全文获取类型
收费全文 | 6623篇 |
免费 | 783篇 |
国内免费 | 1篇 |
专业分类
7407篇 |
出版年
2021年 | 75篇 |
2019年 | 82篇 |
2018年 | 68篇 |
2017年 | 58篇 |
2016年 | 103篇 |
2015年 | 157篇 |
2014年 | 182篇 |
2013年 | 228篇 |
2012年 | 334篇 |
2011年 | 292篇 |
2010年 | 164篇 |
2009年 | 169篇 |
2008年 | 225篇 |
2007年 | 236篇 |
2006年 | 218篇 |
2005年 | 230篇 |
2004年 | 211篇 |
2003年 | 227篇 |
2002年 | 225篇 |
2001年 | 193篇 |
2000年 | 212篇 |
1999年 | 152篇 |
1998年 | 88篇 |
1997年 | 72篇 |
1996年 | 81篇 |
1995年 | 80篇 |
1994年 | 67篇 |
1993年 | 76篇 |
1992年 | 152篇 |
1991年 | 136篇 |
1990年 | 137篇 |
1989年 | 167篇 |
1988年 | 130篇 |
1987年 | 125篇 |
1986年 | 100篇 |
1985年 | 118篇 |
1984年 | 117篇 |
1983年 | 91篇 |
1982年 | 79篇 |
1981年 | 76篇 |
1980年 | 77篇 |
1979年 | 109篇 |
1978年 | 88篇 |
1977年 | 63篇 |
1976年 | 71篇 |
1975年 | 70篇 |
1974年 | 81篇 |
1973年 | 79篇 |
1972年 | 56篇 |
1971年 | 53篇 |
排序方式: 共有7407条查询结果,搜索用时 15 毫秒
31.
P. N. Hoffman E. M. Cooke D. P. Larkin L. J. Southgate R. T. Mayon-White J. V. Pether A. E. Wright D. Keenlyside 《BMJ (Clinical research ed.)》1988,297(6640):34-36
Twenty general practices in four areas in Britain were surveyed to establish their needs for and practices of sterilising and disinfecting equipment. Of the 327 items of equipment and instruments examined in the survey, 190 were satisfactorily decontaminated, 100 were treated in a way judged to result in doubtful decontamination, and in 37 cases treatment was considered unsatisfactory. Decontamination apparatuses (autoclaves, hot air ovens, and hot water disinfectors) were generally in good working order, but the use of chemical disinfectants was often inappropriate. Recommendations were made on appropriate methods of decontamination for various items in common use in general practice. By virtue of the large numbers of patients treated by general practitioners there is a substantial possibility of transmitting infection; having appropriate methods for decontaminating instruments and equipment is therefore imperative. 相似文献
32.
A. Gallais A. J. Wright 《TAG. Theoretical and applied genetics. Theoretische und angewandte Genetik》1980,57(2):81-87
Summary Genetic effects for varietal value are defined at the level of the population of k-parent synthetic varieties. A simple expression for the total variance among synthetics arises directly from these definitions. A general expression for the covariance among related synthetics is given. Genetic effects are also defined in a completely general way so as to allow for any system of testing and used to derive an expression for the genetic advance in recurrent selection for varietal value. Covariances between relatives evaluated in the system of testing and in varietal combination are introduced, allowing a direct expression of the genetic advance in varietal development when parents are selected either individually or in groups. Some general implications for plant breeding are outlined.Dedicated to Professor F.W. Schnell on the occasion of his 65th birthday 相似文献
33.
34.
