Section 5 — Mutagenesis 



(bead coix). The chromosomes of Coi.x aquatica 

 (n = 5) are long ( 1 10-75|Ji) with terminal knobs 

 and are differentiated into eu- and hetero- 

 chromatic regions, the latter flanking the centro- 

 mere on both sides. The chiasmata have been 

 found confined only to dichromatic segments. 

 All the five bivalents, two of which are nucleolar, 

 are mostly of the ring type with a terminalization 

 coefficient of 0.753. 



The inflorescences were irradiated with two 

 series of 250 kVp X-ray doses (i) 2.5 r, 5 r, 10 r, 

 20 r, 40 r and 80 r and (ii) 250 r, 500 r, 750 r and 

 1000 r. The chiasma frequency at diakinesis fol- 

 lowing irradiation was determined 24 and 48 hr 

 after irradiation. Translocations, both ring and 

 chain types, involving (i) two nucleolar bivalents 

 (ii) nucleolar and non-nucleolar bivalents and 

 (iii) two non-nucleolar bivalents were scored. 



While the doses from 2.5 r to 20 r failed to 

 produce any observable translocations, those 

 from 40 r and 80 r were only 2 and 4 per cent re- 

 spectively. The frequency of interchanges obser- 

 ved in the high dose series showed that they were 

 proportionately the highest within nucleolar; 

 lowest among non-nucleolar and intermediate 

 between nucleolar and non-nucleolar chromo- 

 somes. A linear relationship was found to exist 

 between the interchanges and the radiation dose. 



Bivalents without any visible aberrations, such 

 as translocations, had nearly the same frequency 

 of chiasmata as in the control, for the entire 

 dosage range. However, there was a slight de- 

 crease in chiasma frequency in bivalents involved 

 in translocations. Terminalization coefficient was 

 found to be higher with the higher doses. 



ments revealed (1) that the minimum mitotic 

 cycle duration in irradiated meristems was 

 essentially the same as the unirradiated control, 

 (2) that at a given daily exposure rate the per- 

 centage of anaphase cells with visibly damaged 

 chromosomes increased with increased cycle 

 duration, and (3) that the irradiation reduced the 

 number of cells per meristem below that of 

 controls. The decrease in the number of cells per 

 meristem can probably be attributed to repro- 

 ductive cell death due to genetic loss or unbalance 

 since reduction in cell number varied directly 

 with the percentage of cells showing chromosome 

 damage. 



The results with Pisum suggest that (1) for 

 chronic exposures (up to 1000 r/day) the prob- 

 ability of a chromosome being damaged in- 

 creased with increasing duration of the mitotic 

 cycle, (3) the accumulated exposure per cycle re- 

 quired to produce a measurable decrease in the 

 number of cells per meristem approaches a con- 

 stant value, and (3) the accumulated exposure 

 per cycle required to produce an equivalent 

 number of cells with damaged chromosomes in 

 meristems having different minimum mitotic 

 cycle durations also approaches a constant value. 

 Thus in quantitating the factors responsible for 

 cellular radiosensitivity or cell survival the dura- 

 tion of the mitotic cycle must be considered, at 

 least for chronic exposures. 



Research carried out at Brookhaven National 

 Laboratory under the auspices of the U.S. Atomic 

 Energy Commission. 

 1 . Sparrow and Evans, Brookhaven Symposia in 



Biology, No. 14, 76-100, 1962. 



5.98. The Influence of Mitotic Cycle Duration on 

 Chromosome Damage and Cell Survival in Chro- 

 nically Irradiated Pisum Root Meristems. 



J. van 't Hof and A. H. Sparrow (Upton, 

 New York, U.S.A.). 



An hypothesis has been advanced that for 

 chronic exposures the duration of the mitotic 

 cycle is an important factor contributing to cel- 

 lular radiosensitivity 0). To test his hypothesis, 

 experiments were performed with root meristem 

 cells of seedlings of Pisum sativum in which 

 duration of the mitotic cycle was controlled by 

 temperature. Seedlings were exposed to Co 60 

 gamma-rays at exposure rates of 185, 250, 500 

 and 1000 r per day for 3 days. Following expo- 

 sure, measurements were made to determine the 

 mitotic cycle duration, the number of cells having 

 bridges or fragments at anaphase, and the 

 number of cells per meristem. These measure- 



5.99. Mutagenic Action of Contrast Media for 

 Roentgen Diagnosis. A. Barthelmess and M. 

 Bauchinger (Munchen, Germany). 



In the course of investigations on the cyto- 

 genetic action of substances which come in con- 

 tact with gonadal tissues or gametes in the human 

 body we tested 3,5-dijodid-4-pyridone-N-acetic 

 acid-diethanolamin (I) and N,N'-adipine-di-3- 

 amino-2,4, 6-trijodoben-zoic acid-methylgluka- 

 min (II). Both substances are (or have been) used 

 as contrast media in urography, arteriography 

 and venography (I and II) and hystero-salpingo- 

 graphy (II). 



Root tips of Allium cepa have been immersed 

 4 and 24 hr in the solution and fixed at intervals 

 of 0, 24 and 48 hr after the end of treatment. 

 Both substances caused numerous breaks and 

 structural rearrangementsof chromosomes, mi- 



90 



