CHROMOSOME ABERRATIONS IN ANIMALS G95 



High doses of radiation were used in all such experiments ; in the studies 

 just cited the minimum dose required to abolish structural viscosity 

 completely in a 0.2 per cent solution was 22,400 r. It may well be ques- 

 tioned, therefore, whether the comparatively low doses required to effect 

 adhesion and pycnosis of chromosomes in living cells — in some cases as 

 little as 25 r — operate in a similar manner. It may also be asked whether 

 depolymerization, which leads to a decrease in viscosity of solutions of 

 nucleic acid in vitro, would bring about an increase in stickiness of the 

 materials of the chromosomes. 



In an effort to answer these questions, cytochemical studies have been 

 made of the effect of the exposure to ionizing radiations on methyl green 

 stainability of chromosomes. Purified methyl green, when used under 

 suitable experimental conditions, stains polymerized but not depolymer- 

 ized desoxyribonucleic acid (Kurnick, 1947, 1950; Pollister and Leuchten- 

 berger, 1949). Methyl green stainability of meiotic cells of Trillium 

 erectum was not impaired, as determined by spectrophotometric methods, 

 when the living buds were exposed to doses as high as 20,000 r, although 

 such doses reduce the viscosity of solutions of desoxyribonucleic acid 

 (Moses, DuBow, and Sparrow, 1951). It is apparent that depolymeriza- 

 tion of desoxyribonucleic acid (which can be produced by treatment of 

 fixed cells with hot water or with the enzyme desoxyribonuclease) is not 

 induced in the living cells of this plant by exposure to X rays. Similar 

 results were obtained by the author in studies of plant and animal cells 

 in which X-ray treatment had produced deformed and adherent meta- 

 phase and anaphase chromosomes. No reduction of methyl green stain- 

 ability of these chromosomes was effected by doses as high as 16,000 r. 

 These results agree with those of Himes (1950), who found that the 

 "stickiness" of chromosomes caused by the "sticky" gene in maize and 

 by chemical treatment of root tips of onion is not due to depolymerization 

 of the desoxyribonucleic acid. 



In the light of these experimentally derived data it appears that the 

 attempts to interpret the effects of X rays on chromosomes in terms of 

 depolymerization of desoxyribonucleic acid are premature and inconclu- 

 sive. Attachment and detachment of desoxyribonucleic acid are not 

 simple coupling and uncoupling phenomena, since it has been shown that 

 the chromosome is a complex aggregate of both desoxyribose and ribose 

 nucleic acids, intimately associated with each other and with histones and 

 more acidic proteins (Kaufmann, McDonald, and Gay, 1951). Some 

 studies have also indicated that there is very little increase in the amount 

 of desoxyribonucleic acid between mid-interphase and metaphase (e.g., 

 Swift, 1950; Lison and Pasteels, 1950 ; Pasteels and Lison, 1950), although 

 other experiments (e.g., those of Ogur et al, 1951, on Lilium) suggest that 

 the amount of desoxyribonucleic acid increases during mitosis as well as 

 during interphase. It has also been reported that changes occur during 



