 1973),
p.142.]

FIGURE 92. DNA structure resembles a ladder in which the side pieces consist of alternat-
ing units of deoxyribose and phosphate. The rungs are formed by the bases paired in a special
way, A with T and G with C, and held together respectively by two and three hydrogen bonds.
[From Hanawalt and Haynes, The Chemical Basis of Life, p. 142.]