44 



MAURICE J. BESSMAN 



polymer does not show the hysteresis typical of DNA preparations after 

 heating and cooling (Fig. 15). It has been suggested that the regularly 

 alternating sequence of deoxyadenylate and deoxythymidylate in each 

 strand increases the opportunity for correct alignment of the bases in 

 opposite strands during the cooling period (Marmur and Hoty, 1959; 

 Schachman et al., 1960). 



1.4 



o 



> 



1.2 



O 



c 

 oi.l 



'■^ 



a 

 o 



1.0 



40 



50 60 70 



Temperature in ^C. 



80 



Fig. 15. Melting of d-AT copolj-mer. Plotted on the ordinate is the relative 

 absorbancy at 260 m/i at t° compared to the value at 25°C. The abscissa gives the 

 temperature. The first heating cycle is indicated by open circles, and the solution 

 after coohng in the stoppered Beckman cell was heated a second time, giving the 

 data indicated. (From Schachman et al., 1960.) 



Attempts to elucidate the mechanism of synthesis have so far met 

 with little success. For example, in the unprimed synthesis of d-AT 

 polymer, at any stage during the synthesis only two types of molecules 

 are observed. One type has a molecular weight in the order of 10' and 

 is obviously made up of the original triphosphate molecules, and the 

 other has a molecular weight in the order of 10'^ and must represent 

 completed molecules. No intermediates have been found between these 

 two classes of molecules, as indicated in Table XXI where it can be 

 seen that the sedimentation coefficients and reduced viscosities are 

 constant throughout the early and later phases of the synthesis. 



Recently, Sueoka (1961) reported the presence of a minor com- 

 ponent in the DNA of crab testes {Cancer borealis) which had an 



