174 G. B. Brown 



We originally approached the problem of the differences between 

 glycine and adenine because Dr. Hammarsten reported in 1948 that 

 glycine went into DNA to a certain extent. When we did our adenine 

 experiments, we found that adenine wasn't incorporated and we were 

 curious for some time. I think about five laboratories have recently 

 obtained data on the incorporation of purine precursors into DNA. 

 Our experimental approach was to quit worrying about differences in 

 experimental conditions between laboratories and to give two tracers 

 in the same set of animals, ^^C adenine and i^N glycine. While we had 

 essentially no incorporation of intact adenine into the DNA, we had 

 an appreciable incorporation of glycine, as mentioned after Dr. Howard's 

 paper yesterday (p. 150). 



Hammarsten: I didn't mean to say anything about contamination of 

 your material, Dr. Heidelberger; I meant another thing, the extent to 

 W'hich this DNA was extracted from the cell nuclei. Cell nuclei are very 

 difficult to extract when not mechanically completely disintegrated, and 

 a lot of extraction work is done with incompletely disintegrated cell 

 nuclei; and even with weak alkali, extraction of DNA is not complete 

 unless the nuclei are disintegrated. We always shake the nuclei with 

 glass beads at 100 vibrations per second; then they are completely 

 disintegrated in 24 hours and can be completely extracted. If you don't 

 do that we might get different fractions of DNA. We don't know, but 

 it is a possibility. 



Heidelberger: I don't wish to take up time with our experimental 

 procedures, which are published, but I would like to add that in every 

 case, in checking the purity of our DNA samples, whatever they might 

 be, for contamination with PNA, in addition to the colorimetric tests, 

 we also test for uracil on the chromatograms. In every case there was 

 considerably less uracil in the DNA samples than the sugar analyses 

 indicated. We have thus used the amount of uracil in the DNA as an 

 additional measure of RNA contamination. 



Brown: If there are two DNA's the chemical separation and charac- 

 terization is going to be an extremely difficult problem. Dr. Bendich* 

 in our laboratory has an experiment in progress in which labelled 

 formate has been administered in quantity to rats. The DNA is isolated, 

 and then an attempt is made to fractionate it into components with 

 different isotope values; in other words, a fractionation based upon 

 differential metabolism of the sample before it is isolated for study. 

 I don't know any better method to work on a small sample of DNA 

 and try to show that it is inhomogeneous. At the present moment he 

 has two fractions of DNA, obtained by salt extraction, in which the 

 purine label is essentially identical in the two. The thymine label 

 is about 30 per cent less in one than in the other, and that seems hopeful. 

 I don't say it is evidence for the presence of two kinds of DNA. 



♦Bendich, A.. Abstracts, American Chemical Society, Boston; April, 1951, 

 p. lie. 



