Pyrimidine Moieties i?i Animals, Plants, and Bacteria 79 



purine or a pyrimidine base connected in nucleoside linkage to 

 deoxyribose-5 '-phosphate (Fig. 3). The purine and pyrimidine 

 bases are adenine (A), guanine (G), cytosine (C), and thymine (T) 

 (Fig. 4). In addition, methyl cytosine (MC) may partly replace 

 cytosine in the DNA of certain plant and animal cells and glu- 

 cosylated hydroxymethylcytosine may replace cytosine in the 

 DNA of the T-even bacteriophages. The four bases A, G, C, and 

 T are the symbols of the genetic alphabet, just as dot and dash 

 are the symbols of the Morse code. Triplets of bases, such as TTT 

 or TGC, may be the letters of the genetic alphabet and each 

 tri]3let may specify a particular amino acid of a protein chain. 

 Tims, the sequence of triplets along a polynucleotide chain would 

 determine the amino acid sequence of a protein. 



The two DNA chains are held together by hydrogen bonds 

 between the bases, each base being joined to a companion base 

 on the other chain (Fig. 5). The pairing" of the bases is specific, 



HO 



OH 



o /^ HO-P = 



0=d-OH X r -\j ^ ^ 



VvO^ — ,v 



HO H 



o 



T A H OH 



OH 



I HO 



0»d-OH 



O x2 



E I 



_x cf V°V 



HO H I 



-a M 



>>/ 



G C H OH 



Fig. 5. Hydrogen bonding between deoxyadenylic acid and thymidylic acid and 

 between deoxyguanylic acid and deoxycytidylic acid. 



