压弯成形回弹预测方法

为了研究压弯成形中回弹的预测方法,提高压弯成形的精度.采用解析法研究了压弯回弹预测问题.根据压弯件变形的特点,将压弯成形中弯曲板分为等曲率弹塑性段、弹塑性自由段、纯弹性自由段、无变形自由段.将弹塑性自由段、纯弹性自由段简化为悬臂梁结构,利用材料理想弹塑性模型,建立了相应的挠曲线数学模型,并对弯曲板的回弹分段进行了研究,给出了回弹解析的方法.研究结论为进一步进行压弯参数的确定和优化提供了理论依据.     

DNA bending at adenine . thymine tracts

Intrinsic bending of DNA molecules results from local structural polymorphism in regions of homopolymeric dA . dT which are at least 4 base pairs long; the A . T tracts must be repeated in phase with the helix screw. Bending, in the direction of base-pair tilt rather than roll, occurs at the junctions between the A . T tract and adjacent B-DNA, with a larger angle at the 3' than at the 5' end of the A tract.     

Clustered DNA damage induced by protons radiation in plasmid DNA

Clustered DNA damage is considered as a critical type of lesions induced by ionizing radiation, which can be converted into the fatal or strong mutagenic complex double strand breaks (DSBs) during damage processing in the cells. The new data show that high energy protons produce more potentially lethal DSBs than low LET radiation. In this study, plasmid DNA were used to in-vestigate and re-evaluate the biological effects induced by the protons with the LET of ～3.6 keV/μm at the molecular level in vitro, including single strand breaks (SSBs), DSBs, isolated and clustered base damages. The results of complex DNA damage detections indicated that protons at the given LET value induce about 1.6 fold more non-DSB clustered DNA damages than the prompt DSB. The DNA damage yields by protons were greater than that by γ-rays, specifically by 6 fold for the isolated type of DNA damage and 14 fold for the clustered damage. Furthermore, the spectrum of damages was also demonstrated to be depended on the radiation quality, with protons producing more DSBs relative to clusters than do γ-rays.     

The locus of sequence-directed and protein-induced DNA bending

The bending locus of trypanosome kinetoplast DNA, identified by gel electrophoresis, has tracts of a simple repeat sequence (CA5-6 T) symmetrically distributed about it, with a repeat interval of 10 base pairs. The analogous bending induced when catabolite gene activating protein binds to its recognition sequence near the promoter of the Escherichia coli lac operon is centred on a site about 5-7 base pairs away from the centre of the protein binding site.     

Frequent chromosomal translocations induced by DNA double-strand breaks

The faithful repair of DNA damage such as chromosomal double-strand breaks (DSBs) is crucial for genomic integrity. Aberrant repair of these lesions can result in chromosomal rearrangements, including translocations, which are associated with numerous tumours. Models predict that some translocations arise from DSB-induced recombination in differentiating lymphoid cell types or from aberrant repair of DNA damage induced by irradiation or other agents; however, a genetic system to study the aetiology of these events has been lacking. Here we use a mouse embryonic stem cell system to examine the role of DNA damage on the formation of translocations. We find that two DSBs, each on different chromosomes, are sufficient to promote frequent reciprocal translocations. The results are in striking contrast with interchromosomal repair of a single DSB in an analogous system in which translocations are not recovered. Thus, while interchromosomal DNA repair does not result in genome instability per se, the presence of two DSBs in a single cell can alter the spectrum of repair products that are recovered.     

求解接点网络问题的DNA算法

利用DNA的二级结构——发卡构形,给出了求解接点网络问题的DNA算法．首先用DNA分子编码接点网络问题,然后利用DNA分子的自组装和形成二级结构的能力来求解问题．算法具有自动化实现计算的特点,计算所需的实验操作比Lipton提出的算法少,同时计算所需的DNA量也比Lipton提出的算法少．     

Local conformation transitions of linear DNA induced by cisplatin

Cisplatin is the most successful anti-tumor drug, and its pharmacological property is generally considered to derive from the modification of DNA molecules. Structural modifications of short DNA induced by cisplatin have already been investigated. However, the conformation transitions induced by cisplatin are not clear. In the present letter, we have studied the effect of low-concentration cisplatin on DNA conformation by using AFM imaging. We observed formations of micro-rod structures of linear DNA induced by cisplatin. A method is presented to quantitatively analyze the occurrence of micro-rod structures. We found that the formation of micro-rod structures depends on the DNA sequence. Based on the results, we proposed a physical mechanism to explain the local conformation transitions of DNA molecules under the influence of cisplatin.     

