ISOLATION AND COMPOSITION OF NUCLEI AND NUCLEOLI 149 



nucleic acid is attached to protein. ^^^ Depolymerized DNA will stain pink, 

 while high-polymer PXA will stain blue or purple (see Chapter 17). 



6. Conclusions 



A possible error in interpreting the methyl green-pyronin staining of 

 nucleoli has just been pointed out. Another possible error which certainly 

 could have been made in some of the earlier histochemical studies with en- 

 zymes is the use of ribonuclease contaminated with protease or deoxyri- 

 bonuclease or both, since for a time the necessity of resorting to special 

 procedures for removing these contaminants from the ribonuclease was not 

 realized. Another possible source of error that has been detected from work 

 in the A\Titer's laboratory is the failure to realize that if Feulgen staining of a 

 particle such as a nucleolus is not exceptionally intense, this color can easily 

 be lost in the mass of stronger color produced by the surrounding mass of 

 nuclear material. All samples of isolated liver cell nucleoli obtained in the 

 writer's laboratory were found to be Feulgen-positive. 



What is to be concluded concerning the composition of nucleoli with re- 

 spect to nucleic acid? All studies thus far published appear to indicate the 

 presence of PNA in rather low concentration, amounting however at least 

 to some 2 to 5 % of the nucleolar mass. Protein therefore seems to constitute 

 the bulk of the nucleolus. Considering the relatively small percentage of 

 nuclear volume occupied by the nucleolus, which at the most cannot amount 

 to more than a few per cent, it seems clear that the amount of PXA found 

 in the nuclei of rat liver, for example, could not be accounted for as nucleo- 

 lar PXA unless the nucleoli were composed entirely of PX"A. Even then 

 there might not be enough space in the nucleoli to contain all of the nu- 

 clear PXA. To conclude, as some have done, that all nuclear PXA is lo- 

 cated in nucleoli would therefore be unjustified. It would likewise be unjusti- 

 fied to conclude that the PX^A found in isolated chromosomes is confined 

 to the nucleoli present in these preparations. It is probable that PX^A is an 

 intrinsic constituent of chromosomes as well as of nucleoli. 



Concerning the presence or absence of DX'^A, some special comment is 

 required. In the first place, it seems likely that many nucleoli do not con- 

 tain DXA. The nucleoli of Vincent were shown by direct analysis to con- 

 tain none, and the work of PoUister and Leuchtenberger cited above, and 

 that of many other investigators, supports this statement. And yet the 

 nucleoli isolated from hver cell nuclei in the wTiter's laboratory have been 

 shown by direct analysis to contain DXA, or a very similar material. This 



i9« N. B. Kurnick, /. Natl. Cancer Inst. 10, 1345 (1949-50). 

 '" N. B. Kurnick, J. Gen. Physiol. 33, 243 (1950). 

 ^'8 N. B. Kurnick, Exptl. Cell Research 3, 649 (1952). 

 19' M. Alfert, Biol. Bull. 103, 145 (1952). 



