共查询到20条相似文献，搜索用时 15 毫秒
1.
Onoa B Dumont S Liphardt J Smith SB Tinoco I Bustamante C 《Science (New York, N.Y.)》2003,299(5614):1892-1895
Mechanical unfolding trajectories for single molecules of the Tetrahymena thermophila ribozyme display eight intermediates corresponding to discrete kinetic barriers that oppose mechanical unfolding with lifetimes of seconds and rupture forces between 10 and 30 piconewtons. Barriers are magnesium dependent and correspond to known intra- and interdomain interactions. Several barrier structures are "brittle," breakage requiring high forces but small (1 to 3 nanometers) deformations. Barrier crossing is stochastic, leading to variable unfolding paths. The response of complex RNA structures to locally applied mechanical forces may be analogous to the responses of RNA during translation, messenger RNA export from the nucleus, and viral replication. 相似文献
2.
Reaction path methods provide a powerful tool for bridging the gap between electronic structure and chemical dynamics. Classical mechanical reaction paths may usually be understood in terms of the force field in the vicinity of a minimum energy path (MEP). When there is a significant component of hydrogenic motion along the MEP and a barrier much higher than the average energy of reactants, quantal tunneling paths must be considered, and these tend to be located on the corner-cutting side of the MEP. As the curvature of the MEP in mass-scaled coordinates is increased, the quantal reaction paths may deviate considerably from the classical ones, and the force field must be mapped out over a wider region, called the reaction swath. The required force fields may be represented by global or semiglobal analytic functions, or the dynamics may be computed "directly" from the electronic structure results without the intermediacy of potential energy functions. Applications to atom and diatom reactions in the gas phase and at gas-solid interfaces and to reactions of polyatomic molecules in the gas phase, in clusters, and in aqueous solution are discussed as examples. 相似文献
3.
中国鹅5个品种遗传多样性的随机扩增多态DNA分析 总被引：15，自引：1，他引：14
用４０个１０－寡核苷酸随机引物对中国鹅的５个品种－狮头鹅,皖西白鹅,太湖鹅,浙东白鹅和四川白鹅进行了随机扩增多态ＤＮＡ（ＲＡＰＤ）分析。结果表明,用筛选出的１４个引物共扩增出１７４个ＤＮＡ片段,其中的４８个（占２８．２％）在５个品种间表现为多态性。根据扩增ＤＮＡ片段的异同,计算了各品种和个体间的遗传多样性指数和遗传距离指数。同时,用分子进化遗传分析软件（ＭＥＧＡ）以ＵＰＧＭＡ法和ＮＪ法对遗传距离指 相似文献
4.
C O Pabo A K Aggarwal S R Jordan L J Beamer U R Obeysekare S C Harrison 《Science (New York, N.Y.)》1990,247(4947):1210-1213
Comparison of a lambda repressor-operator complex and a 434 repressor-operator complex reveals that three conserved residues in the helix-turn-helix (HTH) region make similar contacts in each of the crystallographically determined structures. These conserved residues and their interactions with phosphodiester oxygens help establish a frame of reference within which other HTH residues make contacts that are critical for site-specific recognition. Such "positioning contacts" may be important conserved features within families of HTH proteins. In contrast, the structural comparisons appear to rule out any simple "recognition code" at the level of detailed side chain-base pair interactions. 相似文献
5.
The uncatalyzed edge growth of carbon nanotubes was investigated by first-principles molecular dynamics simulations. At experimental temperatures the open end of single-walled nanotubes closed spontaneously into a graphitic dome, which may explain why these nanotubes do not grow in the absence of transition metal catalysts. On the other hand, chemical bonding between the edges of adjacent coaxial tubes ("lip-lip" interactions) trapped the end of a double-walled nanotube in a metastable energy minimum, thus preventing dome closure. These calculations show that this end geometry exhibits a high degree of chemical activity and easily accommodates incoming carbon fragments, supporting a model of growth by chemisorption from the vapor phase. 相似文献
6.
Pathways through networks of branched DNA 总被引：2，自引：0，他引：2
O Smithies 《Science (New York, N.Y.)》1970,169(948):882-883
7.
The hinge region on the Fc fragment of human immunoglobulin G interacts with at least four different natural protein scaffolds that bind at a common site between the C(H2) and C(H3) domains. This "consensus" site was also dominant for binding of random peptides selected in vitro for high affinity (dissociation constant, about 25 nanomolar) by bacteriophage display. Thus, this site appears to be preferred owing to its intrinsic physiochemical properties, and not for biological function alone. A 2.7 angstrom crystal structure of a selected 13-amino acid peptide in complex with Fc demonstrated that the peptide adopts a compact structure radically different from that of the other Fc binding proteins. Nevertheless, the specific Fc binding interactions of the peptide strongly mimic those of the other proteins. Juxtaposition of the available Fc-complex crystal structures showed that the convergent binding surface is highly accessible, adaptive, and hydrophobic and contains relatively few sites for polar interactions. These are all properties that may promote cross-reactive binding, which is common to protein-protein interactions and especially hormone-receptor complexes. 相似文献
8.
