220 



RADIATION BIOLOGY 



be seen that it absorbs heavily, as first shown by Kohler (1904). Thus, if 

 the cell is studied \isually on the fluorescent screen or phot(jf^raphed by 

 ultraviolet lifiht, the nucleus is dark and is often fully as sharply con- 

 trasted with the lighter background as it would be if the chromatin had 

 been stained with a basic dye (Fig. (i-ID). Of the many observers who 

 wvvv im})iessed with the strikinj^ contrast shown by chromatin under 

 ultra\iolet examination, none seems to have realized that this was due to 

 a physical property of luicleic acid until Caspersson published his thesis 

 on the chemical composition of structures of the cell nucleus (1930). In 



2200 



2400 



2600 2800 

 WAVE LENGTH, A 



3000 



220 



300 



Fig. 6-3. Ultraviolet absorption curve of a 

 0.02 per cent deoxyribonucleic acid solu- 

 tion (I) compared with that of a 0.2 per 

 cent solution of serum albumen (II). 

 Curves were measured with a Beckman 

 spectrophotometer in a cuvette 1 mm thick. 

 (After Thorell, 1947.) 



240 260 280 



WAVE LENGTH, m/^ 



Fig. 6-4. Ultraviolet absorption curves 

 of the salivary gland of Dro.sophila. 

 Curve I is through a chromosome band ; 

 II is through an adjacent part of the 

 cell outside the chromosome; III is 

 computed for a 10 per cent solution of 

 nucleic acid. (After Caspersson, 1936.) 



this landmark in cytochemistry, Caspersson pointed out that nucleic 

 acid has so strong a natural specific ultraviolet absorption that it can 

 account for the great contrast of ultraviolet pictures (Chap. 5, this vol- 

 ume; Table 6-3 and Fig. 6-3); he showed that the absorption curve 

 through a single chromosome (Fig. 6-4) closelj^ resembles that of nucleic 

 acid, not protein; and he confirmed the opinion that protein could account 

 for but little of the absorption, by digesting the protein with little effect 

 upon the ultraviolet contrast. Xumerous subseriuent publications of 

 Caspersson and his coworkers have made it amply clear that regions of 

 strong ultraviolet contrast in cells are, as a rule, sites of high nucleic acid 

 con('entration (as seen in Figs. 6-2C, D). However, it is a mistake to sup- 

 pose that such ultraviolet contrast is necessarily an accurate reflection of 

 the intracellular nucleic acid distribution, for in each region the density 



