NATURE OF THE GENETIC EFFECTS 383 



meiosis in the maturing germ cells but in the ordinary body cells at all 

 times, as shown by the fact that homologous chromosomes even in 

 somatic cells and early germ cells show a strong tendency to lie side by 

 side, with homologous parts in apposition. Hence, if these were the 

 forces which also produced the position effect, it would be expected to be 

 especially pronounced in just this group of organisms. 



An important corollary to the relative insignificance of the position 

 effect in most organisms is the inference that the genes must be discrete 

 units or segments of the chromosome, sharply demarcated from one 

 another rather than forming one chemical continuum having no distinc- 

 tion between intra- and intergenic connections. If, as certain authors 

 have speculated, the genes are not discrete entities but only regions 

 exerting given biochemical effects, contained in one long, essentially 

 unsegmented molecule, then it should not be possible freely to break that 

 molecule at practically any point and patch it together again in a different 

 alignment without radically altering the chemical structure and behavior 

 of the parts in the neighborhood of the breaks and new attachments. 

 This is especially to be expected in view of the evidence showing that 

 such a large proportion of the genes is important for life or reproduction. 

 It would therefore seem justified to continue to regard them as separable 

 units, even though, in certain organisms, they or their immediate prod- 

 ucts do exert influences, extending over a short distance, on each other. 



10. INFLUENCE OF STAGE OF CELL AT TIME OF EXPOSURE 

 ON THE CONSEQUENCES OF CHROMOSOME BREAKAGE 



The likelihood of production of structural change by a given exposure 

 to radiation, and the type of change produced, depends in considerable 

 measure upon what stage of the cell cycle was treated, i.e., vipon the 

 condition of the chromosomes at the time. If a cell at the time of 

 irradiation is (as would usually be the case) in the so-called "metabolic" 

 or "interphase" stage (also miscalled the "resting stage" to distinguish 

 it from the stages of mitosis), then its chromosomes are in a greatly 

 extended, widely dispersed condition, and are usually undergoing only 

 small-range movements. In this situation, even if several different 

 chromosome breaks have been produced, it is very probable that any 

 broken end will, by its Brownian movement, come into contact with the 

 other broken end derived from the same break, thus accomplishing 

 restitution, long before it has a chance to meet an end derived from a 

 different break. And, even in those cases in which it does fail to make a 

 restitutional contact, it is likely not to meet with any of the other broken 

 ends at all until finally, after each of the pieces has reproduced to give 

 two chromatid fragments in preparation for the next mitosis, the adjacent 

 homologous broken ends of each pair of identical twin chromatid pieces 



