ISOLATION AND COMPOSITION OF NUCLEI AND NUCLEOLI 127 



it is unlikely that the rate of degradation could always exactly match the 

 rate of synthesis. Moreover, it has been pointed out that the PNA content 

 of different types of nuclei can differ greatly, and that starvation may tend 

 to lower the quantity of PNA in the nuclei of liver cells, at least, while 

 increased rate of mitosis may tend to elevate it. Therefore, in future studies 

 of the PNA content of cell nuclei, account should be taken of species, type of 

 tissue studied, rate of cell division, diet of the animal, etc., in order to avoid 

 the mistake of attempting to generalize from insufficient data. 



(2) Intranuclear Distribution of PNA . A small amount of PNA is found 

 in isolated chromosomes,^" and, since this remains firmly bound in the 

 residual chromosomes after removal of the DNA and histone, it seems likely 

 that PNA is a true chromosomal constituent. The results of numerous histo- 

 chemical studies have indicated that there is a relatively high concentra- 

 tion of PNA in the nucleolus. However certain instances of Feulgen-posi- 

 tive liver nucleoli have been reported,^' and it is very common to observe a 

 heavy ring of Feulgen-positive material around the nucleoli. Nucleoli 

 isolated from starfish eggs'^ have been reported to contain a small amount 

 of PNA but no DNA, whereas nucleoli isolated from liver cell nuclei were 

 found to contain much DNA and only a few per cent PNA.*^ Hence it can 

 be stated that PNA is in all probability very generally present in nucleoli, 

 but it is unsound to conclude, as some have done, that all of the nuclear 

 PNA is in the nucleolus. Actually, it is likely from consideration of the ratio 

 of nucleolar volume to nuclear volume that a very small percentage of the 

 total nuclear PNA is located as a rule in the nucleolus. The nuclear sap of 

 amphibian oocytes seems not to contain PNA.^^ n [^ not known whether 

 the nuclear sap of somatic cell nuclei contains PNA. 



{S)' Turnover Studies as Applied to PNA. As previously mentioned, it 

 has been found that the PNA of cell nuclei apparently has a considerably 

 higher rate of turnover than the PNA of any of the cytoplasmic particles, 

 as judged by the rate of incorporation of radioactive phosphorus.*^ •"•^*-*^ 

 This finding, taken together with the observed diminution in the amount 

 of cytoplasmic PNA in the denucleated amoeba, ^^'^^ suggests the possi- 

 bility that nuclear PNA may be a precursor for at least part of the cyto- 

 plasmic PNA. This question is discussed in Chapters 26 and 28. However, 

 since there appears to be a chemical difference between nuclear and cyto- 



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