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are of the proper shape. On the basis of chemical data and size con- 

 siderations, Crick and Watson showed that the chains were made up of 

 -sugar->phosphate-sugar-^phosphate- and so forth, units. Between 

 the two chains, as rungs along a helical step ladder, were strung pairs of 



Figure 12. Double chain of DNA. 



hydrogen-bonded bases of the form purine-H-pyrimidine. These rungs 



are about 11 A long. A piece of a pair of chains is shown in Figure 12. 



This type of unit is repeated into a long double chain. The entire 



double chain is then twisted to form the double helix shown in Figure 13, 



