130 CELL HEREDITY 



Host DNA 



FIGURE 5.11. A possible structure of the gal region of the chromosome in on 

 E. coli heterogenote. 



sion, each leaves behind a nonmotile cell which forms a colony at that 

 site. The motile sister swims away and divides again, repeating the 

 process. Thus it leaves a trail of separate, small colonies in its wake. 

 Trail formation can actually be observed under the microscope. When 

 the abortively transduced DNA carries into an auxotrophic host a gene 

 causing svnthesis of the grtnvth factor, a line is formed which increases 

 arithmeticallv in number, instead of exponentially. Such abortive trans- 

 ductions gi\ e rise to minute colonies. 



Another kind of association sometimes occurs in which the transduced 

 piece of DNA mav duplicate in synchrony with the host DNA, but does 

 not replace the homologous gene of the host. The bacterium transduced 

 in this way is a heterozygote for the transduced region; strictly speaking, 

 it is a heterogenote, because only a small bit of genotype is represented 

 twice. Heterogenotes are formed by a temperate phage called lambda 

 (A), which is found in E. coli. This phage can locate itself at only one 

 site on the bacterial chromosome, at a place very close to the gal locus. 

 Although other phages may transduce many E. coli genes, X can trans- 

 duce only gal, to which its prophage is linked. As a matter of fact, X 

 can onlv transduce when it is formed directly from prophages. When 

 X is grown on a sensitive cell that is gal~^ but without having taken up a 

 lysogenic association with that gene, it cannot transduce it. It would 

 appear that when the prophage is released from the chromosome upon 

 induction, it may take a little more than itself along. Phages that have 

 picked up the neighboring gal locus reduce to a defective state, so that 

 once they have established themselves as prophages in the new hosts 

 they do not form infectious particles. Nor does the complex replace the 

 equivalent loci in the host; both the transduced gal locus and that in 

 the host are present together, forming the heterogenote (Figure 5.11). 

 At a rate of ca. 10~^ per cell per generation these heterogenotic associa- 

 tions may break down, and haploid bacteria segregate with the original 

 gal character, with the transduced character, or with recombinations of 

 the several gal markers that may have been involved in the original trans- 

 duction. Very rarely bacteria segregate which become homozygous at 

 the gal locus. The rare phage produced spontaneously by heterogenotes. 



