长大桥梁结构按多点输入的竖向地震反应分析     

屈铁军  范友 《东北林业大学学报》1989,(6)

本文采用土与结构相互作用的计算模型分析了长大桥梁结构的竖向地震反应。在相互作用的计算中,利用圆心位移影响函数法求得地基的动态复刚度,采用子结构技术建立结构的动态方程,通过快速傅立叶变换在频域上求解。将计算结果与刚性地基计算模型的计算结果以及同时同位相输入的计算结果比较表明:长大桥梁应采用土与结构相互作用计算模型,按多点输入计算,并进行竖向反应分析。  相似文献   

N-Cadherin, a cell adhesion molecule involved in establishment of embryonic left-right asymmetry     

García-Castro MI  Vielmetter E  Bronner-Fraser M 《Science (New York, N.Y.)》2000,288(5468):1047-1051

Within the bilaterally symmetric vertebrate body plan, many organs develop asymmetrically. Here, it is demonstrated that a cell adhesion molecule, N-cadherin, is one of the earliest proteins to be asymmetrically expressed in the chicken embryo and that its activity is required during gastrulation for proper establishment of the left-right axis. Blocking N-cadherin function randomizes heart looping and alters the expression of Snail and Pitx2, later components of the molecular cascade that regulate left-right asymmetry. However, the expression of other components of this cascade (Nodal and Lefty) was unchanged after blocking N-cadherin function, suggesting the existence of parallel pathways in the establishment of left-right morphogenesis. Here, the results suggest that N-cadherin-mediated cell adhesion events are required for establishment of left-right asymmetry.  相似文献   

番茄“树”营养液栽培技术研究   总被引：1，自引：0，他引：1  

曲明山  贺建德  刘彬 《北京农业》2007,(30):25-28

在双层充气法国式现代温室中,试验研究了番茄树的营养液栽培综合配套技术,温室综合环境"温度、湿度、光照等"的调控,番茄树深液流栽培系统的建立,番茄品种的选择、营养生长和生殖生长合理调控,营养液的管理。通过这些技术的试验研究,初步探讨了在京郊的现代化温室中,水培番茄树栽培的可行性和高产的改进措施。  相似文献   

An association between the kinship and fertility of human couples     

Helgason A  Pálsson S  Gudbjartsson DF  Kristjánsson T  Stefánsson K 《Science (New York, N.Y.)》2008,319(5864):813-816

Previous studies have reported that related human couples tend to produce more children than unrelated couples but have been unable to determine whether this difference is biological or stems from socioeconomic variables. Our results, drawn from all known couples of the Icelandic population born between 1800 and 1965, show a significant positive association between kinship and fertility, with the greatest reproductive success observed for couples related at the level of third and fourth cousins. Owing to the relative socioeconomic homogeneity of Icelanders, and the observation of highly significant differences in the fertility of couples separated by very fine intervals of kinship, we conclude that this association is likely to have a biological basis.  相似文献   

Neuroscience. Zif and the survival of memory     

Izquierdo I  Cammarota M 《Science (New York, N.Y.)》2004,304(5672):829-830

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机动车安全技术检测多媒体交互式CAI教学软件的开发     

孙成起  杨瑞  曲洪亮  白传栋 《东北林业大学学报》2001,29(2):123-126

计算机多媒体教学软环境研究的主要内容是通过计算机技术、多媒体技术，集图、文、声、像于一体，创造出一种现场教学的情景和气氛，增加了学生的学习兴趣且有助于理解和记忆，使教学效果明显提高。计算机多媒体CAI教学软环境的研究，是通过计算机技术、多媒体技术将专业的理论与实践知识，生动而形象地展示给学生。机动车安全技术检测CAI教学软环境的研究主要展示机动车安全检测设备的结构、种类、工作原理、使用方法、维护手段以及使用中的注意事项。计算机多媒体交互式CAI教学，是教学改革的探索，是对以往教与学模式的突破。  相似文献   

Extensive gene traffic on the mammalian X chromosome     

Emerson JJ  Kaessmann H  Betrán E  Long M 《Science (New York, N.Y.)》2004,303(5657):537-540

Mammalian sex chromosomes have undergone profound changes since evolving from ancestral autosomes. By examining retroposed genes in the human and mouse genomes, we demonstrate that, during evolution, the mammalian X chromosome has generated and recruited a disproportionately high number of functional retroposed genes, whereas the autosomes experienced lower gene turnover. Most autosomal copies originating from X-linked genes exhibited testis-biased expression. Such export is incompatible with mutational bias and is likely driven by natural selection to attain male germline function. However, the excess recruitment is consistent with a combination of both natural selection and mutational bias.  相似文献   

Nature and domestic life in the Valle del Cuñapirú (Misiones, Argentina): Reflections on Mbyá-Guaraní ethnoecology     

Marta Crivos  María Rosa Martínez  María Lelia Pochettino  Carolina Remorini  Cynthia Saenz  Anahí Sy 《Agriculture and Human Values》2004,21(2-3):111-125

Through the ethnographic record of the subsistence activities partially or completely performed in the domestic sphere in two Mbyá-Guaraní settlements in Misiones (Argentina), we outline factors important in describing the local natural environment. Data was collected through systematic observation and also through semi-structured interviews. Analysis indicates that the natural environment of the area is characterized by the indigenous community in several different ways. Thus, local people view the environment as made up of different “micro-environments,” and they consequently think of the elements that compose these “micro-environments” as having different, distinct characteristics. In the context of their daily activities, both these “micro-environments” and the elements that compose them are regarded as resources, in that they are viewed in terms of what is in them relevant to the subsistence of these groups in the rainforest. It is the intention of this study to initiate the systematic recording and processing of information on how these indigenous communities know and manage the natural resources available to them in their daily life.  相似文献   

Deciphering the reaction dynamics underlying optimal control laser fields     

Daniel C  Full J  González L  Lupulescu C  Manz J  Merli A  Vajda S  Wöste L 《Science (New York, N.Y.)》2003,299(5606):536-539

Femtosecond high-resolution pump-probe experiments have been used together with theoretical ab initio quantum calculations and wave packet dynamics simulations to decode an optimal femtosecond pulse that is generated from adaptive learning algorithms. This pulse is designed to maximize the yield of the organometallic ion CpMn(CO)3 while hindering the competing fragmentation. The sequential excitation and ionization of the target ion are accomplished by an optimized field consisting of two dominant subpulses with optimal frequencies and time delays.  相似文献   

A bacterial toxin that controls cell cycle progression as a deoxyribonuclease I-like protein     

Lara-Tejero M  Galán JE 《Science (New York, N.Y.)》2000,290(5490):354-357

Many bacterial pathogens encode a multisubunit toxin, termed cytolethal distending toxin (CDT), that induces cell cycle arrest, cytoplasm distention, and, eventually, chromatin fragmentation and cell death. In one such pathogen, Campylobacter jejuni, one of the subunits of this toxin, CdtB, was shown to exhibit features of type I deoxyribonucleases. Transient expression of this subunit in cultured cells caused marked chromatin disruption. Microinjection of low amounts of CdtB induced cytoplasmic distention and cell cycle arrest. CdtB mutants with substitutions in residues equivalent to those required for catalysis or magnesium binding in type I deoxyribonucleases did not cause chromatin disruption. CDT holotoxin containing these mutant forms of CdtB did not induce morphological changes or cell cycle arrest.  相似文献