No abstract

No abstract

No abstract

Mature pollen of Tradescantia paludosa and T. bracteata was irradiated with monochromatic x-rays of quantum energy 3 kev and 1"5 kev. Chromatid aberrations were scored at metaphase of the pollen-tube mitosis (second post-meiotic mitosis).It was found that: (1) 1"5 key x-rays are efficient in producing aberrations; (2) the yield of aberrations with 3 key x-rays is linearly proportional to dose ; (3) the oxygen dose-modification factor with 3 kev x-rays is about 3 for true aberrations and 1"8 for single gaps.It is concluded that there is no longer any basis for the estimate that some 17 'ionizations' are required for efficient 'breakage' of a chromatid. Instead, most of the facts are explicable in terms of ' breakage ' by a single energy-loss event in a micro-structure of a ckromatid, for example, a short length of a single macromolecule of DNA.

Thin films of dry DNA (-30 per cent water content) from T7 bacteriophage were irradiated by charged particles with track segment LET , values up to 255 keV/µm . Mean numbers of single-and double-strand breaks per molecule were determined by boundary sedimentation in an analytical ultracentrifuge. For both types of break, the number per unit dose increased only slightly up to about LET =100 keV /pm, but more rapidly beyond that point ; the ratio of the numbers of the two types of breaks did not change systematically throughout the whole range of LET,, . The data have been analysed to deduce the separate contributions of the primary track cores and the delta-rays to total dose and to total single-or double-strand breakage . Breakage by the primary cores per unit dose from cores begins to increase markedly above a core LET1oa around 50 keV/pm. The increase could be explained by effective cooperation between two or more events of small size occurring together at high LET but which would be rather ineffective when occurring singly at low LET . On the basis of estimates of core diameters currently adopted in radiation chemistry, it appears that one intersection of the DNA molecule by a core with high LET100 produces several single-strand breaks, implying long-range intramolecular transfer of energy. This conclusion could be avoided by postulating that core diameters are larger by an order of magnitude than the presently accepted values .Int J Radiat Biol Downloaded from informahealthcare.com by University of Alberta on 11/19/14For personal use only.