ir. REPLICATION OF DNA IN CHROMOSOMES 87 



contained chromosomes with only one of the original labeled subunits. 

 By the use of the genetic markers phage could also be recovered with 

 the densities expected if they contained a single hea\y subunit that had 

 undergone an exchange between the two markers. The markers used 

 were mi (at one end of the genetic map) and c which is 5 units away. 

 The remainder of the map is 12.5 units. The density of both the con- 

 served particles (containing unreplicated chromosomes) and the semi- 

 conserved particles with a crossover between the markers was that 

 predicted if the chromosome lost an average of about 14% of its heavy 

 kibel, i.e., breaks occurred at random between the loci c and mr. 



Kellenberger et al. (1961) found mutants of phage A having different 

 densities. The density differences were apparently due to insertion of 

 extra DNA at the b2 locus in one mutant and at the b5c locus in the 

 other. The markers are 7.5 units apart, i.e., about 43% of the map. The 

 two mutant phages could be separated by centrifugation in a cesium 

 cliloride gradient. Recombination between the two markers yielded par- 

 ticles denser than either parent and also lighter than either parent. 

 When they produced the light phage, labeled its DNA with P^-, and 

 crossed it with the heavy phage, most of the P'^- transmitted remained 

 with the light parental particles. However, the particles recombinant for 

 the density markers contained considerably more P'^- than non-recom- 

 binant hea\y particles. They interpreted the results to show clearly a 

 breakage and exchange of DNA associated with recombination. They 

 also interpreted the evidence to indicate that most of the P'^- transferred 

 to the particles non-recombinant for the density markers was received 

 by breakage and exchange distal to the two markers. 



These experiments make a significant contribution to our understand- 

 ing of the mechanism of recombination in phage chromosomes and 

 perhaps in larger chromosomes as well. The experiments are comparable 

 to the classic experiments performed by Stern (1931) and Creighton and 

 McClintock (1931) which demonstrated that genetic recombination in 

 higher organisms involved an exchange of chromosome segments. They 

 go further than the chromosome experiments in that they demonstrate 

 that copy choice in replication is not the only mechanism of recom- 

 bination. In fact there is still no direct evidence that copy choice occurs. 

 However, it is not ruled out by the experiments and is still a convenient 

 mechanism to explain aberrant segregation, perhaps as a part of break- 

 age and exchange events. By putting together the evidence available 

 on physical exchange between chromatids (sister chromatid exchange 

 made visible in tritium-labeled chromosomes) and these recently re- 

 ported data on phage chromosomes, we can formulate tlie hypothesis that 



