J. D. WATSON AND F. H. C. CRICK 



The third difficulty involves the necessity for the two complementary chains 

 to unwind in order to serve as a template for a new chain. This is a very funda- 

 mental difficulty when the two chains are interlaced as in our model. The two 

 main ways in which a pair of helices can be coiled together have been called 

 plectonemic coiling and paranemic coiling. These terms have been used by 

 cytologists to describe the coiling of chromosomes (Huskins, 1941; for a review 

 see Manton, 1950). The type of coiling found in our model (see Figure 4) is 

 called plectonemic. Paranemic coiling is found when two separate helices are 

 brought to lie side by side and then pushed together so that their axes roughly 

 coincide. Though one may start with two regular helices the process of pushing 

 them together necessarily distorts them. It is impossible to have paranemic 

 coiling with two regular simple helices going round the same axis. This point 

 can only be clearly grasped by studying models. 



There is of course no difficulty in "unwinding" a single chain of DNA coiled 

 into a helix, since a polynucleotide chain has so many single bonds about which 

 rotation is possible. The difficulty occurs when one has a pair of simple helices 

 with a common axis. The difficulty is a topological one and cannot be sur- 

 mounted by simple manipulation. Apart from breaking the chains there are 

 only two sorts of ways to separate two chains coiled plectonemically. In the 

 first, one takes hold of one end of one chain, and the other end of the other, and 

 simply pulls in the axial direction. The two chains slip over each other, and finish 

 up separate and end to end. It seems to us highly unlikely that this occurs in 

 this case, and we shall not consider it further. In the second way the two chains 

 must be directly untwisted. When this has been done they are separate and side 

 by side. The number of turns necessary to untwist them completely is equal 

 to the number of turns of one of the chains round the common axis. For our 

 structure this comes to one turn every 34 A, and thus about 150 turns per million 

 molecular weight of DNA, that is per 5000 A of our structure. The problem of 

 uncoiling falls into two parts: 



(1) How many turns must be made, and how is tangling avoided? 



(2) What are the physical or chemical forces which produce it? 



For the moment we shall be mainly discussing the first of these. It is not easy 

 to decide what is the uninterrupted length of functionally active DNA. As a 

 lower limit we may take the molecular weight of the DNA after isolation, say 

 fifty thousand A in length and having about 1000 turns. This is only a lower 

 limit as there is evidence suggesting a breakage of the DNA fiber during the 

 process of extraction. The upper limit might be the total amount of DNA in a 

 virus or in the case of a higher organism, the total amount of DNA in a chromo- 

 some. For T2 this upper limit is approximately 800,000 A which corresponds to 

 20,000 turns, while in the higher organisms this upper limit may sometimes be 

 1000 fold higher. 



The difficulty might be more simple to resolve if successive parts of a chromo- 

 some coiled in opposite directions. The most obvious way would be to have 

 both right and left handed DNA helices in sequence but this seems unlikely as 

 we have only been able to build our model in the right handed sense. Another 



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