DNA loops induced by cooperative binding of lambda repressor

It has been shown by Hochschild and Ptashne that lambda repressors bind cooperatively to operator sites separated by five or six turns of the helix. Cooperative binding is not observed if the sites are separated by a nonintegral number of turns, unless a four-nucleotide gap is introduced into one of the strands between the two sites. These and other facts suggested that repressors at the separated sites touch each other, the DNA bending smoothly so as to accommodate the protein-protein interaction. Here we use electron microscopy to visualize the predicted protein-DNA complexes.     

集装箱板冷弯裂纹形成分析

针对广州珠江钢铁公司生产的厚规格集装箱板易在外弧面出现微裂纹的现象,采用金相显微镜和扫描电镜对其大量冷弯裂纹试样进行了观察和分析.结果表明,冷弯裂纹试样主要存在以下3种缺陷:距表层10～30 μm处的皮下微裂纹、皮下链状夹杂物和表层氧化层与基体结合部位的残余元素(Cu、Ni、As)偏析,其中以皮下微裂纹缺陷的几率居多.     

DNA sequence determinants of CAP-induced bending and protein binding affinity

The sites of DNA bending induced by binding catabolite activator protein are identified and shown to coincide with positions where DNA grooves face the protein. The bendability of DNA with different sequences at these bend centres parallels the bending preference of the sequences in nucleosomal DNA. Anisotropic DNA bendability significantly affects the structure and strength of regulatory protein-DNA complexes.     

Protection against UV-induced pyrimidine dimerization in DNA by triplex formation

Cyclobutane and [6-4]-pyrimidine dimers are major photoproducts of ultraviolet-irradiated DNA. The yield of these photoproducts is dependent on the sequence and structure of the DNA. By analysing the photofootprints of fragments produced by cleavage of the DNA chain near [6-4]-pyrimidine dimers, we show here that a homopurine-homopyrimidine insert (with either d(TC)x or d(C)n) in plasmid pUC19 is, as expected, a good target for UV-induced pyrimidine-dimer formation. But we find that dimerization is virtually completely suppressed when the pyrimidine oligonucleotides d(TC)y or d(C)m are added to DNA carrying d(TC)x- or d(C)n-containing inserts, respectively. This effect is dependent on the type of oligonucleotide used and is site-specific. The protection occurs under acidic conditions that favour the formation of intermolecular triplexes between the homopurine-homopyrimidine inserts and homologous oligopyrimidines. We therefore conclude that triplex formation effectively protects the DNA duplex from UV-induced damage (pyrimidine dimerization). This observation makes the photofootprinting assay a very promising method for studying intermolecular and intramolecular triplexes (H-form DNA) both in vitro and in vivo.     

Telomeric DNA dimerizes by formation of guanine tetrads between hairpin loops

The telomeric ends of eukaryotic chromosomes are composed of simple repeating sequences in which one DNA strand contains short tracts of guanine residues alternating with short tracts of A/T-rich sequences. The guanine-rich strand is always oriented in a 5'-3' direction towards the end of the chromosome and is extended to produce a 3' overhang of about two repeating units in species where the telomeric terminus is known. This overhang has been implicated in the formation of several unusual intra-and intermolecular DNA structures, although none of these structures has been characterized fully. We now report that oligonucleotides encoding Tetrahymena telomeres dimerize to form stable complexes in solution. This salt-dependent dimerization is mediated entirely by the 3'-terminal telomeric overhang (TT-GGGGTTGGGG) and produces complexes in which the N7 position of every guanine in the overhangs is chemically inaccessible. We therefore propose that telomeric DNA dimerizes by hydrogen bonding between two intramolecular hairpin loops, to form antiparallel quadruplexes containing cyclic guanine base tetrads. These novel hairpin dimers may be important in telomere association and recombination and could also provide a general mechanism for pairing two double helices in other recombinational processes.     

