. Cell heredity. Cytogenetics. GENETIC CONTROL OF CELL INTEGRATION 337 inducer, and continues for some time, eventually stopping unless inducer is added. With the reciprocal cross (cross 2), however, no j3-galactosi- dase whatsoever is formed in the absence of inducer. These results have been interpreted by Pardee, Jacob, and Monod, who did the experiments, to demonstrate that the i" factor produces nothing which interferes with /3-gaIactosidase formation, so that as soon as the z"^ factor enters, enzyme formation may begin. Since the z"*" and i'^ factors are closely linked
. Cell heredity. Cytogenetics. GENETIC CONTROL OF CELL INTEGRATION 337 inducer, and continues for some time, eventually stopping unless inducer is added. With the reciprocal cross (cross 2), however, no j3-galactosi- dase whatsoever is formed in the absence of inducer. These results have been interpreted by Pardee, Jacob, and Monod, who did the experiments, to demonstrate that the i" factor produces nothing which interferes with /3-gaIactosidase formation, so that as soon as the z"^ factor enters, enzyme formation may begin. Since the z"*" and i'^ factors are closely linked, and virtually enter together, it must also be postulated that z"*" acts much faster than does i"*". They propose that the i"*" factor produces a re- pressor of /3-galactosidase, and that the reason why i"*" strains are in- ducible, is that inducer must be added to antagonize the preformed repressor. In the z^ i"^ cells of cross 2, which are the F~ receptors, the cytoplasm is postulated to contain this repressor, and, consequently, the entrance of the z~ i~ genes is of no avail. The results also show that i"*" is dominant to i".. FIGURE Formation of ^-galactosidase in E. coli zygotes from the cross Hfrz^ i^ X F~ z' i~. Enzyme formation begins as soon as the z^/^ region en- ters the F~ cell, and persists for some time in the absence of inducer. Phage 76 and streptomycin are added to kill the Hfr cells (which are 76s str-s). In the reciprocal cross, no enzyme is formed in the absence of inducer (after Pardee, Jacob, and Monod, 1959, J. Molec, Biology, 1:165).. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Sager, Ruth; Ryan, Francis J. (Francis Joseph), 1916-. New York, Wiley
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