共查询到20条相似文献，搜索用时 31 毫秒
1.
为研究适用于公路路面的水泥混凝土材料粘弹性特征,通过室内制备道路水泥混凝土试件,采用控制位移法进行一次加载、循环加载试验,分别测试了C30水泥混凝土试件分别在0.1、0.3、0.5 mm·min~(-13)种加载速率下的σ-ε曲线。通过对3种加载速率下的σ-ε曲线变化规律并采用割线模量E_s表征水泥混凝土的粘弹性。研究结果表明:水泥混凝土试件σ≤0.8σ_(max)时,σ-ε曲线近似线性增长,该范围可认为是水泥混凝土的弹性工作阶段;当σ≥0.8σ_(max)时,试件出现出较大的粘性变形特征,当应力超出屈服应力后发生脆性破坏。加载速率的不同会影响σ-ε曲线的弹性模量E_c和最大破坏应力σ_(max);E_c随着加载速率的增大而减小,σ_(max)随着加载速率v的增大而增大。 相似文献
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在改善输出波形特性的基础上,提出了一种新型的脉宽调制方法,该方法基于三相线电压之间的基本关系来获得开关定时和开关脉冲图,并利用GTO逆变器和TP86A单板机在电压型逆变器上进行了实验。从理论分析和实验结果得出:这种新型PWM技术输出波形的谐波分量低,控制具有规律性,在交流调速和逆变电源方面是一种值得推广的方法。 相似文献
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为了研究高压脉冲电场对种子活性的影响,结合脉冲功率技术和计算机辅助设计技术,研制了满足最新高空核爆电磁脉冲模拟标准MIL-STD-461F的纳秒级气体火花开关。在分析开关放电原理的基础上,考虑火花隙电阻和开关结构电容的影响,基于Simulink建立了开关的动态放电模型,仿真分析了负载、开关电感、开关电容等因素对放电脉冲特性的影响,为开关的设计提供了参考。在此基础上,试制了纳秒级气体火花开关样品,在50Ω负载上输出脉冲峰值达到25 kV,上升时间2.5 ns,脉冲宽度18 ns,为进一步研究脉冲电场种子效应奠定了基础。 相似文献
4.
为了研究硼对铝胁迫下杉木生长、生理和光合特性的影响,揭示硼介导的杉木耐铝调控机制,为进一步利用养分管理手段减轻杉木铝毒害提供参考,以3月龄幼苗为材料,共设置4个处理,对照(CK)、缺硼(-B)、缺硼加铝(-B+Al)和加硼加铝(+B+Al),分析不同处理间杉木生长、叶片光合和抗性生理指标的差异。结果表明：(1)与CK相比,胁迫处理均能显著抑制苗高生长,苗高降幅介于31.83%～55.56%之间,而且铝胁迫下,加硼(+B+Al)能显著缓解缺硼(-B+Al)引起的苗高生长、叶片丙二醛(MDA)、过氧化氢(H2O2)和光合色素含量下降;与-B+Al处理相比,+B+Al处理的苗高、MDA、H2O2、叶绿素和类胡萝卜素含量分别增加48.02%、24.18%、16.39%、17.16%和17.78%。(2)与CK相比,不同胁迫处理显著提高叶片超氧化物歧化酶、过氧化物酶、过氧化氢酶和抗坏血酸过氧化物酶活性,其活性大小均表现为：+B+Al>-B+Al>-B>CK,增幅分别介于17.10%～128.... 相似文献
5.
Zarrin AA Del Vecchio C Tseng E Gleason M Zarin P Tian M Alt FW 《Science (New York, N.Y.)》2007,315(5810):377-381
Antibody class switching in activated B cells uses class switch recombination (CSR), which joins activation-induced cytidine deaminase (AID)-dependent double-strand breaks (DSBs) within two large immunoglobulin heavy chain (IgH) locus switch (S) regions that lie up to 200 kilobases apart. To test postulated roles of S regions and AID in CSR, we generated mutant B cells in which donor Smu and accepter Sgamma1 regions were replaced with yeast I-SceI endonuclease sites. We found that site-specific I-SceI DSBs mediate recombinational IgH locus class switching from IgM to IgG1 without S regions or AID. We propose that CSR evolved to exploit a general DNA repair process that promotes joining of widely separated DSBs within a chromosome. 相似文献
6.
