314 



C. LEVINTHAL 



Fig. 7. A possible scheme by which heterozygotes could be formed by an interaction 

 between two DNA molecules without breaking either of the parental molecules in the 

 process. In (1) a particle which is in the process of duplicating pairs with another of 

 different genetic composition. The pairing might be due to regions of the molecules in 

 which the hydrogen bonds connecting the two chains are momentarily open. (2) One of 

 the two growing strands switches and starts copying along a homologous region of the 

 other parent. (3) Gro-ni.h of the two new strands continues with each of them copying 

 from a different parent. (4) After a period of continued growth and perhaps some 

 separation of the two major chains, the second growing strand switches and starts 

 copying along the other neivly formed strand. (5) The two strands are pulled apart by 

 thermal agitation accompanied by unwinding of the newly formed double helices and re- 

 winding of the new one. This process requires no over-all energy expenditure and releases 

 free energy due to the separation. (6) One of the parents is completely restored to its 

 original state. The rest of the growi.h is completed, either as shown or with the second 

 growing strand (a) copying along the free parental next to (b). Finally the process is 

 finished (7), with the recombinant being made of completely new atoms and the parental 

 structm-es unchanged. 



A double or triple recombinant could occvir in two ways. After state (4), when the 

 second chain has been copying along the dashed parent for a short time, it might revert 

 back and copy for a short additional period along the solid parent. It would still have to 

 retmix and complete the process, as indicated, before separation of the two parental 

 structures could occur. This would lead to multiple switches for one of the chains occur- 

 ring over a short region of the map. However, this cluster of switches would always 

 contain an odd number of switches. A multiple recombinant could also be formed if a 

 second major switch occurred after stage 6. 



