Watson—Crick double helix

The most famous stracture in aU chemistry is the Watson-Crick double helix for DNA (figure 12.3). The discovery of this structure by James Watson and Francis Crick in 1953 was the beginning of molecular biology. An amazing number of insights about the nature of life have been derived from this structure. 

The Watson-Crick double helix is the outcome of three lines of work. The first is the discovery by Erwin Chargaff of Chargaff s rules." Specifically, for all normal DNAs, A = T, G = C and A + G = C + T. The actual content of each base in DNA varies from species to species over a wide range. Despite this variation, the content... 

Figure 2.1. A polydeoxynucleotide chain and the structure of DNA. (A) A polydeoxynucleo-tide chain composed of four different deoxynucleotides covalently linked to one another by a phosphodiester bonds (note orientation of the 5 and 3 end of the molecule) (B) Watson-Crick double helix (top) and simple representation of the antiparallel strands (bottom). [Adapted from Volgelstein, B., and Kinzler, K. W. (Eds.), The Genetic Basis of Human Cancer, McGraw-Hill, New York, 1998.]...

Levels of DNA Structure. A DNA molecule has several levels of structure ranging from the primary structure of the sequence of bases to the secondary structure of the Watson-Crick double helix to the tertiary structure resulting from folding or supercoiling the double helix to even higher order structures involved in the condensation of DNA in the cell nucleus. To serve as a basis for understanding the interaction of platinum complexes with DNA, we first describe some of the more important features of DNA structure. 

GGTTACGT. The convention for indicating polarity is that the 5 -end of the sequence is written to the left. The two chains of the Watson-Crick double helix are antiparallel, so the correct complementary sequence is not TGCATTGG. 

When replication occurs, the two strands of the Watson-Crick double helix must separate so that each can serve as a template for the synthesis of its complement. Since the two strands are complementary to one another, each bears a definite sequence relationship to the other. When one strand acts as a template, it directs the synthesis of its complement. The product of the synthesis directed by each template strand is therefore a duplex molecule that is identical to the starting duplex. The process is accurate because of the specificity of base pairing and because the protein apparatus that catalyzes the replication can remove mismatched bases. 

Abstract The physical aspects of DNA structure and function are overviewed. Major DNA structures are described, which include the canonical Watson-Crick double helix (B form), B , A, Z duplex forms, parallel-stranded DNA, triplexes and quadruplexes. Theoretical models, which are used to treat DNA, are considered with special emphasis on the elastic-rod model. DNA topology, supercoiling and their biological significance are extensively discussed. Recent developments in the understanding of molecular interactions responsible for the stability of the DNA double helix are presented. 

DNase-sensitive fiber with dimensions compatible with that of a Watson-Crick double helix. 

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. 

This generalization, which is based upon analysis of a variety of DNA preparations, is a requirement for the Watson-Crick double helix model for DNA. It can be seen in (XVI) and (XVII) that the total number of free amino groups should equal the total number of keto groups on C-6 of the purines and pyrimidines and that adenine = thymine or (A/T = 1), guanine = cytosine or (G/C = 1) (231). When unnatural bases are incorporated into DNA tliey replace the natural base which they most closely resemble, for example 5-bromouracil for thymine, without deviation from the above rules (838, 833). 

The discovery of the liquid-crystalline phases of RNA and DNA solutions. Whilst preparing materials for X-ray diffraction studies (which eventually led to the Watson-Crick double helix model), Spencer et al. noted that liquid crystal... 

---