THE NUCLEIC ACIDS 



abundant in the former type of nucleic acid. Michaelis^^ has shown 

 that basic stains adsorbed on RNA are in the form of single molecules, 

 but that the metachromatic staining of cartilage matrix, for instance, 

 involves polymerization of the dye. In this interesting paper the author 

 touches on a fundamental topic in this field. The living nucleus is not 

 stained by methylene blue. When the stain is absorbed, the nucleus is 

 dead; Michaelis believes that the dye has then displaced the protein 

 from nucleic acid. In living cells are the whole nucleoproteins ; their 

 free components are produced by fixation. Monne,^^ however, claims 

 to have demonstrated a transient vital staining of the nucleus in 

 Amoeba by micro-injecting dyes into the cytoplasm. These penetrated 

 the nuclear membrane. The coloration within the nucleus soon 

 disappeared ; the cycle was more rapid with acid dyes than with basic. 



SPECTROPHOTOMETRIC METHODS 



Our final topic in the discussion of recent advances in the cytochemistry 

 of the nucleic acids is the most important of the developments in this 

 field, and is mainly responsible for the interest aroused in this subject in 

 recent years. We refer to the development of spectrophotometric 

 methods apphed to the microscope, very largely by Caspersson and 

 his school, which they use mainly in the ultraviolet range. At 2,600 A, 

 which corresponds to the resonance frequency of the pyrimidine and 

 purine ring structure, the absorption of the nucleic acids is far more 

 intense than that of other cell constituents in which these configurations 

 are not found (Figure 3). The absorption band at this wavelength 

 is equally a property not only of both DNA and RNA and their con- 

 stituent nucleotide units, but also of the other adenylic acids and the 

 co-enzymes. Thus ultraviolet techniques like any other cytochemical 

 procedures are not alone sufficient to study the location of the nucleic 

 acids within the cell. 



A number of workers have demonstrated by photomicrography the 

 presence of ultraviolet-absorbing substances within the cells and tissues; 

 the special contribution of the Stockholm school is the development of 

 quantitative methods by which, it is claimed, the extinction coefficient 

 of an area one micron in diameter within a biological preparation can 

 be measured to an accuracy of i per cent (Caspersson '"). Such 

 precision goes much beyond that obtainable by photographic methods ; 

 it is necessary to make direct photo-electric measurements through a 

 range of wavelengths at a series of points within one single cell. 



From a complete ultraviolet absorption curve, information can be 

 obtained concerning the presence not only of nucleic acids but also of 

 proteins which contain the aromatic amino acids, namely tyrosine, 

 tryptophane, and phenylalanine. These show an absorption maximum 



II 



