The Stage of Mitosis at which Chromosomes are Rendered Less Sensitive to X-rays by Ammonia

On the basis of data obtained in previous experiments, it has been postulated that the marked sensitivity to X-rays of chromosomes at the onset of the prophase may be attributed to the neutralization by negative electrons of positive charges carried by the adjacent surfaces formed upon division of the chromonemata. 1 Later experiments showed that in keeping with this hypothesis, treatment with CO2 did not alter the reaction of chromosomes to X-rays, while NH4OH greatly reduced the number of chromosome abnormalities produced by a given dose of X-rays. 2 The theory further required that in normal mitosis the positive charges appear at the onset of the prophase and then disappear during the course of the prophase, presumably as a result of the combination of the positively charged groups with nucleic acids. In accordance with this hypothesis a penetrating base such as NH4OH would be expected to reduce the X-ray sensitivity of chromosomes primarily at the onset of the prophase. The experiments to be described were designed to determine whether these expectations would be fulfilled.

Onion seedlings∗ were irradiated with X-rays and at various intervals after irradiation the root tips examined for the percent anaphases which had no chromosome abnormalities. Similar seedlings were immersed in 0.0025 N NH₄OH for 30 minutes and then irradiated. The technic used has already been described. 2 Both groups of seedlings were given 80 roentgens of X-rays. The source of radiation was an oil-cooled X-ray tube† with a tungsten target which was operated at 200 K.V. and 25 ma. with 2 valve rectification. The filtration by the glass, oil and bakelite was equivalent to 2 mm Al. and 0.25 mm Cu. The effective wave-length was 0.235Å.