Summary. In 1885 Carl Rabl published his theory on the internal structure of the interphase nucleus. We have tested two predictions of this theory in fibroblasts grown in vitro from a female Chinese hamster, namely (1) the Rabl-orientation ofinterphase chromosomes and (2) the stability of the chromosome arrangement established in telophase throughout the subsequent interphase. Tests were carried out by premature chromosome condensation (PCC) and laser-UV-microirradiation of the interphase nucleus. Rabl-orientation of chromosomes was observed in G1 PCCs and G2 PCCs. The cell nucleus was microirradiated in G1 at one or two sites and pulse-labelled with SH-thymidine for 2 h. Cells were processed for autoradiography either immediately thereafter or after an additional growth period of 10 to 60h. Autoradiographs show unscheduled DNA synthesis (UDS) in the microirradiated nuclear part(s). The distribution of labelled chromatin was evaluated in autoradiographs from 1035 cells after microirradiation of a single nuclear site and from 253 cells after microirradiation of two sites. After 30 to 60 h postincubation the labelled regions still appeared coherent although the average size of the labelled nuclear area fr increased from 14.2% (0 h) to 26.5% (60 h). The relative distance dr, i.e. the distance between two microirradiated sites divided by the diameter of the whole nucleus, showed a slight decrease with increasing incubation time. Nine metaphase figures were evaluated for UDS-label after microirradiation of the nuclear edge in GI. An average of 4.3 chromosomes per cell were labelled. Several chromosomes showed joint labelling of both distal chromosome arms including the telomeres, while the centromeric region was free from label. This label pattern is interpreted as the result of a V-shaped orientation of these particular chromosomes in the interphase nucleus with their telomeric regions close to each other at the nuclear edge. Our data support the tested predictions of the RaN-model. Small time-dependent changes of the nuclear space
SUMMARYCells of an euploid strain of the Chinese hamster synchronized in the GI phase were microirradiated in the nucIeus with a laser UV microbeam (A=257 nm) and pulse-labelled with [3H]thymidine. In autoradiographs of cells fixed immediately after the pulse unscheduled DNA synthesis (UDS) was found restricted to tbe microirradiated part of the nueleus. Tbe rate of UDS varied with the UV energy applied and the post-irradiation incubation time. In other experiments chromosome preparations were established after an additional chase and a subsequent growth period. In 28 mitotic cells autoradiographic label was found concentrated on a few chromosomes which lay adjacent to each other in one part of the metaphase plate. The distribution of label on the chromosomes could clearly be distinguished from patterns wh ich originate from semi-conservative DNA synthesis within S phase. The label on chromosomes of microirradiated cells thus represents UDS. Dur findings support the following ideas on the arrangement of interphase chromosomes: (1) Decondensed interphase chromosomes may occupy rather compact territories. (2) Chromosomes do not necessarily exhibit a elose and permanent association with their respective homologues.Incorporation of [3H]thymidine into non-S phase cells (unscheduled DNA synthesis, UDS [1]) has been observed in cells of different origin after treatment with DNA damaging agents [2]. UDS observed after UV microirradiation of tissue culture cells [3][4][5][6][7][8][9] has recently been introduced as a tool to investigate the correlation of the chromosome arrangement in metaphase and in interphase. Sakharov et al. [8,9] performed UV microirradiation of anaphase chromosomes and used UDS to relocate the irradiated chromatin in the nuclei of daughter cells. An investigation of the arrangement of interphase chromosomes using this approach is, however, limited for two reasons: (a) In general chromosomes can be only very poody identified within a living mitotic cell; and (b) the arrangement established after irradiation might be abnormal due to UV-induced alterations (e.g. stickiness) of the irradiated chromatin [7]. These limitations do not apply to an approach in which the cell is irradiated du ring interphase and the irradiated chromatin is identified in chromosome preparations made in the following metaphase. In this case the cell is allowed to establish the interphase arrangement of chromosomes prior to irradiation. The identification of the irradiated chromatin in the next metaphase would allow to investigate this arrangement. This analysis Exp Cell Res /24 (/979)
The temperature-humidity-bias (THB) test is the standard for accelerated stress testing with respect to corrosion and other humidity driven degradation mechanisms. Usually, 1000 hrs tests at 85 degree Celsius and 85 percent relative humidity (85/85) are used to predict up to 25 years of operation. With regard to the respective standards asking for limited selfheating, the bias was commonly limited to 80 V. Nevertheless, recent THB tests on 1.7 kV IGBT modules have shown that higher bias is a more severe test condition. Failure analysis confirmed corrosion of the Al chip-metallization as well as Cuand Ag-dendrites as the relevant failure mechanisms. In order to determine the acceleration due to bias, 1.2 kV IGBTmodules were tested in THB at 65 percent and 90 percent of their nominal voltage V nom , respectively. A characteristic degradation consisting of three phases has been identified. The 2 nd phase seems to be determined by Al corrosion and a factor of about two has been estimated for the acceleration between the aforementioned test-voltages. Within the 3 rd phase, the devices stabilized probably due to localized self-heating. Thus, this degradation mechanism is kind of self-limiting, but the higher leakage increases the risk of thermal runaway, i.e. catastrophic breakdown especially when biased close to V nom .
SummarySynchronized and asynchronously growing cells of a V79 sub-line of the Chinese hamster were either whole-cell irradiated (~,, 254 nm) or laser-UV-microirradiated ()~, 257 nm). Post-incubation with caffeine (1-2 mM) often resulted in chromosome shattering, which was a rare event in the absence of this compound. In experiments with caffeine, the following results were obtained.Shattering of all the chromosomes of a cell (generalized chromosome shattering, GCS) was induced by whole-cell irradiation at the first post-irradiation mitosis when the UV fluence exceeded a "threshold" value in the sensitive phases of the cell cycle (.(31 and S). GCS was also induced by laser-UV-microirradiation of a small part of the nucleus in G1 or S whereas microirradiation of cytoplasm beside the nucleus was not effective. An upper limit of the UV fluence in the non-irradiated nuclear part due to scattering of the microbeam was experimentally obtained. This UV fluence was significantly below the threshold fluence necessary to induce GCS in whole-cell irradiation experiments. In other cells, partial nuclear irradiation resulted in shattering of a few chromosomes only, while the majority remained intact (partial chromosome shattering, PCS). G1/early S was the most sensitive phase for induction of GCS by whole-cell and partial nuclear irradiation. The frequency of PCS was observed to increase when partial nuclear irradiation was performed either at lower incident doses or at later stages of S. We suggest that PCS and GCS indicate 2 levels of chromosome damage which can be produced by the synergistic action of UV irradiation and caffeine. PCS may be restricted to microirradiated chromatin whereas
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