Direct and indirect control of the initiation of meiotic recombination by DNA damage checkpoint mechanisms in budding yeast

Meiotic recombination plays an essential role in the proper segregation of chromosomes at meiosis I in many sexually reproducing organisms. Meiotic recombination is initiated by the scheduled formation of genome-wide DNA double-strand breaks (DSBs). The timing of DSB formation is strictly controlled because unscheduled DSB formation is detrimental to genome integrity. Here, we investigated the role of DNA damage checkpoint mechanisms in the control of meiotic DSB formation using budding yeast. By using recombination defective mutants in which meiotic DSBs are not repaired, the effect of DNA damage checkpoint mutations on DSB formation was evaluated. The Tel1 (ATM) pathway mainly responds to unresected DSB ends, thus the sae2 mutant background in which DSB ends remain intact was employed. On the other hand, the Mec1 (ATR) pathway is primarily used when DSB ends are resected, thus the rad51 dmc1 double mutant background was employed in which highly resected DSBs accumulate. In order to separate the effect caused by unscheduled cell cycle progression, which is often associated with DNA damage checkpoint defects, we also employed the ndt80 mutation which permanently arrests the meiotic cell cycle at prophase I. In the absence of Tel1, DSB formation was reduced in larger chromosomes (IV, VII, II and XI) whereas no significant reduction was found in smaller chromosomes (III and VI). On the other hand, the absence of Rad17 (a critical component of the ATR pathway) lead to an increase in DSB formation (chromosomes VII and II were tested). We propose that, within prophase I, the Tel1 pathway facilitates DSB formation, especially in bigger chromosomes, while the Mec1 pathway negatively regulates DSB formation. We also identified prophase I exit, which is under the control of the DNA damage checkpoint machinery, to be a critical event associated with down-regulating meiotic DSB formation

Experimental analysis of damage creation and permanent indentation on highly oriented plates

This paper presents an experimental investigation concerning low-velocity impact and quasi-static indentation tests on highly oriented laminates used in aeronautical and aerospace applications. The damage observed in such laminates is very particular. Post mortem analysis were carried out which helped to define an impact damage scenario. Microscopic observations led to explain the mechanism of permanent indentation formation which is a fundamental point of damage tolerance justification. Equivalence between static and dynamic is also discussed

Optical doping and damage formation in AIN by Eu implantation

AlN films grown on sapphire were implanted with 300 keV Eu ions to fluences from 3×1014 to 1.4×1017 atoms/cm2 in two different geometries: “channeled” along the c-axis and “random” with a 10° angle between the ion beam and the surface normal. A detailed study of implantation damage accumulation is presented. Strong ion channeling effects are observed leading to significantly decreased damage levels for the channeled implantation within the entire fluence range. For random implantation, a buried amorphous layer is formed at the highest fluences. Red Eu-related photoluminescence at room temperature is observed in all samples with highest intensities for low damage samples (low fluence and channeled implantation) after annealing. Implantation damage, once formed, is shown to be stable up to very high temperatures.FCT - POCI/FIS/57550/2004FCT - PTDC/FIS/66262/2006FCT - PTDC/CTM/100756/200

Damage Averaging and the Formation of Class Action Suits

Within a class action suit, similarly injured individuals can collectively obtain compensation through the justice system. Damage averaging occurs when the compensation awarded by the court to individual members is partly or completely determined by the average damage of the class. The key role of damage averaging in influencing the identity of the individual that will initiate the class action suit is illustrated in a waiting game. If there is complete averaging, the individual with the lowest damage will initiate the class action suit, while if there is less damage averaging, other individuals may do so. Grâce au recours collectif, des individus ayant subi des dommages d'ampleur différente mais de même nature peuvent obtenir compensation en cour. Il est possible que le montant accordé à un individu par la cour ne soit pas strictement une compensation pour les dommages qu'il a subis, mais qu'il réflète aussi, en partie, la moyenne des dommages subis par tous les participants au recours collectif. Envisageant la formation d'un recours collectif comme un jeu d'attente, nous montrons que l'usage de la moyenne des dommages par la cour est un déterminant important de l'identité de celui qui initiera le recours collectif. Si seule la moyenne des dommages est utilisée par la cour dans l'établissement des compensations, alors l'individu ayant subi les plus petits dommages initiera le recours collectif. Si la cour utilise également les dommages individuels dans l'établissement des compensations, alors d'autres individus pourraient vouloir l'initier.Class action suit, damage averaging

