192 BO THORELL 



rearrangements which causes the individual carrying them to be variegated 

 for characters associated with genes located at one of the rearrangement 

 points. The other point of rearrangement is in one of the heterochromatic 

 regions. If the bands of the gene loci in question in a normal chromosome 

 show values for ultraviolet absorption at 260 m/x of about 6 %, the same 

 bands translocated close to a heterochromatic region absorb about 20 %. 

 This may indicate that, like the genetically known position effect, the ultra- 

 violet absorption of a chromosome band, presumably to some extent con- 

 ditioned by its nucleic acid content, is likewise dependent on its position in 

 the chromosome. 



The orientation of the nucleic acids in the chromosomes has been the 

 object of discussion on the basis of the high dichroic ratio shown by poly- 

 merized DNA.^^'^"" Oriented DNA films show dichroic ratios up to 4.7 

 depending on the humidity.'''^ •^''^ Except in certain materials, such as 

 sperm heads, the orientation of the nucleic acids giving a measurable di- 

 chroism is very low in cytological material including Drosophila chromo- 

 somes'"^. Hence it is technically difficult to estimate. No results have so far 

 appeared on this important structural problem, but special instruments 

 have been designed.^"* 



The use of the ultraviolet microscope has offered unique possibilities for 

 the localization of nucleic acids in the cell. But it has become generally 

 recognized that the interpretation of the ultraviolet absorption spectra is 

 not at all unequivocal. In the chromosomes there are certainly ultraviolet- 

 absorbing substances other than DNA but quantitatively as important as 

 the DNA. For example, a great proportion of PNA must be considered. As 

 the quantitative microoptical determinations can now be performed with a 

 high precision, it is also necessary to consider how far the absorbing sub- 

 stances correspond with the chemically defined compounds (polymerized 

 nucleic acids, nucleotides, other aromatic compounds, unsaturated fatty 

 acids, etc.). In this respect, complementary information can be obtained 

 by infrared microspectroscopy, as mentioned above, and also by studies 

 involving digestion with specific enzymes under the proper conditions. 



III. Changes in the Chromosomal Nucleic Acids During the Cell Cycle 



1. Quantitative Changes 



Two main processes connected with mitosis can be distinguished in re- 

 lation to the chromosomal nucleic acids. Firstly, at some stage during the 



«« B. Commoner and D. Lipkin, Science 110, 41 (1949). 

 10" M. Wilkins, Discussions Faraday Soc. 9, 368 (1950). 

 I'll B. Thorell and F. Ruch, Nature 167, 815 (1951). 

 "2 W. Seeds, Progr. Biophys. and Biophys. Chem. 3, 27 (1953). 

 103 x. Caspersson, Chromosoma 1, 605 (1940). 

 '"* F. Ruch, personal communication. 