禽呼肠孤病毒血清学相关性及S3基因序列同源性分析 总被引：1，自引：0，他引：1
采用血清学交叉中和试验方法,分析了4株禽呼肠孤病毒株CL(鸡源禽呼肠孤病毒)、YH和YB(鸭源禽呼肠孤病毒)及S1133(禽呼肠孤病毒标准株)之间抗原的相互关系;并对CL株的S3基因进行克隆测序,用DNAsis和Prosis等分析软件进行序列比较及推导氨基酸序列分析.结果表明:4株禽呼肠孤病毒株之间具有共同的群特异抗原,但不同病毒株间存在中和抗原位点的不对称性;血清学相关性与S3基因序列及其推导的σB蛋白氨基酸序列同源性不一致,表明禽呼肠孤病毒诱导机体产生特异性中和抗体的病毒蛋白不止σB蛋白一个,σB氨基酸序列同源性与血清学相关性没有必然的直接联系. 相似文献
7.
Duffy KR Wellard CJ Markham JF Zhou JH Holmberg R Hawkins ED Hasbold J Dowling MR Hodgkin PD 《Science (New York, N.Y.)》2012,335(6066):338-341
In response to stimulation, B lymphocytes pursue a large number of distinct fates important for immune regulation. Whether each cell's fate is determined by external direction, internal stochastic processes, or directed asymmetric division is unknown. Measurement of times to isotype switch, to develop into a plasmablast, and to divide or to die for thousands of cells indicated that each fate is pursued autonomously and stochastically. As a consequence of competition between these processes, censorship of alternative outcomes predicts intricate correlations that are observed in the data. Stochastic competition can explain how the allocation of a proportion of B cells to each cell fate is achieved. The B cell may exemplify how other complex cell differentiation systems are controlled. 相似文献
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9.
Critical roles of activation-induced cytidine deaminase in the homeostasis of gut flora 总被引：1，自引：0，他引：1
Fagarasan S Muramatsu M Suzuki K Nagaoka H Hiai H Honjo T 《Science (New York, N.Y.)》2002,298(5597):1424-1427
Activation-induced cytidine deaminase (AID) plays an essential role in class switch recombination (CSR) and somatic hypermutation (SHM) of immunoglobulin genes. We report here that deficiency in AID results in the development of hyperplasia of isolated lymphoid follicles (ILFs) associated with a 100-fold expansion of anaerobic flora in the small intestine. Reduction of bacterial flora by antibiotic treatment of AID-/- mice abolished ILF hyperplasia as well as the germinal center enlargement seen in secondary lymphoid tissues. Because an inability to switch to immunoglobulin A on its own does not lead to a similar phenotype, these results suggest that SHM of ILF B cells plays a critical role in regulating intestinal microflora. 相似文献
10.
Lander GC Tang L Casjens SR Gilcrease EB Prevelige P Poliakov A Potter CS Carragher B Johnson JE 《Science (New York, N.Y.)》2006,312(5781):1791-1795
Bacteriophages, herpesviruses, and other large double-stranded DNA (dsDNA) viruses contain molecular machines that pump DNA into preassembled procapsids, generating internal capsid pressures exceeding, by 10-fold, that of bottled champagne. A 17 angstrom resolution asymmetric reconstruction of the infectious P22 virion reveals that tightly spooled DNA about the portal dodecamer forces a conformation that is significantly different from that observed in isolated portals assembled from ectopically expressed protein. We propose that the tight dsDNA spooling activates the switch that signals the headful chromosome packing density to the particle exterior. 相似文献
11.