可调谐48 nm弯致应变光纤光栅

为了满足光纤通信和光纤传感对大范围波长调谐的迫切需要,该文对弯致应变光纤光栅波长调谐的公式进行了推导,给出了解析表达式,并研制了弯致应变光纤光栅波长调谐的实验装置,实现了高达48nm的波长调谐范围。在波长调谐过程中,3dB带宽展宽约为0.1nm;在误差允许范围内,波长调谐的实验值与解析表达式得出的理论曲线相吻合。     

Arbuscular mycorrhizal formation of crucifer leaf mustard induced by flavonoids apigenin and daidzein

Flavonoids from legume root secretion may probably act as signal molecules for expression of Rhizobial “nod” nodulation genes and AM fungal symbiotic gene. Leaf mustard is a non-mycorrhizal plant; it does not contain flavonoids and other signal molecules. AM fungi could not infect the roots of leaf mustard and form a symbiont in nature,when it was treated with flavonoids (apigenin or daidzein).The results of trypan blue staining showed that two kinds of AM fungi (G intraradices and G mosseae) successfully infected the roots of non-mycorrhizal plant leaf mustard. AM fungi grew towards and colonized the roots of leaf mustard,producing young spores and completing the course of life.AM fungi are the only one kind of fungi with ALP activity.The result of ALP staining has also proved that AM fungi infected successfully the roots of leaf mustard. AM fungi (G intraradices and G mosseae) that existed in the roots of non-mycorrhizal plant leaf mustard were probed by nested PCR and special molecular probes. The above-mentioned proof chains have fully proved that flavonoids induced AM fungi (G intraradices and G mosseae) to infect non-mycorrhizal plant and establish symbiotic relationship.     

Homodimer formation of retinoid X receptor induced by 9-cis retinoic acid.

Retinoid response pathways are mediated by two classes of receptors, the retinoic acid receptors (RARs) and the retinoid X receptors (RXRs). A central question is whether distinct response pathways are regulated by these two classes of receptors. The observation that the stereoisomer 9-cis-retinoic acid binds with high affinity to RXRs suggested that this retinoid has a distinct role in controlling RXR activity, but it was almost simultaneously discovered that RXRs function as auxiliary receptors for RARs and related receptors, and are essential for DNA binding and function of those receptors. Hence, although RARs seem to operate effectively only as heterodimeric RAR/RXR complexes, RXRs themselves apparently function predominantly, if not exclusively, as auxiliary receptors. Here we report that 9-cis-retinoic acid induces RXR homodimer formation. Our results demonstrate a new mechanism for retinoid action by which a ligand-induced homodimer mediates a distinct retinoid response pathway.     

Electrically induced structure formation and pattern transfer

The wavelength of light represents a fundamental technological barrier to the production of increasingly smaller features on integrated circuits. New technologies that allow the replication of patterns on scales less than 100 nm need to be developed if increases in computing power are to continue at the present rate. Here we report a simple electrostatic technique that creates and replicates lateral structures in polymer films on a submicrometre length scale. Our method is based on the fact that dielectric media experience a force in an electric field gradient. Strong field gradients can produce forces that overcome the surface tension in thin liquid films, inducing an instability that features a characteristic hexagonal order. In our experiments, pattern formation takes place in polymer films at elevated temperatures, and is fixed by cooling the sample to room temperature. The application of a laterally varying electric field causes the instability to be focused in the direction of the highest electric field. This results in the replication of a topographically structured electrode. We report patterns with lateral dimensions of 140 nm, but the extension of the technique to pattern replication on scales smaller than 100 nm seems feasible.     

Functional proteomic identification of DNA replication proteins by induced proteolysis in vivo

Evolutionarily diverse eukaryotic cells share many conserved proteins of unknown function. Some are essential for cell viability, emphasising their importance for fundamental processes of cell biology but complicating their analysis. We have developed an approach to the large-scale characterization of such proteins, based on conditional and rapid degradation of the target protein in vivo, so that the immediate consequences of bulk protein depletion can be examined. Budding yeast strains have been constructed in which essential proteins of unknown function have been fused to a 'heat-inducible-degron' cassette that targets the protein for proteolysis at 37 degrees C (ref. 4). By screening the collection for defects in cell-cycle progression, here we identify three DNA replication factors that interact with each other and that have uncharacterized homologues in human cells. We have used the degron strains to show that these proteins are required for the establishment and normal progression of DNA replication forks. The degron collection could also be used to identify other, essential, proteins with roles in many other processes of eukaryotic cell biology.