Failure analysis of CFRP laminates subjected to Compression After Impact: FE simulation using discrete interface elements

This paper presents a model for the numerical simulation of impact damage, permanent indentation and compression after impact (CAI) in CFRP laminates. The same model is used for the formation of damage developing during both low-velocity / low-energy impact tests and CAI tests. The different impact and CAI elementary damage types are taken into account, i.e. matrix cracking, fiber failure and interface delamination. Experimental tests and model results are compared, and this comparison is used to highlight the laminate failure scenario during residual compression tests. Finally, the impact energy effect on the residual strength is evaluated and compared to experimental results

RNase H enables efficient repair of R-loop induced DNA damage.

R-loops, three-stranded structures that form when transcripts hybridize to chromosomal DNA, are potent agents of genome instability. This instability has been explained by the ability of R-loops to induce DNA damage. Here, we show that persistent R-loops also compromise DNA repair. Depleting endogenous RNase H activity impairs R-loop removal in Saccharomyces cerevisiae, causing DNA damage that occurs preferentially in the repetitive ribosomal DNA locus (rDNA). We analyzed the repair kinetics of this damage and identified mutants that modulate repair. We present a model that the persistence of R-loops at sites of DNA damage induces repair by break-induced replication (BIR). This R-loop induced BIR is particularly susceptible to the formation of lethal repair intermediates at the rDNA because of a barrier imposed by RNA polymerase I

Nickel(II)-catalysed oxidative guanine and DNA damage beyond 8-oxoguanine

Oxidative DNA damage is one of the most important and most studied mechanisms of disease. It has been associated with a range of terminal diseases such as cancer, heart disease, hepatitis, and HIV, as well as with a variety of everyday ailments. There are various mechanisms by which this type of DNA damage can be initiated, through radiation and chemical oxidation, among others; however, these mechanisms have yet to be fully elucidated. A HPLC-UV-EC study of the oxidation of DNA mediated by nickel(II) obtained results that show an erratic, almost oscillatory formation of 8-oxoguanine (8-oxoG) from free guanine and from guanine in DNA. Sporadic 8-oxoG concentrations were also observed when 8-oxoG alone was subjected to these conditions. A HPLC-MS/MS study showed the formation of oxidised-guanidinohydantoin (oxGH) from free guanine at pH 11, and the formation of guanidinohydantoin (GH) from DNA at pH 5.5

Giant cell formation produced by laser microbeam irradiation of chromatin in Chinese hamster cells

A pulsed laser microbeam of wavelength 532 nm was used to produce visible small lesions in the nucleoplasm or in the cytoplasm of V79 Chinese hamster cells. Transmission electron microscopy (TEM) of microirradiated nuclei showed that the lesions were produced within the nucleus and comprised between 0.2 and 0.5% of the total chromatin. Serial sections above and below the lesion site did not reveal any detectable chromatin damage, indicating that a visible lesion was restricted to the focal point of the beam. Whereas cells microirradiated anywhere in the cytoplasm showed normal clonal growth with few exceptions, the cells containing nuclear lesions did not enter mitosis at the time of unirradiated controls. Instead they formed giant cells in a high percentage of cases (72/99). The DNA content of these cells was considerably increased suggesting polyploidization. In some cases, division of giant cells was observed resulting in non-viable daughter cells containing micronuclei. Further evidence that the induction of giant cell formation depends on chromatin damage was obtained by microirradiation of chromosomes in anaphase. Here, giant cell formation was observed in the daughter cell which received microirradiated chromatin, whereas microirradiation of cytoplasm between the moving sets of chromosomes did not affect subsequent divisions of both daughter cells. Our data point out that loss of reproductive integrity and giant cell formation can be induced by damage at many sites of the chromosome complement