Chromosome breakage and the nucleic acid cycle

Conclusions

Irradiation affects the time scale of development by which we measure its effects in the breakage of chromosomes. This explains why single-time comparisons have hitherto shown discordant results. Our comparative breakage-time series enable us to measure both effects and to show direct proportionality of dose and breakage in the pollen grain.

Comparison of pollen grains and pollen tubes shows that their chromosomes have a similar breakability and also inherent rejoinability (as deduced from the frequencies of minutes). But actual rejoining, as between different chromosomes or chromosome arms, such as is revealed in the empirical coefficient of reunion, is frustrated by the close packing of the nucleus.

The pollen-tube nuclei show extreme overlapping of B′ and B″ owing to extreme divergence in rates of nuclear development. They also show a greater primary effect with greater stickiness than in the pollen and (perhaps correlated with this) SR of unbroken ends, which overlaps with the regular SR of broken ends. The general result agrees with that inferred from progeny tests after X-rayingDrosophila sperm while of course showing many other changes that would not appear in any progeny.