Automatic Adjustment in Bacterial Cells 321 



controversy has centred round the mode of couphng. The 

 uncompromising attribution of autosynthetic properties to 

 nucleic acid alone seems an improbable oversimplification. 

 On general grounds a complex mutual interdependence of all 

 the synthetic processes seems much more likely. 



A model for the DNA molecule and a scheme for its replica- 

 tion have been suggested by Watson and Crick (1953). They 

 claim that the DNA molecule in the cell is composed of two 

 coiled chains held together by hydrogen bonds between the 

 bases, a single base from one chain being hydrogen bonded 

 to a single base from the other. From evidence which will 

 not be detailed here they conclude that the only possible 

 pairs of bases are adenine with thymine and guanine with 

 cytosine. The phosphate-sugar backbone of the model is 

 completely regular but any sequence of the pairs of bases can 

 fit into the structure. In a long molecule, therefore, many 

 different permutations are possible, and according to Watson 

 and Crick it seems likely that the precise sequence of the bases 

 is the code which carries the genetical information. Moreover, 

 if the actual order of the bases on one of the chains were given, 

 one could write down the exact order of the bases on the other, 

 because of the specific pairing. In other words, one chain is 

 the complement of the other. According to this scheme, 

 duplication of the DNA molecule involves the breakage of the 

 hydrogen bonds and the unwinding and separation of the 

 two chains. Each chain then acts as a template for the 

 formation on to itself of a new companion chain. 



In the chromosomes of Crepis capillaris, however. Plant and 

 Mazia (1956) find that the parental DNA is not equally dis- 

 tributed between the products of chromosomal duplication as 

 the Watson and Crick scheme would predict, and suggest that 

 it is necessary to find an alternative mechanism for DNA 

 duplication which would eliminate the permanent separation 

 of the two strands of the double helix, or to assume that only 

 part of the DNA is involved in the reproductive process. 

 Arley (1955) finds difficulty in the Watson-Crick scheme for 

 DNA replication in that the energy required to break the 



CELL METAB. — 11 