Three phenotypically distinct processes-somatic hypermutation, gene conversion, and switch recombination-remodel the functionally rearranged immunoglobulin (Ig) loci in B cells. Somatic hypermutation and switch recombination have recently been shown to depend on the activation-induced deaminase (AID) gene product. Here, we show that the disruption of the AID gene in the chicken B cell line DT40 completely blocks Ig gene conversion and that this block can be complemented by reintroduction of the AID complementary DNA. This demonstrates that the AID master gene controls all B cell-specific modifications of vertebrate Ig genes. 相似文献
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Mandal M Lee M Barrick JE Weinberg Z Emilsson GM Ruzzo WL Breaker RR 《Science (New York, N.Y.)》2004,306(5694):275-279
We identified a previously unknown riboswitch class in bacteria that is selectively triggered by glycine. A representative of these glycine-sensing RNAs from Bacillus subtilis operates as a rare genetic on switch for the gcvT operon, which codes for proteins that form the glycine cleavage system. Most glycine riboswitches integrate two ligand-binding domains that function cooperatively to more closely approximate a two-state genetic switch. This advanced form of riboswitch may have evolved to ensure that excess glycine is efficiently used to provide carbon flux through the citric acid cycle and maintain adequate amounts of the amino acid for protein synthesis. Thus, riboswitches perform key regulatory roles and exhibit complex performance characteristics that previously had been observed only with protein factors. 相似文献
14.
Identifying the properties of gene networks that influence their evolution is a fundamental research goal. However, modes of evolution cannot be inferred solely from the distribution of natural variation, because selection interacts with demography and mutation rates to shape polymorphism and divergence. We estimated the effects of naturally occurring mutations on gene expression while minimizing the effect of natural selection. We demonstrate that sensitivity of gene expression to mutations increases with both increasing trans-mutational target size and the presence of a TATA box. Genes with greater sensitivity to mutations are also more sensitive to systematic environmental perturbations and stochastic noise. These results provide a mechanistic basis for gene expression evolvability that can serve as a foundation for realistic models of regulatory evolution. 相似文献
15.
从猪卵巢上获取的卵母细胞经体外培养成熟42～44 h后去除颗粒细胞,选取成熟的卵母细胞进行孤雌激活培养.试验采用不同的场强、脉冲时程和脉冲次数进行对照试验,并比较了细胞松弛素B(CB)辅助激活对孤雌胚胎发育的影响.结果显示:1)当场强为0.8 kV.cm-1,选取4个脉冲时程(80、100、120、140μs)参数分别进行1次、2次脉冲激活时,100μs、1次脉冲囊胚率最高,为(42.22±5.38)%,但同其他组无显著差异(P>0.05).2)脉冲时程为100μs,选取4个场强(0.8、1.0、1.2、1.4 kV.cm-1)参数分别进行1次、2次脉冲激活时,1.4 kV.cm-1、1次脉冲囊胚率最高,为(44.04±0.68)%,但同其他组无显著差异(P>0.05).3)场强为0.8 kV.cm-1和1.4 kV.cm-1,选取4个脉冲时程(80、100、120、140μs)进行1次脉冲激活时,1.4 kV.cm-1+80μs组囊胚率(54.60±2.86)%均高于其他组,且极显著高于1.4 kV.cm-1+140μs、0.8 kV.cm-1+80μs和0.8 kV.cm-1+140μs组(P<0.01).4)用1.4 kV.cm-1、80μs、1次脉冲激活卵母细胞后,使用CB辅助激活4 h的囊胚率为(54.07±3.12)%,极显著(P<0.01)高于不使用CB组.以上结果说明,电激活中脉冲次数对猪孤雌胚胎发育无显著影响,且在本试验条件下,采用1.4 kV.cm-1的场强、80μs的脉冲时程,并使用CB辅助激活4 h,能够得到较高囊胚率的猪孤雌激活效率. 相似文献
16.
为提高光伏大棚太阳能利用率及逆变桥臂故障后逆变器不能正常使用等问题,提出新型三相多电平T型准Z源逆变器(T型SL-q ZSI)。新型逆变器拓扑结构由两个对称改进型准Z源网络和三相T型逆变器组成。该逆变器采用较小直通占空比即可获得较高升压因子,调制比范围较大,输出多电平电压,独立为光伏大棚供电,提高农光互补系统电能利用率。当桥臂开关发生管故障时,通过改变调制策略,逆变器可正常工作。同时采用相移正弦脉冲宽度调制(PS-PWM)方法,仿真和试验验证新拓扑正确性和优越性。该逆变器可保证光伏大棚内部电力供应且具有Z源升压、输出多电平电压、开关管容错能力强、输出电能质量高等优点。 相似文献
17.
