THE MITOTIC CYCLE 



in the ultraviolet between 2,700 and 2,900 A. The position of that of 

 tyrosine is shifted to the upper limit of this range at high />H's (Holi- 

 day 'i) and also according to Caspersson'^^ 73 ]^y ^j^g presence of 

 histones. A shift in the protein maximum to this region of the ultra- 

 violet absorption curve is used as evidence by the Stockholm workers 

 for the presence of basic proteins. Caspersson and Thorell'* claim 

 that a protein peak in this region is seen very clearly in embryonic cells 

 at stages when the proportion of histone to non-basic protein is high. 

 Hyden^^ has devised an independent cytochemical test for basic 

 proteins which depends on their capacity for binding acid dyes at low 

 />H's and claims that the results obtained by these two methods on 

 nerve cells are consistent. Little use seems yet to have been made of 



; 



30 



25 



20 



15 



2.200 



2,m 2,S00 2,800 



3.000 2,000 2,200 

 Wavelength [K] 



2,¥00 2.600 2,800 3,000 3,200 



Figure "i a AbsoqDtion curves of (i) polynucleotides and (2) serum globulin, and 

 b of (i) adenine (2) tryptophane (3) tyrosin (4) histidine (5) phenylalanine. 

 (Adenine J%, all others 1%; i cm thickness of layer.) From Caspersson'* {By 

 courtesy of W. W. Norton & Company) . 



this method of Hyden's. The effect of the presence of histones on these 

 ultraviolet absorption curves is still in some doubt; Pollister and Ris'^ 

 quote the observations of Mirsky and Pollister^^ that the peak for 

 relatively pure histones is in the usual protein region, and not shifted 

 towards 2,900 A; Caspersson'^ however, says that the displacement 

 only occurs in the presence of nucleic acids in addition. An illustration 

 of the uncertainty which thus still surrounds the recognition of histones 

 within the cell is that different workers do not agree on the question of 

 their distribution within the chromosomes during mitosis (pi 04). On this 

 subject, Caspersson''^ has recently admitted that 'the absorption of 

 the nucleic acids is ... in these dense structures so very large that even 



12 



