THE MITOTIC CYCLE 



preparations of cathepsin which digest dissolved proteins and nucleo- 

 proteins. Daly et alii^'^^^ find that isolated histones are readily attacked 

 by pepsin. Caspersson^^*' suggested that the proteins associated with 

 metaphase chromosomes are mainly of the histone type and that during 

 telophase their heterochromatic regions synthesize histones, while the 

 euchromatin produces non-basic protein. Thus a protein cycle during 

 mitosis is postulated in which the amount of histones keeps parallel 

 with that of the Feulgen-positive material. The young nucleus in 

 telophase is believed to produce relatively large quantities of protein, 

 to which, Caspersson maintains, the swelling of the chromosomes and 

 the nucleus at that time is due. He considers that much of this protein 

 disappears during prophase (p 80). Caspersson's recent admission 

 has already been quoted (p 12) that in metaphase chromosomes, the 

 character and amount of the proteins cannot be determined by ultra- 

 violet microspectrometry alone. 



Ris^'i maintains that the relative proportions of histone and non- 

 basic protein remain the same from interphase to prophase in the cells 

 of the onion root-tip, and from pre-leptotene to pachytene in the 

 spermatocytes of the grasshopper. His method was an indirect one, 

 based on the Millon reaction for tyrosine, which is present in both 

 histones and non-basic proteins. Chromosomes were first shown to be 

 Millon-positive by Heine. ^^^ Ris applied the reaction in the presence 

 first of trichloroacetic acid which does not dissolve histones, and then 

 of sulphuric acid in which basic proteins are soluble (Mirsky and 

 PoLLiSTER^^^). Microphotometric readings on the chromosomes were 

 made in each case. 



It is clear that much remains to be learnt about the changes in the 

 chromosomal proteins during mitosis. We may expect that different 

 types of cell will be found to vary in this respect; the differences be- 

 tween the histones of intermitotic nuclei which are being disclosed by 

 the work of the Stedmans^^^ may well apply also to dividing cells. 



Inorganic chemistry 



We turn now to the inorganic chemistry of the chromosomes, a 

 subject which in J. H. Newman's phrase, has 'a vagueness of an equally 

 intense kind'. Nearly all the evidence in this field comes from studies in 

 which the technique of micro-incineration has been used. This 

 technique and its applications- have been described by Horning^®* in 

 a well-known review. After micro-incineration of a section or a smear, 

 the residual ash of the cells remains sufficiently undisturbed for the 

 whole structure of the tissue to be recognizable under the microscope 

 with suitable illumination. The method is extremely sensitive; the 

 quantities of the substances involved lie beyond the limit of micro- 

 chemical tests. Unfortunately, at present the method suffers from grave 



104 



