Crick (1953) proposed a molecular model which is widely accepted as 

 representing the natural state of the DNA molecule. Essentially, this 

 consists of two pentose-phosphate chains in a double entwined helical 

 arrangement bound together by base pairs presumably linked by hydro- 



34A 



,S — T • H ■ A — S 

 P P 



'S— C- H-G-S^ 



Figure 2-5. Schematic Representation of the Watson-Crick Model of the 

 DNA Molecule. The molecule consists of two sugar-phosphate chains 

 (-P-S-P-) entwined to form a helix which is held together by hydrogen bonds 

 (•H*) between the companion bases of the two chains. A, adenine; T, thy- 

 mine; G, guanine; C, cytosine. The horizontal solid lines represent comple- 

 mentary bases held together by hydrogen bonding in other parts of the helical 

 molecule. (Redrawn from Watson, J. D., and Crick, F. H. C, 1953. "Ge- 

 netical Implications of the Structure of Deoxyribonucleic Acid," Nature, 

 171. Fig. 2, p. 965.) 



gen bonds (Figure 2-5). Each turn of the helix is about 34 A and there 

 are about 10 base pairs for every gyre. The diameter is about 20 A, 

 this dimension corresponding to the minimal space required to fit in the 

 base pairs, which on the basis of these measurements would be adenine- 



GENERAL MORPHOLOGY AND CHEMISTRY OF THE CELL / 19 



