392 AN INTRODUCTION TO MODERN GENETICS 



the mutation constant. The values actually obtained for it are very 

 small in experiments on mutation in germinal tissue; Timofeeff- 

 Ressovsky gives figures varying from sensitive volumes which contain 

 from 1 5650 to as few as 75 atoms. Larger values have been obtained 

 by Haskins^, who measured the mutation rate from eosin to white in 

 somatic cells, where there is a greater chance of detecting all mutations 

 which occur. Marschak^ measured not particular mutations rates but 

 the frequency of the less restricted class of visible chromosome aber- 

 rations produced by X-rays in Gasteria, and thus calculated the "sensi- 

 tive width" (i.e. width which must be traversed by an ion) of the 

 chromatid as 5 m/z. If this is taken as the diameter of tlie sensitive 

 volume of individual genes, this comes out as about a hundred times as 

 large as the volumes for particular mutation-steps found by Timofeeff- 

 Ressovsky. (Fig. 160, p. 399). 



E. THE CHROMOSOME AND THE GENE 



I. The Chemical Nature of Chromosomes 



Chromosomes as such have never been chemically analysed; they 

 are too small for present methods. The nearest material which can be 

 collected in quantities large enough for ordinary chemical investigation 

 is sperm, particularly fish sperm. The head part of the sperm consists 

 almost entirely of nuclear material, and analyses^ of this show that the 

 two main constituents are thymonucleic acid (about 60 per cent) and 

 simple proteins of the kind known as protamines (35 per cent).^ Nucleic 

 acid combines very easily with proteins to form complex nucleo- 

 proteins, and it probably occurs in this combined form in the nucleus. 



The distribution of nucleic acid in the nucleus can be investigated 

 by means of ultra-violet spectroscopy,^ since it has a characteristic 

 strong absorption at wave-lengths near 2600 A. During division stages, 

 when the chromosomes are contracted and can be seen as separate 

 bodies, almost all the nucleic acid is attached to the chromosomes; it 

 may also be in the chromosomes in resting stages, but the fully extended 

 chromonemata are not separately distinguishable from the nuclear sap, 

 and the evidence is therefore not clear. The metaphase chromosomes 

 can also be shown to contain protein, since they are attacked by proteo- 

 lytic enzymes. In the saUvary gland nuclei, the chromosomes contain 



^ Cf. Haskins and Enzman 1936. 



2 Marschak 1935. ^ Quoted Caspersson 1936. 



* Caspersson and Hammarsten 1935. '" Caspersson 1936. 



