NUCLEIC ACIDS IN CHROMOSOMES AND MITOTIC DIVISION 183 



Mirsky, Pollister, and Ris^'* carried out painstaking morphological 

 studies to prove that the threadlike structures isolated by them in fact 

 were chromosomes from the interphase nuclei. Their conclusions have, 

 however, been disputed® by the claim that the threads are not preformed 

 structures in the nucleus but artifacts, produced by drawing out the more 

 or less fragmented nuclei during the treatment. 



Denues,^ in a series of publications, has scrutinized the different phases 

 of the isolation procedure with the aid of the electron microscope. Many 

 characteristic features, such as a certain differentiation along the axis of 

 the threads, coiled structures, and occasional visible doubling, are cited as 

 evidence for the chromosome nature of the threads. A certain conmion 

 pattern can also in many cases be found in threads isolated from different 

 organs of the same species. The morphological evidence that the chromatin 

 threads isolated are true chromosomes of interphase nuclei has been sum- 

 marized by Ris and Mirsky (1951).* The threads constitute a morphologi- 

 cal fraction which can be isolated readily, and much of it can no doubt be 

 identified as interphase chromosomes or chromosome pairs. ^ 



In any consideration of the identity of the structures isolated, it must 

 be kept in mind that the process of isolation may involve contamination of 

 the particles or extraction of material from them. Such contamination or 

 loss may seriously affect the nucleic acid composition of isolated chromo- 

 somes. In some material, such as bird erythrocytes, cytoplasmic contami- 

 nation will add very little to the nucleic acid figures. On the other hand, 

 in tissues such as liver the apparent PNA content of isolated chromosomes 

 will be substantially increased if they are contaminated with cytoplasmic 

 particles. The same risk also holds for isolated nuclei.^" Therefore, an ade- 

 quate morphological control preferably by phase-contrast microscopy (or 

 electron microscopy) during the isolation procedure is essential. Isolation 

 in widely differing media, e.g., those used by Stern and Mirsky,^' can give 

 information about undesirable extraction conditions. 



About 90 % of the mass of isolated chromosomes can be extracted with 

 1 M NaCl.^^ The extracted material consists mainly of nucleohistone, of 

 which 45 % is deoxyribonucleic acid and 55 % is low-molecular-weight pro- 

 tein of the histone type. The insoluble residue after NaCl extraction still 

 shows threadlike structures ("residual chromosomes") which are, however, 



* A. E. Mirsky and H. Ris, J. Gen. Physiol. 31, 1 (1947) ; 34, 475 (1951). 



« W. G. P. Lamb, Nahire 164, 109 (1949) ; Exptl. Cell Research 1, 571 (1950). 



7 A. R. T. Denues, Exptl. Cell Research 3, 540 (1952); 4, .333 (1953). 



8 H. Ris and A. E. Mirsky, Exptl. Cell Research 2, 263 (1951). 

 3 A. W. Pollister, Exptl. Cell Research Suppl. 2, 73 (1952). 



'" R. Y. Thomson, F. C. Heagy, W. C. Hutchinson, and J. N. Davidson, Biochem. J . 



53,460(1953). 

 '• H. Stern and A. E. Mirsky, /. Gen. Physiol. 37, 177 (1953). 

 12 A. E. Mirsky and H. Ris, J. Gen. Physiol. 31, 7 (1947) ; iVafwre 163, 666 (1949) 



