共查询到20条相似文献，搜索用时 31 毫秒
1.
王振生 《福建农林大学学报(自然科学版)》1981,(1)
本文介绍了按给定位置进行四连杆机构的尺度综合问题。分析了采用半角转动法求中心点和圆周点以进行四杆机构综合的原理和方法.文中详细地阐明了回转极的性质,扼要地谈及了中点曲线和圆点曲线的概念及其一些性质;导出了相对回转极点、中心点和圆周点座标的数学表达式;最后针对给定主、从动构件四个相对位置的尺度综合作了简要的概括。 相似文献
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Beck M Förster F Ecke M Plitzko JM Melchior F Gerisch G Baumeister W Medalia O 《Science (New York, N.Y.)》2004,306(5700):1387-1390
Nuclear pore complexes (NPCs) are gateways for nucleocytoplasmic exchange. To analyze their structure in a close-to-life state, we studied transport-active, intact nuclei from Dictyostelium discoideum by means of cryoelectron tomography. Subvolumes of the tomograms containing individual NPCs were extracted in silico and subjected to three-dimensional classification and averaging, whereby distinct structural states were observed. The central plug/transporter (CP/T) was variable in volume and could occupy different positions along the nucleocytoplasmic axis, which supports the notion that it essentially represents cargo in transit. Changes in the position of the CP/T were accompanied by structural rearrangements in the NPC scaffold. 相似文献
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Dunkle JA Wang L Feldman MB Pulk A Chen VB Kapral GJ Noeske J Richardson JS Blanchard SC Cate JH 《Science (New York, N.Y.)》2011,332(6032):981-984
During protein synthesis, the ribosome controls the movement of tRNA and mRNA by means of large-scale structural rearrangements. We describe structures of the intact bacterial ribosome from Escherichia coli that reveal how the ribosome binds tRNA in two functionally distinct states, determined to a resolution of ~3.2 angstroms by means of x-ray crystallography. One state positions tRNA in the peptidyl-tRNA binding site. The second, a fully rotated state, is stabilized by ribosome recycling factor and binds tRNA in a highly bent conformation in a hybrid peptidyl/exit site. The structures help to explain how the ratchet-like motion of the two ribosomal subunits contributes to the mechanisms of translocation, termination, and ribosome recycling. 相似文献
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Structural rearrangements are an essential property of atomic and molecular glasses; they are critical in controlling resistance to flow and are central to the evolution of many properties of glasses, such as their heat capacity and dielectric constant. Despite their importance, these rearrangements cannot directly be visualized in atomic glasses. We used a colloidal glass to obtain direct three-dimensional images of thermally induced structural rearrangements in the presence of an applied shear. We identified localized irreversible shear transformation zones and determined their formation energy and topology. A transformation favored successive ones in its vicinity. Using continuum models, we elucidated the interplay between applied strain and thermal fluctuations that governs the formation of these zones in both colloidal and molecular glasses. 相似文献
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本文密切联系当今一系列教育观念的更新,以及上海农业转型升级的形势,阐述了构建素质教育的新思路,并围绕再创上农人才特色,对素质教育的抓手、阵地和枢纽进行了回顾与探讨.提出应把综合素质和人才特色的养成贯穿于理论教学、实践教学和校园文化活动之中;要把图书馆、学生宿舍同课堂一样视作重要教育阵地;并充分发挥教研室、班主任和学生社团的枢纽作用,使素质教育得以全方位强化. 相似文献
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Status of women microbiologists 总被引：1，自引：0，他引：1
The general picture that emerges from this study is that the woman microbiologist, upon entering the professional job market, faces (i) slower advancement; (ii) restricted extramural recognition; and (iii) fewer positions of a supervisory or administrative nature, when compared to men. Most striking is the salary differential, which increases with increasing educational level, with increasing rank, and with increasing seniority. From the beginning of her professional training, the woman microbiologist feels handicapped by lack of encouragement and proper role models. She generally receives little advice regarding her professional future and rarely feels pushed to take the most challenging position. Should she be married, she feels that her mobility is severely restricted. Even though the subjective nature of these feelings may be interpreted as projections of failure, subtle inducements for women to stay at lower levels may well exist, in addition to more objective