BIOLOGICAL ORDER 



The two purine bases, adenine and guanine, and the two pyrim- 

 idine bases, thymine and cytosine, are represented in Figure 4. 

 Each base is attached to a molecule of desoxyribose in position 1 

 carrying a phosphorus in position 3. This is a unit, a desoxyribonu- 

 cleotide (Figure 1). In the nucleic acid, nucleotides are linked by 

 phosphodiester bonds. The result is a long chain. (See also Figure 1.) 



adenine 



thymine 



guanine 



\^~^c- 



-H-K 



\-H- 



N C / 



('sugars nh 



cytosine 



H 

 ' H 



\-/ 



{ Vh 



\ / 



C — N 

 // \ 



O'^UGAF 



Figure 4. The Four Nucleic Bases in the Desoxyribonucleic Acid. 



Each pvrimidine base is bound to a purine base by hydrogen bonds. Adenine 

 is bound to thymine, guanine to cytosine. 



Thanks to James Watson and Francis Crick, the structure of DNA 

 has been disclosed. DNA is a double polynucleotide chain. The two 

 chains are held together by hydrogen bonds between the bases. 

 A pvrimidine base is bound to a purine base: adenine to thymine, 

 guanine to cytosine. Thus in the double DNA helix, the ratio of 

 adenine/thymine and of guanine/cytosine is equal to 1. The two 

 chains run in opposite directions. They are twined around each 

 other (Figure 5). 



[20] 



