Chromosome chromatid rearrangements resulting from the mitotic crossing-over between sister-chromatids in ring chromosomes of Creptis capillaris

Sister chromatid exchanges in ring chromosomes following X‐irradiation of human lymphocytes

PURPOSE

To examine whether X-rays induce sister chromatid exchanges (SCE).

MATERIALS AND METHODS

Peripheral lymphocytes irradiated in vitro or in vivo were cultured and treated with okadaic acid to generate premature chromosome condensation (PCC). When identical spreads were analysed using conventional Giemsa staining and pan-centromeric fluorescence in situ hybridization painting, ring chromosomes were observed.

RESULTS

In PCC preparations, cells in the late G(2) phase and late M phase were observed. In late M phase cells, 17-20% of ring chromosomes lacked one chromatid (single-chromatid ring), irrespective of dose. Both the distribution patterns of centromeres in rings and intercentromere distances in dicentric rings indicate that a considerable number of single-chromatid rings might be formed by SCE occurring in a chromosome-type ring, thereby joining strands of two rings, followed by a transformation into one ring. These single-chromatid rings were less stable in vivo than chromosome-type rings.

CONCLUSION

Single-chromatid rings visualized clearly using PCC techniques indicate SCE in the respective rings. Contrary to the conventional SCE-detecting technique, this approach does not require the use of bromodeoxyuridine, which itself leads to SCE. Some of the observed SCE might be secondary products resulting from the repair of radiation-induced DNA damage, while others may be spontaneous.

Arrangement of chromosomes in the interphase nucleus of plants

Chromosomal arrangement in the interphase nucleus has two main aspects: (1) arrangement of chromosomes with respect to nuclear polarity and to other nuclear components, and (2) arrangement of chromosomes with respect to one another. The latter aspect consists of two main types of spatial relationships; (1) relationships between different members of one chromosomal set, (b) relationships between different chromosomal sets. Data concerning various aspects of chromosomal arrangement in the interphase nucleus are presented and discussed and the genetic control as well as subcellular mechanisms which are involved in nuclear organization, are elucidated. Evidence is presented indicating that, in common wheat, the gene system that determines the specific pattern of chromosomal arrangement in the nucleus is operating via the microtubular elements of the spindle system. The significance of ordered arrangement of chromosomes in the nucleus for the regularity of genetic activity and chromosomal behavior, is pointed out.