Deoxyribonucleic acid antiparallel strands

During the past half a century, fundamental scientific discoveries have been aided by the symmetry concept. They have played a role in the continuing quest for establishing the system of fundamental particles [7], It is an area where symmetry breaking has played as important a role as symmetry. The most important biological discovery since Darwin s theory of evolution was the double helical structure of the matter of heredity, DNA, by Francis Crick and James D. Watson (Figure 1-2) [8], In addition to the translational symmetry of helices (see, Chapter 8), the molecular structure of deoxyribonucleic acid as a whole has C2 rotational symmetry in accordance with the complementary nature of its two antiparallel strands [9], The discovery of the double helix was as much a chemical discovery as it was important for biology, and lately, for the biomedical sciences. 

The structure of deoxyribonucleic acid was determined in 1953 by Watson and Crick to consist of two antiparallel strands of deoxyribonucleic acid coiled around a common axis in a double helix (Fig. 6.6). The purine and pyrimidine bases are on the inside of the helix, whereas the phosphate and deoxyribose units are on the outside. The planes of the bases are perpendicular to the axis of the helix. The planes of the sugars are at approximately 70° to those of the bases. The helix diameter is 20 A adjacent bases are separated by 3.4 A along the... 

Polynucleotides include ribonucleic acid (RNA) and deoxyribonucleic acid (DNA). Under physiological conditions, DNA normally exists in the p-form, which consists of two antiparallel strands the Watson-Crick double helix. Interstrand hydrogen bonding connects the two strands. An adenosine in one strand can hydrogen bond with a thymine in the second, and a guano-sine can hydrogen bond with a cytosine in the other strand. The strands must be antiparallel 32, 52, 63, 64. 

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