STRUCTUEAL AND CHEMICAL ARCHITECTURE OF HOST CELLS 33 



200 A. Tlie electron microscope, when used by the most skillful investigators, 

 is resolving structures down to 10 to 15 A. Improved methods of calibration 

 of these instruments have facilitated a precise analysis of structural dimen- 

 sion. 



It may be noted that a 10-fold increase in the resolving power of the 

 electron microscope would permit an atom-by-atom chemical analysis of a 

 given molecule. However, at present the instrument can distinguish between 

 electron-scattering and non-scattering structures, concentrates of the nucleic 

 acids tending to belong to the former group and proteins to the latter. The 

 development of techniques to magnify these effects and to permit the assign- 

 ment of chemical composition to electron-dense regions is being actively 

 studied. For example, electron-dense iron can, by binding to nucleates, 

 increase the density of these materials to the electron beam and assist the 

 localization of these materials, as in the demonstration of the nucleic acids 

 within virus particles and in bacteria (Bernstein, 1956). Although a variety 

 of techniques are known to the X-ray crystal! ograp her for associating 

 metaUic ions with specific functional groups to determine their position in a 

 protein crystal, e.g., Hg++ to protein SH, these procedures have not yet been 

 systematically exploited in staining protein groups for electron microscopy. 



In bacteria, rich in electron-dense RNA, the central region possesses a 

 relatively low electron-scattering power. Bradfield (1954) has reported on the 

 electron micrography of bacteria in which the aldehyde group of deoxy- 

 ribose liberated from DNA deposits metaUic Ag from ammoniacal AgOH. 

 By this technique, even as with more conventional Feulgen staining (to be 

 discussed below), the DNA is found to be concentrated in these central 

 bodies, which are therefore termed bacterial nuclei. However, sectioning has 

 not revealed nuclear membranes at all comparable to those observed in plant 

 and animal cells. Also, Bradfield considers that these bacterial nuclei divide 

 amitotically. Spiegelman et at. (1958) have presented evidence to suggest that 

 the DNA itself may constitute the limiting element of the nuclear body of 

 Bacillus megatherium. 



The use of ultraviolet microscopy in the microchemical analysis of cells 

 was first exploited by Caspersson and his associates (Caspersson, 1950). 

 Using the marked ultraviolet absorption of the nucleic acids at 2600 A, as a 

 function of their content of purines and pyrimidines, the Swedish school has 

 made many observations on the distribution of these moieties in nuclear and 

 cytoplasmic structures. Although the qualitative observations of these 

 workers have generally been confirmed and accepted, many difficulties exist 

 in tlie interpretation of the quantitative data. In addition to the effects of 

 hght scattering, superimposed protein absorption, or even the orientation of 

 nucleic acid within a structure, it may occasionally happen that a marked 

 ultraviolet absorption with a peak at about 2600 A is not due to nucleic acid 



