NUCLEIC ACIDS IN CHROMOSOMES AND MITOTIC DIVISION 191 



of these structures shows high ultraviolet absorption of the chromosome 

 bands with a maximum at 260 mn}'''^^ On the other hand, the ultraviolet 

 absorption of the interband spaces was very low and shows a maximum at 

 280 m^u. On the basis of these findings, Caspersson^^ -^^ '^^ estimated that the 

 nucleic acid content in the bands was as high as 10 to 30 %. The interband 

 spaces consisted only of protein containing tyrosine and tryptophan in 

 concentrations of 5 % and 2 %, respectively/^ ■** '^^ 



A correction of this interpretation, however, was later made by Eng- 

 strom and Ruch,^^ who showed by X-ray microradiography that, in fact, 

 most of the chromosomal mass is locahzed in the bands and that the inter- 

 band spaces are almost empty. As a result of their high content of organic 

 substance, the bands can be expected to absorb ultraviolet Ught to a high 

 extent, both specifically and unspecifically. 



The ultraviolet-absorbing substances in the division chromosomes from 

 the salamander can be almost completely extracted by hot trichloroacetic 

 acid. Since the spindle fibers and asters also lose their ultraviolet absorp- 

 tion after extraction, Pollister and Ris^"'^^ have concluded that both chro- 

 mosomes, spindle fibers, and asters contain substantial amounts of nucleic 

 acids. Walker, Davies, and Yates, ^^-^■* in extensive studies on tissue culture 

 cells, confirmed the specific absorption at 260 m/z of the interphase and 

 mitotic nuclei. In presenting their results they state with caution that the 

 amount of nitrogenous bases has been calculated from the ultraviolet ab- 

 sorption "as if they were combined as nucleic acid, without prejudice to 

 their actual conformations." They find that the absorption spectra in their 

 material are, in general, of nucleoprotein type and that the aromatic amino 

 acid contribution near the nucleic acid peak is very small, like the apparent 

 (unspecific) absorption at 312 m/x. Their results are concerned with changes 

 in the "nucleic acid absorption" during growth and division of the tissue 

 culture cells and will be referred to in greater detail in Section III. 



In this connection some cytogenetical experiments performed on Droso- 

 phila giant chromosomes by Schultz in collaboration with Caspersson may 

 be noted. ^^'^* There is in Drosophila melanogaster a group of chromosome 



*' T. Caspersson, Nalurwissenschaften 28, 514 (1940). 



8'T. Caspersson, Chromosoma 1, 605 (1940). 



*' T. Caspersson, Naturwissenschaften 29, 29 (1941). 



90 A. Pollister and H. Ris, Cold Spring Harbor Sijmposia Quant. Biol. 12, 147 (1947). 



»• H. Ris, Cold Spring Harbor Symposia Qxiant. Biol. 12, 158 (1947). 



92 P. M. B. Walker and H. G. Davies, Discussions Faraday Soc. 9, 461 (1950). 



" P. M. B. Walker and H. B. Yates, Symposia Soc. Exptl. Biol. 6, 265 (1952). 



94 P. M. B. Walker and H. B. Yates, Proc. Roy. Soc. (London) BUG, 274 (1952). 



95 J. Schultz and T. Caspersson, Arch, exptl. Zellforsch. Gewebeziicht. 22, 650 (1939). 

 9« J. Schultz, Cold Spring Harbor Symposia Quant. Biol. 9, 55 (1941). 



9'' J. Schultz, Cold Spring Harbor Symposia Quant. Biol. 12, 179 (1947). 

 9« T. Caspersson and J. Schultz, Nature 142, 294 (1938). 



