CELLS IN DIVISION 



chromosomes when the daughter groups separate in anaphase; in 

 general, there is some doubt whether the values obtained are independent 

 of the degree of concentration of the Feulgen-positive material within the 

 nucleus, for in corresponding measurements on interphase nuclei Ris 

 and MiRSKY^'^* lay some stress on the fact that reproducible results 

 depend on the even dispersion of DNA throughout the nucleus by 

 means of hypertonic sucrose. 



Evidence from a different type of investigation supports the conclu- 

 sion that DNA is synthesized in resting cells which will later undergo 

 mitosis, Howard and Pelg^''^ have shown that when seeds of Viciafaba 

 are germinated in culture solutions containing P^^, the isotope is in- 

 corporated into meristematic nuclei during interphase, probably in the 

 form of DNA (Plate IV). The work of these authors clearly shows the 

 possibilities of their autoradiographic technique in this field. 



The relative proportions of DNA and RNA in the chromosomes 

 during the division cycle appears to be approximately constant in 

 Ris's microcolorimetric measurements on onion root nuclei and grass-, 

 hopper spermatocytes stained by the Unna technique. Jacobson and 

 Webb,!'^' however, have shown that in chick cells in tissue culture, the 

 chromosomes first become methylene blue-positive at the time when the 

 nucleolus breaks up at the end of prophase; this suggests that its ribonu- 

 cleoproteins are then rapidly transferred to the chromosomes. The loss 

 of RNA from the daughter chromosomes in early anaphase is equally 

 rapid, for they leave behind them a track of methylene-blue-staining 

 material in the interzonal region of the spindle. Such observations 

 could be readily extended to other material, and these authors have 

 confirmed by digestion experiments the accuracy of their localization 

 of RNA. 



Proteins 



The proteins of the chromosomes during the division cycle must now 

 be discussed. In earlier pages (12, 47), it has been mentioned that 

 two types are associated with nucleic acids, namely basic proteins of 

 comparatively low molecular weight, the protamines and histones ; and 

 secondly, larger and non-basic proteins containing appreciable amounts 

 of tryptophane. Mazia's^'^ evidence on the proteolytic digestion of 

 chromosomes suggests that histones are associated with the chromone- 

 mata and non-basic proteins with the matrix. Chromosomes shrink 

 when treated with pepsin, which according to Mazia does not attack 

 histones, while trypsin digests them completely. This shrinkage is seen 

 most clearly in an isolated and stretched salivary chromosome, and 

 mainly affects the interband regions. In a later paper, Mazia^'^ has 

 shown that the fibrous nature of the proteins of chromosomes modifies 

 their behaviour towards enzymes. In particular, they are resistant to 



103 