measurements, such as lower salary levels and slower professional advancement. Despite these handicaps, professional women continue to work. As a group, they work for the same reasons that men do, they work as long and as hard as men do, and they remain at their positions as long as men do. Women and men rate themselves equally as to job performance, degree of independence, and publication rate. On the basis of this study, it should not be surprising that women professionals are less visible than men and that only a small proportion of women become what is considered successful by the usual external criteria. If women were to receive continued encouragement, scientific contact, and professional recognition at each stage of their professional lives, they would undoubtedly become more visible. The lack of encouragement and selfconfidence leading to isolation, which then leads to lack of recognition, is a vicious circle that must be broken for the woman professional. This can be done most easily for the beginning student. For older women, there must be increased placement in positions of responsibility and visibility. Protective practices that discourage women from entering arenas of competition can only be viewed as discrimination on the basis of sex, since women professionals are rarely given the choice between being protected and being independent. Unexpectedly, this study illustrates the lower status of another group of individuals who are considered deviants from the expected roles of the established society-single men with doctorates, who were found in the positions predominately filled by women. In conclusion, this study of a select group of scientists probably has general applicability to all women professionals in their roles vis-à-vis men. Examination and documentation of discriminatory practices based on sex points to the areas in which women must direct their demands for equality. 相似文献
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Germline organization of the murine T cell receptor beta-chain genes 总被引：11，自引：0，他引：11
H S Chou C A Nelson S A Godambe D D Chaplin D Y Loh 《Science (New York, N.Y.)》1987,238(4826):545-548
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晋西黄土地区水土保持林地土壤入渗性能的研究 总被引：26，自引：2，他引：26
通过对累积入渗量的分析，发现林地土壤入渗性能最佳，荒草地次之，而农耕地最差。方差分析表明，在流域内不同地貌部位之间土壤入渗性能无明显差异，而不同地类之间差异显著，即水土保持林对提高土壤入渗能力有显著作用。多重比较分析说明不同水土保持林地土壤入渗性能也有明显区别；同时发现水土保持林对入渗的空间分布亦有重要影响。 相似文献
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The mouse homolog (c-sis) of the transforming gene of the simian sarcoma virus was mapped to chromosome 15 by the Southern blot analysis of DNA's from hamster-mouse somatic cell hybrids. Alterations in c-sis expression may thus play a role in the various murine neoplastic diseases characterized by rearrangements or duplications of chromosome 15. 相似文献
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Cantley LC 《Science (New York, N.Y.)》2002,296(5573):1655-1657
Phosphorylated lipids are produced at cellular membranes during signaling events and contribute to the recruitment and activation of various signaling components. The role of phosphoinositide 3-kinase (PI3K), which catalyzes the production of phosphatidylinositol-3,4,5-trisphosphate, in cell survival pathways; the regulation of gene expression and cell metabolism; and cytoskeletal rearrangements are highlighted. The PI3K pathway is implicated in human diseases including diabetes and cancer, and understanding the intricacies of this pathway may provide new avenues for therapuetic intervention. 相似文献
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Insertion sequence duplication in transpositional recombination 总被引：15，自引：0，他引：15
Insertion sequences (IS) are discrete segments of DNA that can transpose from one genomic site to another and promote genetic rearrangements. A question that is central to understanding the mechanism of transpositional recombination is whether genetic rearrangements are accompanied by duplication of the IS that promotes them. Analysis of adjacent deletions mediated by IS903 provides the strongest evidence to date than any IS-mediated transpositional recombination can occur by an efficient replicative mechanism. 相似文献
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Predicting soil depth in a large and complex area using machine learning and environmental correlations 
