The Phenomenon of "Vesting" in Ring Chromosomes and Its Role in the Mutation Theory and Understanding of the Mechanism of Crossing-Over

After X-ray irradiation of Crepis capillaris dry seeds the authors detected, in addition to usual patterns of chromosome mutations, a new phenomenon of normal rod-shaped chromosomes being ``vested,'' or encircled, by ring chromosomes. An analysis of the phenomena has demonstrated that the ring-chromosome production is connected with an X-ray-induced formation of a rigid loop by cross-links of different chromosome segments. The repair mechanism seems to cut DNA strands into three fragments at the cross-link site. When the fragment ends brought close together assume a definite position, the middle fragment forms a ring; the rejoined terminal fragments are within this ring which thus vests the chromosome. After DNA synthesis, two sister chromatids are vested by two sister rings. The appearance of different ``vesting'' patterns is associated with the mitotic crossing-over between sister ring chromatids. At this moment the internal chromatids appear to be capable of slipping out in different combinations from the rings vesting them. It has been shown that the DNA strands are cut into fragments during both the production of chromosome rearrangements and mitotic crossing-over. Due to the effect of chromosome repair cutting, chromosome rearrangements may result from exchange (contact of exchanging chromosomes in a cross-link site) and fragmentation mechanisms when DNA molecules are cut without cross-links or other mechanism assuring contact between different chromosome loci.