90 



Figure V-4.— The two strands of nucleic acids are held together by hydrogen 

 bonds which are specific for the four nucleotides. A "double helix" is thus 

 created and these interactions are referred to as the Watson-Crick pairing rules. 



protein "language." The "dictionary" which establishes the rules by 

 which this translation is accomplished is called the genetic code. The 

 genetic code, shown in table V-l, was thought to be entirely univer- 

 sal until quite recent experiments showed that in mitochondria, one 

 of the stop signals actually codes for one of the amino acids, trypto- 

 phan; another amino acid, methionine, has two codons rather than 

 only one. While these exceptions are rather minor variations on a 

 major theme, they do emphasize the fact that isolated systems can 

 evolve slightly different codes. 



Which Came First: Proteins or Nucleic Acids? 



Several people have proposed that the first genetic system was 

 composed of proteins alone. They point out that it is much easier to 

 synthesize and polymerize the amino acids than the nucleotides. 

 Furthermore, the resulting polymers have some catalytic ability; in 

 the contemporary cell, proteins play the major role as catalysts. 

 Nucleic acids, on the other hand, are not usually thought of as having 

 catalytic activity, although they do act as templates for replication 

 and transcription (and a "bringing together" of reactants is certainly 



