THE NUCLEIC ACIDS 



secondly, if so, whether the reaction can be used quantitatively to 

 measure this substance in absolute amounts. 



All are agreed that aldehyde groups in the hydrolysis products of 

 deoxyribose are responsible for the production of the purple colour; 

 according to Stacey et alii,^^ it is j:«;-hydroxy-laevulinic aldehyde which 

 reacts with the fuchsin-sulphurous reagent (Figure i). The question 

 is whether the coloured substance formed by this reaction stays pre- 

 cisely at the site of formation, or whether it can diffuse into and stain 

 neighbouring zones of the section. Stedman and Stedman** point out 

 that this dye is water-soluble, but is readily adsorbed by chromosomin, 

 the non-basic protein of the nucleus which these authors have dis- 

 covered. The Feulgen reaction according to these authors indicates the 

 position of the chromosomin stained by a dye liberated in a reaction in 

 which deoxyribose took part. 



The Stedmans' argument is not demolished by the demonstration 

 that a granule of DNA* when fixed, embedded in gelatin and sectioned, 

 is Feulgen-positive (Bracket*^), for the liberated dye has then no 

 chromosomin to which it may become attached. It would be useful to 

 compare the result of this experiment with one in which a similar mass of 

 DNA was embedded in gelatin to which chromosomin had been added. 

 At the moment, the most cogent argument which can be brought for- 

 ward to support the validity of the Feulgen reaction is the constancy of 

 the results obtained when a standard procedure is adopted. For instance, 

 the small Feulgen-positive heterochromatic granules of the interphase 

 nucleus are seen in a constant relationship to the residual chromosome 

 threads (Fell and Hughes ;^^ p 33). If in such instances there is 

 diffusion of the liberated dye, it must be restricted to a radius little 

 greater than the limits of resolution of the microscope. 



The subject which we have just discussed is clearly antecedent to the 

 further question of whether the Feulgen reaction can be made the 

 basis of a quantitative microcolorimetric method. Several workers have 

 assumed that all the necessary conditions are satisfied, and their results 

 will be discussed in a further section (p 35). Lesler*' has made a series 

 of model experiments which relate to this question. He prepared a 

 series of mixtures of gelatin with DNA at various concentrations, on 

 which the Feulgen reaction was performed, and the results were judged 

 subjectively. They did not suggest a uniform relationship between the 

 concentration of DNA and the depth of colour produced. 



Comparable debates on the validity of the methods used and the 

 results obtained have occurred in other branches of cytochemistry. It 

 must be borne in mind that the search for methods which will estimate 



* In this book, the usual abbreviations of 'DNA' for deoxyribonucleic acid and 'RNA' 

 for ribonucleic acid have been employed. The latter term is used loosely for all cytoplasmic 

 nucleic acids; some authors prefer to speak of pentose nucleic acids (PNA), for it has not 

 been proved that d-ribose occurs in them all. 