Velocity structure of a gas hydrate reflector 总被引：3，自引：0，他引：3
Seismic reflection profiles across many continental margins have imaged bottom-simulating reflectors (BSRs) parallel to the seabed; these are often interpreted as the base of a zone in which methane hydrate "ice" is stable. Waveform inversion of seismic reflection data can be used to estimate from seismic data worldwide the velocity structure of a BSR and its thickness. A test of this method at a drill site of the Ocean Drilling Program predicts that sediment pores beneath the BSR contain free methane for approximately 30 meters. The hydrate and underlying gas represent a large global reservoir of methane, which may have economic importance and may influence global climate. 相似文献
9.
We reexamined the Late Cretaceous-early Tertiary apparent polar wander path for the Pacific plate using 27 paleomagnetic poles from seamounts dated by (40)Ar/(39)Ar geochronology. The path shows little motion from 120 to 90 million years ago (Ma), northward motion from 79 to 39 Ma, and two groups of poles separated by 16 to 21 degrees with indistinguishable mean ages of 84 +/- 2 Ma. The latter phenomenon may represent a rapid polar wander episode (3 to 10 degrees per million years) whose timing is not adequately resolved with existing data. Similar features in other polar wander paths imply that the event was a rapid shift of the spin axis relative to the mantle (true polar wander), which may have been related to global changes in plate motion, large igneous province eruptions, and a shift in magnetic field polarity state. 相似文献
10.
《Science (New York, N.Y.)》1993,262(5141):1803
The ScienceScope item "NIH shakeup continues" (29 Oct., p. 643) incorrectly described the position of Francis Collins, director of the National Center for Human Genome Research, on the National Institutes of Health's (NIH's) decision 2 years ago to file for patents on gene fragments of unknown function. Collins does not believe it was appropriate for NIH to file for the patents in the first place, but since the applications have been filed, he now believes they should be pursued to a conclusion in order to clarify whether such gene fragments are patentable. 相似文献
11.
Immunoglobulin G (IgG) mediates pro- and anti-inflammatory activities through the engagement of its Fc fragment (Fc) with distinct Fcg receptors (FcgRs). One class of Fc-FcgR interactions generates pro-inflammatory effects of immune complexes and cytotoxic antibodies. In contrast, therapeutic intravenous gamma globulin and its Fc fragments are anti-inflammatory. We show here that these distinct properties of the IgG Fc result from differential sialylation of the Fc core polysaccharide. IgG acquires anti-inflammatory properties upon Fc sialylation, which is reduced upon the induction of an antigen-specific immune response. This differential sialylation may provide a switch from innate anti-inflammatory activity in the steady state to generating adaptive pro-inflammatory effects upon antigenic challenge. 相似文献
12.
Differences in Nuclear DNA Between Male-Sterile and Male-Fertile Lines of Sorghum bicolor 总被引：2，自引：0，他引：2
Cytoplasmic male sterility (cms) is determined by nuclear-cytoplasmic interactions. Up tonow, most studies are focused on the comparison of cytoplasmic DNAs of male-sterile lines and male-fertilelines, and analysis of nuclear DNA has not been documented yet. In order to find out the possible difference innuclear genome of male-sterile line A1 Tx623 and corresponding male-fertile line Tx623 of sorghum, randomamplified polymorphic DNA (RAPD) approach was used to analyze their cytoplasmic and nuclear genomes.Total DNAs of them were amplified at first to screen primers, which were able to generate reproducible bandsspecific to male-sterile line or male-fertile line. Then the selected primers were used to amplify their mitochon-drial DNA (mtDNA) and chloroplast DNA (cpDNA). The origins of all the polymorphic fragments were ana-lyzed. After ruling out those amplified from cytoplasmic DNA, seventeen polymorphic fragments were deter-mined to be amplified from nuclear DNA. These fragments originated from nuclear DNA indicate that diffe-rences in sequence exist between the nuclear DNA of male-sterile line and male-fertile line of sorghum, whichdo not agree with the traditional standpoint that they have identical nucleus. 相似文献
13.
Levin-Zaidman S Englander J Shimoni E Sharma AK Minton KW Minsky A 《Science (New York, N.Y.)》2003,299(5604):254-256
The bacterium Deinococcus radiodurans survives ionizing irradiation and other DNA-damaging assaults at doses that are lethal to all other organisms. How D. radiodurans accurately reconstructs its genome from hundreds of radiation-generated fragments in the absence of an intact template is unknown. Here we show that the D. radiodurans genome assumes an unusual toroidal morphology that may contribute to its radioresistance. We propose that, because of restricted diffusion within the tightly packed and laterally ordered DNA toroids, radiation-generated free DNA ends are held together, which may facilitate template-independent yet error-free joining of DNA breaks. 相似文献
14.
Nakanishi I 《Science (New York, N.Y.)》1978,200(4348):1379-1381
The phase velocity and the quality factor (Q) of mantle Rayleigh waves from the great Kurile Islands earthquake of 1963 have a close azimuthal correlation with each other and differ considerably among different paths. Serious error may be introduced in the interpretation of observed regionality in phase velocity if regional differences in Q are ignored. 相似文献
15.
