CHROMOSOME DUPLICATION AND GENETIC RECOMBINATION 195 



exchanges to those occurring between identical strands. The physical basis 

 of complementarity could be the DN A double helix itself. The complemen- 

 tary strands run in opposite directions in the sense that the phosphate 

 diester bonds connect the C-3 carbon of one sugar with the C-5 of the 

 next, (Figure 1.1) and this alone may exclude end-to-end joining of com- 

 plementary chains. Thus the data from sister-strand exchanges supports 

 the hypothesis that the DNA subunits observed by tritium labeling actu- 

 ally correspond to the two strands of the DNA double helix. 



Recently, DNA replication in intact chromosomes has been studied 

 by exposing cells to thymidine-H'^ for very short intervals. In general, 

 evidence has been obtained of asynchronous replication in plant {Crepis 

 and rye) and in animal material (grasshopper and Chinese hamster). A 

 detailed analysis of Chinese hamster cells in tissue culture has shown 

 that several of the chromosomes have segments which are typically late 

 in duplication. For example, the DNA of the long arm of the X chromo- 

 some is replicated in the last half of the interval of DNA synthesis, 

 while that of the short arm is replicated during the first half of this 

 period. Does the time of DNA replication coincide with the time of 

 chromosome duplication? It has been shown that chromosomal histones 

 are synthesized at approximately the same time as is DNA. Although 

 the analysis has not been pursued with the precision of the DNA label- 

 ing experiments, it is likely that the asynchrony of DNA replication 

 applies also to other components of the newly forming chromosome. 



This information all comes from studies of mitosis. For the analysis 

 of recombination mechanisms, one requires similar observations of 

 chromosome duplication in meiosis, and such experiments are now being 

 carried out. However, the mitotic chromosomes do provide some geneti- 

 cally relevant information: (1) that each chromatid contains two DNA 

 subunits, which are probably the two strands of the double helix; (2) 

 that in general the old subunits remain intact during replication of new 

 strands, but occasionally sister-strand exchanges may occur; and (3) that 

 exchanges involve recombination of all four strands, not at random, but 

 limited to rejoining of like-directioned chains. 



RECOMBINATION REVISITED 



Having surveyed the available knowledge about chromosome organi- 

 zation, we may now return to the central theme of this chapter: possible 

 mechanisms of genetic recombination. The issue was reopened by the 

 results of phage crosses which differed from those observed with higher 