Heterotrimeric guanine nucleotide-binding regulatory proteins (G proteins) dissociate into guanosine triphosphate (GTP)-bound alpha subunits and a complex of beta and gamma subunits after interaction with receptors. The GTP-alpha subunit complex activates appropriate effectors, such as adenylyl cyclase, retinal phosphodiesterase, phospholipase C, and ion channels. G protein beta gamma subunits have been found to have regulatory effects on certain types of adenylyl cyclase. In the presence of Gs alpha, the alpha subunit of the G protein that activates adenylyl cyclase, one form of adenylyl cyclase was inhibited by beta gamma, some forms were activated by beta gamma, and some forms were not affected by beta gamma. These interactions suggest mechanisms for communication between distinct signal-transducing pathways. 相似文献
18.
The Toll-like receptor 2 pathway establishes colonization by a commensal of the human microbiota 总被引：4，自引：0，他引：4
Round JL Lee SM Li J Tran G Jabri B Chatila TA Mazmanian SK 《Science (New York, N.Y.)》2011,332(6032):974-977
Mucosal surfaces constantly encounter microbes. Toll-like receptors (TLRs) mediate recognition of microbial patterns to eliminate pathogens. By contrast, we demonstrate that the prominent gut commensal Bacteroides fragilis activates the TLR pathway to establish host-microbial symbiosis. TLR2 on CD4(+) T cells is required for B. fragilis colonization of a unique mucosal niche in mice during homeostasis. A symbiosis factor (PSA, polysaccharide A) of B. fragilis signals through TLR2 directly on Foxp3(+) regulatory T cells to promote immunologic tolerance. B. fragilis lacking PSA is unable to restrain T helper 17 cell responses and is defective in niche-specific mucosal colonization. Therefore, commensal bacteria exploit the TLR pathway to actively suppress immunity. We propose that the immune system can discriminate between pathogens and the microbiota through recognition of symbiotic bacterial molecules in a process that engenders commensal colonization. 相似文献
19.
Negative feedback regulation ensures the one receptor-one olfactory neuron rule in mouse 总被引：1，自引：0，他引：1
Serizawa S Miyamichi K Nakatani H Suzuki M Saito M Yoshihara Y Sakano H 《Science (New York, N.Y.)》2003,302(5653):2088-2094
In the mouse olfactory system, each olfactory sensory neuron (OSN) expresses only one odorant receptor (OR) gene in a monoallelic and mutually exclusive manner. Such expression forms the genetic basis for OR-instructed axonal projection of OSNs to the olfactory bulb of the brain during development. Here, we identify an upstream cis-acting DNA region that activates the OR gene cluster in mouse and allows the expression of only one OR gene within the cluster. Deletion of the coding region of the expressed OR gene or a naturally occurring frame-shift mutation allows a second OR gene to be expressed. We propose that stochastic activation of only one OR gene within the cluster and negative feedback regulation by that OR gene product are necessary to ensure the one receptor-one neuron rule. 相似文献
20.
钙调神经磷酸酶B样相互作用蛋白激酶(CIPK)蛋白家族是由Ca2+介导的植物信号通路中的关键蛋白家族,在植物抗逆和生长发育中起关键作用。本研究将生物信息学方法和转录组数据分析相结合,挖掘出大麦31个HvCIPK基因家族成员并将其分为5个亚家族。HvCIPKs基因家族成员具有CIPKs典型的N端激酶结构域和C端NAF调节结构域；蛋白质分子量在40302.27~89926.43KDa之间,为亲水性蛋白；启动子总共包含11种与非生物胁迫、激素调控以及生长发育相关的顺式作用元件；蛋白互作网络预测结果显示,HvCIPKs与Na+、K+转运体、ABA信号通路关键蛋白(SOS1、AKT1和ABL2)存在相互作用关系；转录组数据分析发现HvCIPK1、HvCIPK2、HvCIPK6、HvCIPK9、HvCIPK11受盐碱胁迫的诱导表达。该研究为进一步探索大麦HvCIPKs基因家族功能及调控机制提供理论依据。 相似文献