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ERNST FREESE 



of the following considerations should also apply to mutations that occur 

 in such systems. 



B. STRUCTURE AND DUPLICATION OF CHROMOSOMES 



Since tlie hereditaiy information of chromosomes is presumably car- 

 rii'd by DNA it would be desirable to know how the DNA is arranged 

 in a chromosome. One possibility would be to assume that a chromosome 

 contains just one long DNA molecule. But as the DNA apparently has 

 to unwind during its duplication it seems more likely that occasionally 

 one of the two DNA strands is (or becomes) interrupted in order to 

 provide a swivel. The other DNA strand at the swivel site could be 

 either continuous or interrupted by some non-DNA material. Details 

 about the structure and duplication of chromosomes can be found in 

 the article by Taylor (Chapter II). In view of the present lack of 

 knowledge about the precise chromosomal structure it seems wiser to 

 refrain from trying to explain large chromosomal alterations in molec- 

 ular terms and to limit ourselves to the small changes that are limited 

 to a small region of DNA. 



C. CLASSIFICATION OF BASE ALTERATIONS IN NUCLEIC ACIDS 



All possible kinds of base alterations in DNA are outlined in Fig. 4. 

 The corresponding changes of single-stranded nucleic acid can be 



12 3 



7 6 5 4 ! 8 10 



Fig. 4. Classification of tlie base pair changes in DNA. Each circle (#) repre- 

 sents a base. A hollow circle (O) is a base dilTerent from that in the original 

 standard type DNA. 



