361 



FIGURE 39-4. Linear ''^chromosomes''' 



of Vlif strains. Arrows show direction 



of chromosome penetration during 



conjugation. {After A. L. Taylor and 



E. A. Adelberg, I960. See References.) 



ser/gly 



efficiency. Thus, the closer a gene is to O, 

 the greater is its chance for integration. 



In this connection it may be fruitful to 

 recall the observation (cf. p. 345) that when 

 one strand of a double helix of donor DNA 

 is broken via DNAase activity, incorporation 

 of donor DNA into the recipient DNA frac- 

 tion is unaffected but transformation rate is 

 drastically reduced. Accordingly, the fact 

 that the farther a locus is from O, the lower 



is its integration efficiency, may be a direct 

 consequence of the action of an Hfr or Vhf 

 locus which breaks one strand of double-hehx 

 DNA and not the other. In this event, such 

 loci could be producing effects not only on 

 chromosome rupture (by severing both 

 strands of the DNA double chain at the same 

 level), but also on integration (by breaking 

 only one chain of the two at any given level). 

 In cases when penetration of a chromosome 



FIGURE 39-5. Genetic map of 

 Escherichia coli in which distances 

 are expressed in minutes; 

 markers in parentheses are less 

 precisely mapped. (After A. L. 

 Taylor and E. A. Adelberg, I960. 

 See references.) 



