188 BO THORELL 



The results correspond reasonably well with those obtained by analysis of isolated 

 chromosomes (see Sect. II.l). Thus, by using ribonuclease in combination with an 

 acid dye which stains protein but not PNA, the association of PNA with protein in 

 the chromosomes is confirmed.^' '^^ The action of deoxyribonuclease on chromosomes 

 appears to release some protein material both of the histone and nonhistone type. 

 Degradation of one material could thus also result in the disappearance from the 

 chromosomes of other associated substances. 



Digestion of the chromosomal PNA in a cytological preparation seems to affect 

 the DNA in such a waj' that its affinity for methyl green is reduced.*' •** If contamina- 

 tion with deoxyribonuclease can be excluded, this observation suggests an association 

 in situ between PNA and DNA which may be of importance in relation to the state 

 of nucleic acid polymerization.** 



The removal of the nucleic acids from the chromosomes by enzymes or 

 by extraction with trichloroacetic acid leaves a residue of protein, stainable 

 with acid dyes.^' The principal threadlike structure still remains. If these 

 proteins are hydrolyzed by trypsin, the chromosomal structure disinte- 

 grates. This occurs whether the polymerized nucleic acids are intact or 

 precipitated before the digestion of protein." '^^ 



The experimental results obtained by the use of dyes, especially in com- 

 bination with specific enzymic hydrolysis, have thus given a fairly intricate 

 pattern of association between the different chromosomal substances, a 

 pattern which at present cannot very easily be physiologically interpreted. 



Moreover from a purely methodological point of view, many problems 

 remain to be solved, such as the interaction of the different chromosomal 

 substances and its influence on substrate specificity, and the activation 

 and inhibition of the enzymic hydrolysis of an intact cellular structure. 



c. Radiation Absorption Measurements 



As a result of their purine and pyrimidine content, the nucleic acids have 

 a specific absorption in the ultraviolet region around 260 m/i.^^'^° This is 

 discussed in detail in Chapter 14. Their absorption constants make them 

 detectable within the ranges of concentration and layer thicknesses in which 



*' B. P. Kaufmann, M. R. McDonald, and H. Gay, J. Cellular Camp. Physiol. 

 38, Suppl. 1, 71 (1951). 



*2 B. P. Kaufmann, M. R. McDonald, H. Gay, K. Wilson, and R. Wyman, Car- 

 negie Inst. Wash. Yearbook 46, 141 (1947). 



" B. P. Kaufmann, Science 109, 443 (1949). 



" B. P. Kaufmann, M. R. McDonald, and H. Gay, Am. J. Botany 38, 268 (1951). 



"M. R. McDonald, J. Cellular Comp. Physiol. 38, Suppl. 1 (Discussion), 89 (1951). 



S6 A. Mirsky, J. Cellular Comp. Physiol. 38, Suppl. 1 (Discussion), 92 (1951). 



*' T. Caspersson, E. Hammarsten, and H. Hammarsten, Trans. Faraday Sac. 31, 

 367 (1935). 



" T. Caspersson, Skand. Arch. Physiol. 73, Suppl. 8 (1936). 



" C. Dh^r^, Compt. rend. sac. Biol. 60, 34 (1906). 



«» F. Heyroth and J. Loofbourow, J. Am. Chem. Soc. 53, 3341 (1931) ; see also J. Am. 

 Soc. 56, 1728 (1934). 



