2 NUCLEUS 255 



target area, viz., if not all parts of the former are capable of changing their 

 molecular structure by ionization. It may, alternatively, be smaller, if it does 

 not attain the overall dimension of the statically calculated ionization area 

 which, according to Timofeeff-Ressovsky (1940), contains 100-2000 (with 

 a mean of roughly 1000) atoms. The latter possibiHty is, however, discounted 

 by estimations of the size of the gene made by speciaUsts ingenetics.True.it 

 is often stated in the literature that the target area is of the same order of 

 magnitude as that of the gene size found by other methods, but we shall 

 show that this is not so. 



One known estimation of the kind comes from Muller(i93 5). Assuming 

 that a single chromonema thread of the salivary gland chromosomes had the 

 same volume as in the corresponding metaphase chromosomes of normal 

 cells, the following calculation applies to the x'-chromosome of Drosophila. 

 In metaphase its volume is 1/8 /x^, two-thirds of which fall to the share of 

 the chromonema, the length of which in the salivary gland chromosome is 

 200 ^. When completely uncoiled, therefore, a single chromonema thread 

 has the submicroscopic thickness (cf. Metz, 1941) of 0.02 /x. The thread 

 is thinner still if it is assumed that the chromonema is regularly screwed-up 

 in the metaphase chromosome, the diameter of which is \ [x. The length of 

 200 fi gives us 250 windings; consequently, with the chromosome being 2 fx. 

 in length, the chromonema could not be thicker than 0.004 /x. 



In calculating the length of the chromonema section containing a gene, 

 MuLLER was guided by the following consideration: By examining the 

 interchange of factors in cross-breeds, four genes were localized in a given 

 chromomere of 0.5 ju. width in the salivary gland chromosome and the 

 existence of further genes was shown to be improbable. Thus the length 

 covered by a gene on the chromonema thread would be about o.i 25 ^. This 

 is a dimension which lies on the borderline of microscopic resolving power. 

 The chromonema sections which, according to Muller, correspond ap- 

 proximately to one gene, are shown in diagram in Fig. 1 26d and, for com- 

 parison, the target area is indicated by a black circle. It is recognized that the 

 thickness of the chromonema thread is of the same order of magnitude 

 as the diameter of the target area, but never the estimated size of the gene, 

 the volume of which exceeds that of the target area by two to three orders 

 of magnitude ! It can be shown that the sphere of action within a gene has a 

 similar size to the target area. 



Carrier hypothesis (Frey-Wyssling, 1944b). If the volume of the 

 gene is liable to be more than a thousand times larger than the target 

 area, what, it must be asked, are the relations between these two 

 quantities? It will be seen in Fig. i26d how the small, sensitive region 

 is embedded in a large, non-mutating area. It is not known where the 

 sensitive region lies and it may therefore, if desired, be thought of 

 as placed anywhere. The picture is reminiscent of that of enzymes ^ 



