220 The Nature of and Changes in Genes 



places and the breaks occur purely at random. This fact seems 

 to indicate that the string-like chromonema, which is the struc- 

 ture affected by the X-rays, is the same for all parts of all the 

 chromosomes. It is true for both euchromatic or heterochro- 

 matic regions, for the differentiation of the chromosome into 

 these two regions is not due to a difference in the chromonema 

 ''backbone" but to the material which attaches itself to the 

 chromonema. 



Active and Inert Regions of the Chromosome 



As mentioned in Chapter 5, the chemical nature of the active 

 and of the apparently inert regions of a chromosome seems to be 

 somewhat different. The active regions are generally said to be 

 made up of euchromatin (that is, true chromatin) whereas the 

 inactive regions are composed of heterochromatin. The relative 

 proportions of these two substances differ, however, in such 

 chromosomes as the giant salivary gland chromosomes of Dro- 

 sophila and other somatic chromosomes of the same individual. 

 The giant chromosomes have a relatively smaller amount of 

 heterochromatin than the chromosomes of most body cells, and 

 the heterochromatic material at the centromeres of all the sali- 

 vary gland chromosomes of Drosophila is united into the chro- 

 mocenter. Just why the heterochromatic region is proportion- 

 ately larger than the euchromatic region in ordinary mitotic 

 chromosomes is not clear, but Waddington suggests as a tenta- 

 tive hypothesis that there may be an abnormally large concen- 

 tration of nucleic acid on the chromosome in somatic mitoses. 



The property that chromosomes exhibit of staining deeply 

 with "nuclear" stains is apparently the result of the presence on 

 the chromosome of nucleic acid. In the salivary gland chromo- 

 somes of Drosophila, the euchromatin stains more deeply than 

 the heterochromatin because it has more nucleic acid, but in the 

 mitotic chromosomes of the same individuals, both the euchro- 

 matin and the heterochromatin seem to have a high nucleic acid 

 content and to stain uniformly very deeply. Darlington and La 

 Cour showed that although the euchromatin of the plants Paris 

 and Trillium contains a large amount of nucleic acid during 

 metaphase and anaphase of mitosis, it begins to lose some of this 

 nucleic acid during anaphase so that at telophase the amount of 

 nucleic acid is considerably less than it had been at metaphase. 