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下载免费PDF全文 Soil depth is critical for eco-hydrological modeling, carbon storage calculation and land evaluation. However, its spatial variation is poorly understood and rarely mapped. With a limited number of sparse samples, how to predict soil depth in a large area of complex landscapes is still an issue. This study constructed an ensemble machine learning model, i.e., quantile regression forest, to quantify the relationship between soil depth and environmental conditions. The model was then combined with a rich set of environmental covariates to predict spatial variation of soil depth and straightforwardly estimate the associated predictive uncertainty in the 140 000 km2 Heihe River basin of northwestern China. A total of 275 soil depth observation points and 26 covariates were used. The results showed a model predictive accuracy with coefficient of determination (R2) of 0.587 and root mean square error (RMSE) of 2.98 cm (square root scale), i.e., almost 60% of soil depth variation explained. The resulting soil depth map clearly exhibited regional patterns as well as local details. Relatively deep soils occurred in low lying landscape positions such as valley bottoms and plains while shallow soils occurred in high and steep landscape positions such as hillslopes, ridges and terraces. The oases had much deeper soils than outside semi-desert areas, the middle of an alluvial plain had deeper soils than its margins, and the middle of a lacustrine plain had shallower soils than its margins. Large predictive uncertainty mainly occurred in areas with a lack of soil survey points. Both pedogenic and geomorphic processes contributed to the shaping of soil depth pattern of this basin but the latter was dominant. This findings may be applicable to other similar basins in cold and arid regions around the world. 相似文献
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[目的]研究硫酰氟对贮烟害虫的熏杀效果。[方法]设置4个试验仓间,每个仓间均设置上中下3个浓度检测点。熏蒸过程前期每2h检查一次,共检查11次。[结果]硫酰氟熏蒸防治烟草害虫的研究表明,硫酰氟整仓熏蒸22h后,仓问设置的3个浓度监测点的浓度基本一致;均能达到对烟草甲成虫100％的杀灭效果。[结论]为进一步推广应用提供相关依据。 相似文献
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Genomewide DNA rearrangements occur in many eukaryotes during development, but their functions and mechanisms are poorly understood. Previous studies have implicated a sequence-recognition mechanism based on RNA-mediated interactions between nuclei in ciliated protozoa. In this study, we found that the process recognized and deleted a foreign gene integrated in a Tetrahymena chromosome, suggesting an unusual mechanism of genome surveillance. We further found that injection of double-stranded RNA into the cell at specific developmental stages triggers efficient deletion of the targeted genomic regions. Together the results indicate an RNA-based mechanism that directs genomewide DNA rearrangements and serves to disable invading genetic agents. 相似文献
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Fragile sites at 16q22 are not at the breakpoint of the chromosomal rearrangement in AMMoL 总被引：5，自引：0，他引：5
There is much speculation about fragile sites on human chromosomes predisposing to specific chromosome rearrangements seen in cancer. Acute myelomonocytic leukemia is characterized by neoplastic chromosome rearrangements involving band 16q22 in patients who carry the rare fragile site at 16q22. This specific leukemic breakpoint is within the metallothionein gene cluster, which is here shown to be proximal to the rare fragile site (FRA16B) and to a common fragile site (FRA16C) in this region. Hence neither of these fragile sites are at the breakpoint in this leukemic chromosomal rearrangement. 相似文献
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LOEWE S 《Science (New York, N.Y.)》1961,133(3468):1925-1926
A wide variation in chromosome number and morphology was observed among different species and subspecies of lemurine lemurs. Comparative karyotype analysis indicates close phylogenic relationships and strongly suggests that chromosome structural rearrangements may have played an important role in the evolution of this group of primates. 相似文献
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沈瑞珍 《北京林业大学学报》1985,(4)
本文分析研究了高把油锯的操作特点及锯架结构的合理型式,根据实际测试数据提出:在测试高把油锯的振动时,应在左把手前横档处和挡腿管处增加测点,并需在此二处采取相应的隔振措施。 相似文献