CHROMOSOME DUPLICATION AND GENETIC RECOMBINATION 177 



Both can be visualized with DNA-specific staining reactions, as diffusely 

 staining regions. In preparation for mitosis and meiosis, however, the 

 interphase chromosomes condense to form readily visible structures, each 

 with its own characteristic morphology. No analogous condensation has 

 been seen in bacterial cells. 



Replication of DNA 



Recent studies have shown that DNA is replicated in the same manner 

 in bacteria and in higher forms (p. 186). However, chromosome dupli- 

 cation involves synthesis and integration of other constituents besides 

 DNA. The fact that DNA is synthesized almost continuously in multi- 

 plying bacteria, but occurs only at a specified and restricted time in 

 cell division in higher forms, may be related to chromosome complexity. 



Segregation of genetic material 



Mitosis and meiosis represent the orderly systems by which sets of 

 chromosomes are distributed to daughter cells after chromosome dupli- 

 cation has occurred. No comparable systems have been observed in 

 bacteria, although it seems certain that some effective assortment proc- 

 ess is at work, to insure the phenotypic uniformity of clones and main- 

 tenance of linkage relations that are observed. The special features of 

 mitosis and meiosis, such as spindle formation, may be related to the 

 problems presented by the presence of more than one chromosome per 

 haploid set. 



Recombination 



Both sexual and other systems share a fundamental feature: the main- 

 tenance of the linear order of the genes, as evidenced by the constancy 

 of linkage relations observed in mapping. To what extent the mech- 

 anism of recombination of linked genes is the same in all systems is a 

 problem which has not yet been fully clarified. 



How are these outstanding differences in structural organization and 

 integration of the genetic material expressed in genetic behavior? Or, 

 rephrasing the question, how do bacteria differ from higher forms at the 

 level of cell heredity? There are no simple answers to these questions. 

 In this chapter, we shall limit our considerations to DNA, chromosomes, 

 and mechanisms of recombination, leaving questions of DNA-RNA inter- 

 actions and the role of the organized nucleus for later chapters. 



