THE MITOTIC CYCLE 



have already shown that nuclear histones are composite and consist of 

 one main and one or more subsidiary components. The latter vary 

 greatly in arginine content, whereas the proportion of this amino acid 

 in the main histone seems almost constant throughout a number of 

 vertebrates. 



Fractionation 



Not all the proteins of the cell nucleus are basic. Stedman and 

 Stedman^^^ have prepared from various types of nucleus a protein 

 which is neither a protamine nor a histone, which they regard as the 

 main protein of the chromosomes, to which they gave the name 

 'chromosomin'. This substance is prepared by first extracting with acid 

 the basic material from dry lipoid-free nuclei and then fractionating the 

 residual material with dilute alkali. The heads of fish sperm may 

 contain up to 30 per cent of chromosomin. This substance gives 

 strongly positive reactions for tryptophane, tyrosine, arginine and 

 contains sulphur; histones give negative or only very faint reactions 

 for tryptophane. A non-basic protein containing tryptophane has 

 also been identified in the nucleus by Mirsky and Pollister.^^o gy 

 extraction of nuclei with molar sodium chloride these authors separated 

 a complex fibrous product, consisting of nucleic acid, histone and this 

 non-histone protein. To the whole of this they give, not without the risk 

 of confusion, the name of 'chromosin'. The tryptophane containing 

 protein ('Tr. Pr.') is freed from histone and partially from nucleic 

 acids by methods similar to those employed by the Stedmans. 



The isolated 'chromosomes' prepared by Mirsky and Ris^^" (p 64) 

 can also be fractionated into the same three components. Molar sodium 

 chloride causes these threads to swell, and to lose their histone and most 

 of their nucleic acid into solution. The residue which is still micro- 

 scopically recognizable as a thread, consists mainly of a protein which 

 these authors identify with their 'Tr. Pr.' prepared from whole nuclei. 

 Apart then from differences in nomenclature and their rival claims of 

 priority, the Stedmans agree with the Mirsky group both over the 

 non-basic protein of the nucleus, and also in the view that the DNA is 

 evenly spread throughout the resting nucleus, in which they may well 

 both be wrong. 



Although there is this evidence for the existence of non-basic proteins 

 in the cell nucleus, we do not know whether they belong equally to the 

 chromonemata and to the nuclear sap. The chromatographic analysis 

 of the nuclear sap of Xenopus oocytes which we owe to Brown et alii^^^ 

 clearly indicates that a complex non-basic protein is present in this 

 material (Figure 16). 



The proportion of tryptophane-containing protein to DNA in the 

 interphase nucleus which Pollister and Leuchtenberger^^^ calculate 



48 



