NUCLEIC ACIDS IN CHROMOSOMES AND MITOTIC DIVISION 189 



they occur in many cells. Other cellular substances containing the chromo- 

 phoric pyrimidine group show a similar high absorption at 260 mu, e.g., 

 nucleosides and nucleotides. By comparison with the cellular proteins the 

 specific absorption of nucleic acids is much higher (about 100 times), and 

 only those proteins containing the aromatic amino acids have a specific 

 absorption in this wavelength region. ^^'^^ 



The high absorption of the nucleic acids has been used to investigate by 

 microoptical methods their distribution and content in the chromosomes 

 both in the resting nucleus and during the mitotic cycle. 



Absorption in the regions of soft X-rays®^ '^^ and infrared mainly be- 

 tween 3 and 15 m^^ has also been utilized in the same way but to a much 

 lesser extent than ultraviolet absorption. 



The first ultraviolet microscope was constructed about 1900^^'^^ pri- 

 marily to gain a higher resolving power than was available w^ith the ordi- 

 nary fight microscope. By using the new microscope Kohler (1904)^^ and 

 von Schrotter (1906)" noted the richness in detail exhibited by various 

 types of cells, both fiving and fixed. With regard to the high ultraviolet 

 absorption in the chromatin and especially in the chromosomes (Kohler^^), 

 von Schrotter" made the assumption "dass die Absorption ftir ultraviolettes 

 Licht mit dem Nucleingehalte des Gewebes oder der einzelnen Strukturen 

 zunimmt." The "Nuclein" had previously been described by Miescher^ and 

 Kossel.^^ 



In spite of the fact that ultraviolet microscopy was suggested several 

 times as a tool for cytochemical analysis,^^'^^-^" it was during the next three 

 decades mainly used only for structural studies on living or unstained 

 material. It is true that some theoretical considerations and some measure- 

 ments were pubUshed,^^'^^-^^ but no attempt was made to solve the obvious 

 difficulties of introducing absorption measurements in the ultraviolet 

 microscope as an analytical technique. Interesting pictures, however, were 

 shown in the micrographs of living chromosomes in grasshopper spermato- 

 cytes by Lucas and Stark^^ and the mitotic figures in tissue culture cells by 



«' E. R. Holiday, Biochem. J. 24, 619 (1930). 



** A. Engstrom and B. Lindstrom, Biochem. et Biophys. Acta 4, 351 (1950). 



" A. Engstrom and F. Ruch, Proc. Natl. Acad. Sci. U.S. 37, 459 (1951). 



«^ R. Frazer and J. Chayen, Exptl. Cell Research 3, 492 (1952). 



" A. Kohler, Z. wiss. Mikroskop. 21, 129 (1904). 



«« A. Kohler and M. von Rohr, Z. Instrumentenk. 24, 341 (1904). 



" Yl.yox\^c\imiiGT,Virchow's Arch.pathol. Anat. M.P^stoL 183, 343 (1906). 



«8 A. Kossel, Z. phxjsiol. Chem. 7, 7 (1882). 



" A. Kohler and A. Tobgy, Arch. Augenheilk. 99, 263 (1928). 



'« R. W. G. Wyckoff and A. H. Ebeling, J. Morphol. 55, 131 (1933). 



" W. Swann and C. del Rosario, /. FranKlin Inst. 213, 549 (1932). 



'2 F. F. Lucas, /. Franklin Inst. 217, 661 (1934). 



" F. Vies and M. Gex, Arch. phys. biol. 11, 157 (1934). 



'^ F. F. Lucas and M. B. Stark, J. Morphol. 52, 91 (1931). 



