354 RADIATION BIOLOGY 



the activity of given genes, as is shown by the fact that this composition 

 becomes permanently changed by mutations of the genes. Yet it has not 

 to date been possible to demonstrate rigorously that any given enzyme, 

 antigen, or other cell constituent is the direct product, or the partial 

 replica, of some one gene, or that there is, in general, a one gene-one 

 enzyme, or one gene-one antigen relation. In some cases, in fact, the 

 contrary is certainly true. 



2. TRANSMISSION OF THE GENETIC MATERIAL 



As a result of the reproduction of the genes in the chromosomes, iden- 

 tical twin genes and strings of genes, indistinguishable from each other, 

 forming the basis of sister chromatids, come to be present, lying side by 

 side, in the place of each original chromosome. It would probably be 

 more accurate to speak of the two chromatids as "mother" and "daugh- 

 ter" rather than as sisters, although it has not yet been definitely proved 

 that all the original material remains in one (the "mother") gene or 

 chromosome while all the newly gathered material comes to be in the 

 other (the "daughter"). In preparation for cell division by mitosis both 

 these chromatids become tightly coiled by helical spiralization, probably 

 of two or more degrees of fineness, into a compact double mass. This 

 structure is appropriate in shape and consistency for having its two mem- 

 bers pulled cleanly apart, to opposite poles of the spindle-shaped division 

 figure, with the aid of the tractive "spindle fibers," or lines of streaming, 

 which usually become attached during mitosis to each chromatid at a 

 single fixed point, its centromere. In most cases it is only during this 

 contracted mitotic stage that a chromosome may be readily seen and 

 identified. After mitosis, it again undergoes considerable uncoiUng and 

 so becomes largely lost to view. 



The process of exact doubUng and equal distribution, occurring at every 

 somatic mitosis and resulting in the remotest cells of the body receiving 

 chromosomes and genes just like those of the original fertilized egg that 

 existed at the start of development, is interrupted at meiosis, in prepara- 

 tion for a new fertilization. Until this stage, in ordinary biparental 

 diploid organisms, all cells have carried along by mitosis two sets of 

 chromosomes, a condition called diploid; one of these sets, the "mater- 

 nal," was contributed to the original fertilized egg by the mother, and the 

 other, the "paternal," by the father. For each maternal chromosome, 

 then, there exists in these cells a homologous paternal chromosome. And, 

 similarly, each gene within a given chromosome is ordinarily matched 

 (except in the case of differing sex chromosomes) by a homologous gene 

 occupying a corresponding position in the homologous chromosome 

 derived from the other parent. But by means of the processes of meiosis, 

 each functional male or female gamete to be formed comes to have only 



