An X-ray analysis of progressive chromosome splitting

Summary

The chromosomes of a diploidTradescantia became effectively double with respect to X-rays at a time between 30 and 23 hr. before metaphase of the first pollen grain mitosis. During this period there were no differences to be observed between the loci of chromatid and chromosome breaks, which indicates that the splitting of all the chromosomes in any one nucleus, anda fortiori of any single chromosome, is accomplished in a considerably shorter period.

The analysis of fifteen microspores in which chromatid and chromosome breaks were discovered coincidently supplies some evidence for the belief that the splitting of single chromosomes is progressive, beginning near the centromere and proceeding distally. The observations, however, as might be expected with such a small sample were of no real statistical significance.

The proximal regions of the chromosome arms show a higher frequency of breaks, of both types, than the distal portions. This localization of breakage is attributed to the existence of a greater torsional stress in the proximal regions.

Either progressive splitting or differential stress would account for the localization of crossing-over, though the former seems in better agreement with the phenomenon of interference.