Bacteria: Recombination (/) 



347 



Was the increase in transformation rate, 

 obtained in the presence of denatured, homol- 

 ogous DNA, due to the formation of hybrid 

 molecules or did the excess denatured strepto- 

 mycin-sensitive DNA merely cause an in- 

 crease in unions between pairs of streptomy- 

 cin-resistant single strands? The latter pos- 

 sibility cannot be the explanation, since, as 

 mentioned previously, hybrid molecules can 

 be formed between single strands even when 

 these are derived from different (but related) 

 species. It is clear, therefore, that hybrid 

 molecules were produced, that these can trans- 

 form, and that just as only one strand is re- 

 quired for the replication of DNA, only one of 

 the complementary strands is needed to carry 

 all the information for cistronic action con- 

 tained in the normal double helix. (We would 

 expect that the viruses 0X174 and Si 3, cf. p. 

 311, also carry all their genetic information in 

 a single DNA strand.) 



Three additional observations should be 

 made. First, strand separation is accom- 

 plished by heat in a matter of a few minutes 

 or less. This is important since it suggests the 

 direction further work may take in studying 

 how this might occur with adequate rapidity 



in vivo. (It has been suggested that chain 

 separation is normally produced enzymati- 

 cally, through the activity of ravelase.) 

 Strand recombination by slow cooling has as 

 yet no known biological counterpart, al- 

 though it might be a mechanism for genetic 

 exchange between closely related DNA mole- 

 cules. The second point is that the capacity 

 to routinely separate and combine single 

 strands should lead to a better understanding 

 of transformation, in particular the mecha- 

 nism of integration. Experiments along these 

 lines and others would be greatly aided by 

 the use of closely linked marker recons, one 

 on each strand of the hybrid molecule. In 

 fact, such studies have already been reported, 

 in which double transformations have been 

 obtained with a complex of strands containing 

 different markers. Molecular hybrids may 

 also be useful for comparing base sequences 

 in closely related organisms even when 

 genetic recombination between them cannot 

 take place. The final point is that, in bacteria, 

 the recon can be identified as the smallest unit 

 of DNA capable of being integrated or re- 

 placed in a host chromosome subjected to the 

 transformation process. 



SUMMARY AND CONCLUSIONS 



Contrary to the simplifying assumption made on page 329, genetic recombination occurs 

 in bacteria by means of genetic transformation. This process involves a sequence of events 

 in which competent cells permanently bind transiently-bound, large molecular weight 

 particles of DNA. Once the DNA particle has penetrated, it apparently undergoes a 

 synapsis-like process with a corresponding segment of the bacterial chromosome. Trans- 

 formation is completed when a small segment of the DNA particle becomes integrated by 

 replacing a similar segment of the recipient chromosome. 



Transformation provides direct and conclusive evidence that chromosomal DNA is 

 genetic material. In bacteria, the recon is the smallest unit of DNA involved in trans- 

 formation. 



Strand separation and recombination in vitro produces denatured and renatured DNA, 

 respectively. Renatured DNA can transform, even when a hybrid molecule contains one 

 normal and one mutant chain. This proves that all the information for cistronic action 

 can be carried in one of the two strands of double helix DNA. 



