498 MUTATION AND PLANT BREEDING 



stranded structure of the DNA might provide "cover" for miscopy- 

 ing at a specific site in an individual thread; alternatively, matched 

 miscopying in 32 threads simultaneously might allow mutant 

 expression. 



Two other kinds of "critical step" in a mutagenic process can 

 be easily visualized as (1) the deletion or rearrangement of groups 

 of nucleotides within an individual DNA sequence and (2) the 

 breakage of cation bonds between neighboring DNA sequences, 

 possibly followed by the deletion or relative rearrangement of whole 

 sequences. In the higher organisms these chemically different "crit- 

 ical steps" may be operationally indistinguishable so long as they 

 can be recognized only by chromosomal breaks or by gross pheno- 

 typic changes. 



Extra- and Intra-genic Mutation 



Stripped to its bare essentials, the DNA-derived concept of 

 genetic structure is a serial coding system in which the determina- 

 tion of specific information depends more on the spatial relation- 

 ships in the system as a whole than on the properties of its individual 

 components. The latter are only two in number, the base pairs 

 adenine: thymine and guanine rcytosine. Similarly, the number of 

 letters in our alphabet does not exert a serious limitation to the 

 exchange of information; in fact, several letters are superfluous. It 

 is not the number of different kinds of letters which is important 

 but the way in which they are arranged. 



Strauss 1 has likened the nucleotide sequence in DNA to a 

 sentence in which the letters are base pairs and the words, genes. 

 What functions as spaces between words is not clearly understood, 

 though it is possible that certain recurring patterns in the nucleo- 

 tide sequence indicate "stops" between one gene "word" and the 

 next. The ultimate unit is the base pair at a particular site; for 

 example, AT, T-A, G-C, or C-G. Since the same base pairs occur 

 over and over again throughout the sequence, it follows that they 

 can possess no genetic properties per se. They become genetically 

 meaningful only when associated with other base pairs in a local 

 sub-sequence possessing a unique spatial configuration. The sub- 



^trauss, B. S. 1960. An Outline of Chemical Genetics. Philadelphia: W. B. Saunders 

 Co. 