The altered properties of a concanavalin A-resistant Chinese hamster ovary cell line with obvious temperature-sensitive growth properties is described. The variant cell line, CR-7, was shown to have a higher efficiency of colony formation than the parental wild-type population after treatment with various concentrations of concanavalin A (ConA). The variant cells had the properties of a temperature-sensitive cell line as judged by growth studies performed on solid surfaces or in suspension culture at the permissive (34 °C) and non-permissive (39 °C) temperatures; by colony efficiency determinations performed at 34 °C and 39 °C; and by the altered ability to incorporate DNA, RNA, and protein precursors into acid-precipitable material at the non-permissive temperature. Evidence for changes in the membrane properties of CR-7 cells included: a reduced agglutinability in the presence of ConA, an altered cellular morphology on solid surfaces, an enhanced sensitivity to the toxic effects of membrane-active agents, altered and temperature-sensitive adhesiveness properties, and a reduced ability to bind labelled ConA. 相似文献
35.
J.Keith Wright 《Biochemical and biophysical research communications》1978,83(4):1284-1290
Sheep erythrocytes sensitized with intact antibody or reduced and alkylated antibody were lysed by guinea-pig serum indicating that reduced and alkylated antibody bound and activated complement. Reduction of antibody caused erythrocyte lysis to exhibit pseudo-first-order kinetics, while the lysis kinetics of erythrocytes sensitized with intact antibody was sigmoidal. Analysis of erythrocyte lysis by complement according to the von Krogh equation showed that reduction of antibody diminished the von Krogh exponent from 2.8 to 1.3, while the value of remained unchanged at 0.17 (complement dilution). These observations suggested that the sole effect of the reduction of antibody inter-heavy-chain and heavy-light chain disulfide bonds was to diminish the cooperativity of antibody-complement interaction. 相似文献
36.
R. C. Wright 《BMJ (Clinical research ed.)》1967,2(5544):108-109
37.
Two-dimensional 1H-NMR methods have been used to assign heme and amino acid proton resonances in both isomeric states of the carbon monoxide complexes of two Glycera dibranchiata monomeric hemoglobins, HbA and HbB. For each hemoglobin, there are small differences in heme pocket structure in the two isomeric forms. The largest structural perturbations associated with heme isomerism involve residues close to pyrrole rings I and II. The positions relative to the heme of phenylalanine CD1 and the proximal histidine ligand are almost unaffected by heme isomerism. These residues probably play a key role in determining the location of the heme within the heme pocket. 相似文献
38.
The present study was undertaken to evaluate the possible contribution of other cytokines to the lytic activity of NKCF-containing supernatants. We compared some of the functional properties of human NKCF and purified recombinant human rLT and rTNF. It was found that the target cell specificity of rLT was quite different from NKCF in that rLT was neither species specific nor NK specific. Furthermore, antibodies against rLT did not affect the lytic activity of NKCF. These results demonstrate that LT does not significantly contribute to the lytic activity mediated by NKCF. The target specificity of rTNF was found to be related to that of NKCF with the exception of one NK-resistant cell line that was lysed by rTNF in a 20-hr 51Cr-release assay. However, rTNF was not toxic to any of the target cells tested as assessed by trypan blue exclusion in a 20-hr assay unless the targets were labeled with 51Cr. In contrast, NKCF did kill target cells as detected by trypan blue exclusion that were not labeled with 51Cr. Further analysis of this mechanistic difference in the lytic activity of rTNF and NKCF revealed that rTNF in combination with either cycloheximide or mitomycin C but not IFN-gamma could lyse unlabeled U937 target cells. In addition, pretreatment of U937 target cells with nonradioactive Na2CrO4 at concentrations equivalent to that used to 51Cr-labeled cells resulted in their susceptibility to lysis by rTNF as assessed by trypan blue exclusion. These findings suggest that lysis of several susceptible target cells in 20 hr by rTNF requires the presence of additional agents that may be sublethally toxic and/or inhibitory to macromolecular synthesis. Antibody inhibition studies revealed that anti-TNF mediated from partial to complete inhibition of lysis of U937 by unfractionated supernatants containing NKCF. However, fractionation of such supernatants on chromatofocusing columns yielded two distinct peaks of activity eluting in the pH range of 5 to 6 and 7 to 8. Anti-TNF could inhibit the acidic form of NKCF but not the neutral form. It is concluded that NKCF activity is mediated in part by TNF or an antigenically related molecule as well as some other distinct factor(s). The lack of consistent inhibition of NK CMC by anti-TNF suggests that TNF alone is not sufficient to mediate NK activity, or else it is inaccessible to the added antibody. 相似文献
39.
