THE NATURE OF THE GENE 395 



tion due to the nucleic acid. So do fully uncoiled chromosomes/ such 

 as those of salivary glands and zygotene stages ; when the chromonema 

 is presumably coiled in a single coil (e.g. mitotic metaphase), so that 

 it runs perpendicularly to the length of the chromosome, the sign of 

 the double refraction changes and becomes positive in the direction of 

 the chromosome axis, while in meiotic metaphase, where the minor 

 spiral is coiled again in a major spiral, the double refraction reverses 

 again and becomes once more negative in the direction of the chromo- 

 some axis. All these data fit in very well with the idea that the protein 

 and nucleic acid have combined to form composite fibres in which the 

 two constituent fibres he parallel to one another. 



The cytological evidence makes it quite clear that the chromosomes 

 are not homogeneous structures. In the first place, there is a differentia- 

 tion in sahvary gland chromosomes between the darkly staining bands 

 and the non-staining inter-band regions. The property of stainabiUty 

 depends on the content of nucleic acid, and the concentration of this 

 substance in the bands can be demonstrated direcdy by studies of 

 ultra-violet absorption. One must suppose that the proteins in the 

 band regions have a particular affinity for nucleic acid, and Wrinch^ 

 has suggested that this may be due to a higher concentration of basic 

 groups, particularly arginine, which is known to be present in remark- 

 ably high amounts in clupein. The difference between the bands and 

 inter-bands appears, however, rather larger and more sharply defined 

 than would be expeaed if it were due to a merely quantitative difference; 

 but this appearance may turn out to be illusory when actual measure- 

 ments of nucleic acid content become available. 



The extensibility of the bands seems to differ sharply from that of 

 the interbands, the former being much the more rigid. There are two 

 factors to be taken into consideration here. Firstly, the nucleic acid 

 fibre itself appears to be inelastic, and the rigidity of the bands may 

 be due simply to their nucleic acid content. Secondly, while it is easy 

 to see how nucleic acid may combine with fully extended polypeptides, 

 it is not so clear how it can fit on to a globular protein; it is possible 

 then that the proteins of the bands, when combined with nucleic acid, 

 are in the extended form, and thus have themselves lost much of their 

 extensibiUty; but whether this should be regarded as a result or as a 

 contributory cause of their affinity for nucleic acid is as yet quite 

 unknown. 



^ Kuwada and Nakamura 1934&, Nakamura 1937, Schmidt 1936, 1937- 

 * Wrinch 1936. 



