182 BO THORELL 



chromosomes starts. The thin and twisted chromatin threads of the inter- 

 phase nucleus are spirahzed and condensed into a small volume. The mi- 

 totic chromosomes are at the disposal of the nuclear division; they are 

 relatively short and rigid structures and thus easily separable. 



In 1868-72 Miescher' isolated nucleic acid from leucocytes and sperm. 

 The similarity in composition of sperai and pus nuclei led him to foresee 

 the importance of this substance: ". . .the study of the sperm would have 

 a far-reaching significance for the problem of heredity." Miescher's nucleic 

 acid was later identified as deoxypentose nucleic acid (DNA) . By means of 

 the Feulgen nucleal reaction used as a cytological staining procedure 

 (Chapter 17), it was found^ that the DNA present in the cell was mainly 

 located in the chromosomal material and DNA has long been regarded as 

 the characteristic substance of the chromosomes. Its cell-physiological func- 

 tion is still obscure, but collected genetical, biochemical, and cytological 

 evidence indicates that it plays an essential part in the maintenance of 

 heredity. 



The following review gives the main results appearing from biochemical 

 and cytological studies of the chromosomal nucleic acids and their behavior 

 in the interphase nucleus and during mitosis. The chapter is for the sake 

 of simplicity divided according to the different methods of investigation 

 used. In judging the results obtained in this rather intricate field of cyto- 

 chemistry, the validity of the methods used plays a dominant role. Each 

 section, therefore, starts with a short methodological discussion. 



II. The Chromosomal Nucleic Acids 



1. Analyses of Isolated Chromosomes 



The earlier investigations on the isolation and analysis of deoxypentose 

 nucleic acid (DNA) were performed on whole nuclei, cells, or organs. Sub- 

 sequently,^ * chromatin threads, "chromosomes," were isolated in amounts 

 which permitted quantitative chemical analysis. The isolation procedure 

 involves disruption of cell nuclei by such different means as the Waring 

 Blendor or the colloid mill. The significance of the analytical results for the 

 composition of the chromosomes naturally depends on how close the iso- 

 lated material corresponds to the definition of chromosomes. Unfortu- 

 nately, most of the classical cytological material in which the morphological 

 details of the chromosomes can be beautifully demonstrated does not lend 

 itself to isolation in bulk. Instead, nuclei from mammaUan thymus or liver 

 or from fish or bird erythrocytes have been used. 



1 F. Miescher, "Die histochemische und physiologische Arbeiten." Leipzig, 1897. 

 2R. Feulgen and H. Rossenbeck, Z. physiol. Chem. 135, 203 (1924); R. Feulgen, 



Ber. Physiol. 22, 489 (1924). 

 3 A. E. Mirsky and A. W. PoUister, Biol. Symposia 10, 243 (1943). 

 * A. Claude and J. S. Potter, /. Exptl. Med. 77, 345 (1943). 