Summary Brush border membrane vesicles were prepared from mussel gills using differential and sucrose density gradient centrifugation. These vesicles contained both the maximal Na+-dependent alanine transport activity found in the gradient and the maximal activities of -glutamyl transpeptidase and alkaline phosphatase. Electron micrographs showed closed vesicles of approximately 0.1–0.5 m diameter. Transport experiments using these vesicles demonstrated a transient 18-fold overshoot in intravesicular alanine concentration in the presence of an inwardly directed Na+ gradient, but not under Na+ equilibrium conditions. A reduced overshoot (10-fold) was seen with an inwardly directed K+ gradient. Further studies revealed a broad cation selectivity, with preference for Na+, which was characteristic of alanine transport but not glucose transport in these membranes. The apparent amino acid specificity of the uptake pathway(s) was similar to that of intact gills and supported the idea of at least four separate pathways for amino acid transport in mussel gill brush border membranes. The apparent Michaelis constant for alanine uptake was approximately 7m, consistent with values forK
t
determined with intact tissue. 相似文献
40.
Refinement of the crystal structure of wheat germ agglutinin isolectin 2 at 1.8 A resolution 总被引:5,自引:0,他引:5
C S Wright 《Journal of molecular biology》1987,194(3):501-529
The crystal structure of wheat germ agglutinin isolectin 2 has been refined by the restrained least-squares method of Hendrickson & Konnert (1980). The asymmetric unit of the C2 crystals contains two chemically identical promoters related by a non-crystallographic 2-fold screw operation. A total of 2290 protein atoms and 186 ordered water sites refined to a final R-factor of 0.179 and an average B-value of 21.6 A2, using 54% (15,601) of the total possible number of reflections in the resolution range 8 to 1.8 A with Fo greater than 3 sigma (Fo). The final model conforms to stereochemically correct bond distances and angles with root-mean-square (r.m.s.) values of 0.018 A and 3.3 degrees, respectively. Accuracy of this model is estimated to be 0.20 A on the basis of a Luzzati plot. Main-chain atomic positions in the two independent promoters, designated I and II, agree with an r.m.s. deviation of 0.30 A (0.58 A for all atoms), indicating identical backbone conformation. The largest discrepancies are seen at flexible surface residues. One error was detected in the amino acid sequence at position 41 (Ser), which refined satisfactorily as a Trp. Loss of electron density for residue A171 during the course of refinement suggests either disorder or absence of this C-terminal residue. The conformation of the polypeptide chain, which is folded into four homologous 43-residue domains (A, B, C and D), was analyzed in terms of dihedral angles, backbone hydrogen bond lengths and CA-atom positions. The four domains were found to be very similar according to all these criteria and superposition of their CA-atoms yielded r.m.s. distances ranging from 0.36 to 0.72 A for the six possible comparisons [corrected]. Large deviations (greater than 1.0 A) are only seen in the five-residue segments that link adjacent domains and at the N and C termini. Refinement has also allowed critical examination of each of the two unique sugar binding sites, referred to as "primary" and "secondary" sites, in different lattice environments. While the essential tyrosyl side-chain in each of these sites (Y73, Y159) assumes precise orientation for optimum hydrophobic contact with the N-acetyl methyl group of the sugar ligand, side-chains involved in hydrogen bonds (S62, E115; and S148, D29) were found to be relatively flexible and able to adapt their conformation to changes in environment. Ordered water structure present in these binding sites is not completely analogous in the different environments.(ABSTRACT TRUNCATED AT 400 WORDS) 相似文献