408 ANNUAL REPORT SMITHSONIAN INSTITUTION, 196 2 



triplets for other amino acids. Thus if it is assumed that the order of 

 the G, U, and A nucleotides in the glutamic acid code is UGA, then 

 the lysine triplet which is known to contain tAvo A and one U 

 nucleotide must be UAA, thus 



UGA > UAA, 



assuming the mutation responsible resulted from a change of a single 

 nucleotide. 



Study of intra-coding-unit recombinants reveals nucleotide se- 

 quence just as classical crossing over indicates gene order and per- 

 mits the assignment of a unique linear order to genes in a chromosome. 

 This method is being used by Dr. Charles Yanofsky and coworkers. 



There is good reason to believe that the genetic code is degenerate 

 in the sense that a single amino acid may be specified by more than 

 one triplet. 



The task of deciphering the total genetic code is well along toward 

 solution and one may confidently predict that it will be completed in 

 the near future. 



JNIy fourth question concerns the nature of mutation. How is genetic 

 information modified during the replication in a manner that permits 

 organic evolution ? 



During DNA replication, mistakes are occasionally made. Pre- 

 sumably, during replication a nucleotide does not pick up a comple- 

 mentary partner as it should but instead picks up a noncomplementary 

 one. It has been postulated that such mistakes result from an im- 

 probable tautomeric form in which a hydrogen atom is in an improb- 

 able position at the exact moment the nucleotide picks up a partner. 

 A wrong partner is therefore selected. In the next round of replication 

 the "wrong" partner will pick up what is its complementary partner, 

 and this will result in substitution of one nucleotide pair for another. 

 This is somewhat like a typographical error. In typographical errors 

 it is possible to have extra letters, too few letters, one letter substituted 

 for another, or transposed letters. Presumably similar kinds of mis- 

 takes can be made in genetic information during replication. In fact, 

 there is genetic evidence that these four basic types of mistakes do 

 occasionally occur. 



How often do such mistakes occur ? Quite infrequently, we believe. 

 From the time one receives a set of directions in the fertilized egg until 

 one transmits it to the next generation — and remember this is perhaps 

 17 to 20 successive replications of information equivalent to about 

 1,000 printed volumes — a significant and detectable mistake is made 

 perhaps about once in a hundred times. This is clearly a high order 

 of precision. 