Catastrophic volcanic collapse, without precursory magmatic activity, is characteristic of many volcanic disasters. The extent and locations of hydrothermal discharges at Nevado del Ruiz volcano, Colombia, suggest that at many volcanoes collapse may result from the interactions between hydrothermal fluids and the volcanic edifice. Rock dissolution and hydrothermal mineral alteration, combined with physical triggers such as earth-quakes, can produce volcanic collapse. Hot spring water compositions, residence times, and flow paths through faults were used to model potential collapse at Ruiz. Caldera dimensions, deposits, and alteration mineral volumes are consistent with parameters observed at other volcanoes. 相似文献
16.
Polar lobes of eggs of the mud snail Ilyanassa obsoleta, detached at the "trefoil" stage of the first cleavage, are capable of incorporating labeled amino acid into protein. The rate of incorporation per unit volume is about half that of the whole egg. The ability to incorporate at a similar rate persists for at least 24 hours after isolation. The sum of the incorporation of isolated lobes and lobeless fragments approximates that of the whole egg. The results extend to this material (the anucleate polar lobe) evidence for long-lived messenger RNA. They suggest also that the demonstrated morphogenetic influence of the lobe, which is exerted primarily during cleavage, may be correlated with its ability to synthesize protein. 相似文献
17.
作为农业现代化建设和践行乡村振兴战略的中坚力量,农业高校的研究生思政教育工作更具紧迫性和艰巨性。文章在考察我国“立德树人”思政教育理念历史发展脉络的基础上,阐述了新时代农业高校研究生培养的时代使命,并以中国农业大学为案例,分析了当前农业高校新时代思政育人的基本路径在于:实现“隐性教育与显性教育”的有机结合、“请进来与走下去”的有机结合、“线上和线下并行”的有机结合,以此使立德树人潜移默化地融进研究生教育的各个环节,提升学生的素养,培养爱国爱农的情怀。文章认为,这将成为助力人才培养质量的优化升级和农业高等教育事业发展的新常态。 相似文献
18.
蒙顶山名茶游离氨基酸总量及组分的测定分析 总被引：1，自引：0，他引：1
【目的】探讨蒙顶山名茶"鲜香"与"鲜味"特征的产生机制,提供蒙顶山名茶品质评价的理论依据。【方法】试验通过全量法和杯茶法制备茶汤试液,分别采用水合茚三酮比色法和L-8900型全自动氨基酸分析仪测定了4种蒙顶山名茶中的游离氨基酸总量和组分含量。【结果】4种蒙顶山名茶的游离氨基酸总量较高,分别为蒙顶黄芽(5.28±0.57)%,蒙顶石花(5.59±0.51)%,蒙顶甘露(4.65±0.82)%,蒙顶毛峰(5.34±0.96)%;在杯茶法1次冲泡5min条件下,蒙顶山名茶游离氨基酸的浸出率与浸出浓度亦较高,浸出率为55.24%～76.14%,浸出浓度为58.73～78.24mg/100mL;蒙顶山名茶游离氨基酸组分含量的特征为茶氨酸、天冬氨酸、谷氨酸、精氨酸和丝氨酸的含量在1 000mg/kg以上,色氨酸、半光氨酸、组氨酸和络氨酸组分含量为100～500mg/kg。【结论】蒙顶山名茶高氨基酸总量、丰富的组分含量及独特配比是其茶香"鲜香高长"、茶味"鲜浓爽口"的重要物质基础。 相似文献
19.
20.
Genetic Diversity Evaluation of Maize Recurrent Selection Population with RAPD Marker 总被引：5，自引：1，他引：4
Liu Xunjia Zheng Yonglian Liu Jilin 《中国农业科学》2000,(1):17-23
The genetic diversity of maize populations Wuxi (W) from Southwest China, BSSS9(B) from America, Mohuangjiu (M) from Mexico, WBMC0 synthesized by W, B, M as main parents,WBMC1 one cycle selected from WBMC0 were evaluated by RAPD molecular marker. The results showed that :(1) Totally 89 fragments (loci) were amplified by 15 10-mar random primers, the proportion of polymorphic loci were W 76. 4%, B 75. 3%, M 79. 8%,WBMC0 85. 4% and WBMC1 92. 1% respectively; (2) The mean gene heterozygosity based on 89 loci was W 0. 285, B 0. 252, M 0. 296, WBMC0 0. 327 and WBMC1 0. 346; (3) The mean genetic distance based on 89 loci were W 0. 2533, B 0. 2246, M 0. 2481, WBMC0 0. 3006 and WBMC1 0. 3119; (4) The genotypic mean numbers amplified by 15 primers were W 9.1, B 7.8, M 8.5, WBMC0 10. 1 and WBMC1 10. All indexes indicated that the synthesized maize population were more polymorphic than the parent populations in DNA level. One cycle selection did not reduce the variation. The new conception of "genotypic diversity" (the number of genotypes in a population) was provided to describe the genetic diversity for any population being equilibrium or unequilibrium in genetics. The principle and technical system were discussed for evaluating genetic variation of recurrent selection population using RAPD molecular marker. 相似文献