全文获取类型
收费全文 | 2637篇 |
免费 | 287篇 |
专业分类
2924篇 |
出版年
2021年 | 39篇 |
2020年 | 21篇 |
2019年 | 20篇 |
2018年 | 39篇 |
2017年 | 35篇 |
2016年 | 51篇 |
2015年 | 83篇 |
2014年 | 98篇 |
2013年 | 104篇 |
2012年 | 136篇 |
2011年 | 141篇 |
2010年 | 84篇 |
2009年 | 69篇 |
2008年 | 85篇 |
2007年 | 114篇 |
2006年 | 100篇 |
2005年 | 92篇 |
2004年 | 99篇 |
2003年 | 109篇 |
2002年 | 120篇 |
2001年 | 88篇 |
2000年 | 77篇 |
1999年 | 66篇 |
1998年 | 44篇 |
1997年 | 42篇 |
1996年 | 52篇 |
1995年 | 41篇 |
1994年 | 32篇 |
1993年 | 53篇 |
1992年 | 61篇 |
1991年 | 67篇 |
1990年 | 52篇 |
1989年 | 53篇 |
1988年 | 34篇 |
1987年 | 37篇 |
1986年 | 44篇 |
1985年 | 42篇 |
1984年 | 27篇 |
1983年 | 21篇 |
1982年 | 31篇 |
1981年 | 26篇 |
1979年 | 24篇 |
1978年 | 21篇 |
1977年 | 25篇 |
1976年 | 17篇 |
1975年 | 15篇 |
1974年 | 22篇 |
1973年 | 16篇 |
1972年 | 15篇 |
1969年 | 16篇 |
排序方式: 共有2924条查询结果,搜索用时 15 毫秒
81.
Joseph A. Ezzo Clark Spencer Larsen James H. Burton 《American journal of physical anthropology》1995,98(4):471-481
Multielement analysis was performed on bone samples extracted from the femora of 39 adults from three mortuary sites (Johns Mound, Santa Catalina de Guale, and Santa Catalina de Guale de Santa Meria) and time periods (late preagricultural, early contact, and late contact) in the Georgia Bight. This study was used to investigate whether elemental analysis would support or contradict other lines of data regarding diets and dietary change previously generated for the region. The data are in agreement with an earlier interpretation, based on stable isotopes, that dietary maize increases through time but fails to support the idea that marine resources decreased in importance. Rather, it appears that the wild plant food component of the diets decreases as maize increases in importance; throughout the sequence, marine resources comprise a significant portion of the diets. © 1995 Wiley-Liss, Inc. 相似文献
82.
T M Larsen R C Miller G B Spiegelman S Hayashi G M Tener D A Sinclair T A Grigliatti 《Molecular & general genetics : MGG》1982,185(3):390-396
Summary Transfer RNA was extracted from 50–300 mg of adult flies and specifically labeled in vitro. The level of individual isoacceptors was quantitated by efficient annealing to Drosphila tRNA genes carried on recombinant DNA plasmids immobilized on nitrocellulose filters. The level of tRNA
3b
Val
in the tRNA isolated from flies deficient in the major tRNA
3b
Val
loci has been examined. The results show that deletion of the major tRNA
3b
Val
loci resulted in a reduction of approximately 50% in the level of tRNA
3b
Val
but did not produce the Minute phenotype; furthermore the effects of deficiencies at two loci were approximately additive. 相似文献
83.
Summary In a collection of Nigerian serum samples typed for alleles of factor B of the alternative complement pathway, a very high frequency of BfF was found (0.69). In addition, a new variant was found in two samples. This variant (F1.29) moved faster than BfF1. It was hemolytically active.Supported in part by The Medical Research Council of Canada 相似文献
84.
The fucose-containing, sulfated polysaccharides from Ascophyllum nodosum and Fucus vesiculosus were isolated by extraction with water adjusted to pH 2. Pure fractions were carefully separated by fractional precipitation with ethanol from aqueous solutions containing magnesium or calcium chloride. Progress in the fractionation efforts and purity of the fractions isolated were established by free-boundary and cellulose acetate clectrophoresis. Ascophyllan, two “complexes”, and a galactofucan were isolated from A. nodosum. An ascophyllan-like fraction, and a “complex” were isolated from F. vesiculosus. Mild, acid hydrolysis (0.02m hydrochloric acid, 1 h, 80°) converted each of the “complexes” into an electrophoretically faster-moving and a slower-moving component. The “complex” from F. vesiculosus comprised a greater proportion of the extract than did the two “complexes” from A. nodosum. In addition, the Fucus “complex” was richer in fucose*. However, the data suggest that neither species contains a pure fucan sulfate in the native state. 相似文献
85.
86.
I F Larsen 《BMJ (Clinical research ed.)》1977,2(6098):1356-1357
87.
Jørgen Næstoft Niels-Erik Larsen 《Journal of chromatography. B, Analytical technologies in the biomedical and life sciences》1977,143(2):161-169
A method for the determination of ethotoin and its p-hydroxylated and dealkylated metabolites in urine has been developed. Ethotoin and the metabolites were extracted from acidified urine with ethyl acetate and silylated before injection into a combined gas chromatograph—mass spectrometer. Four partly identified metabolites were recorded, but their exact quantitation was not possible as pure reference substances were not available.The limit of sensitivity was far below the amounts of ethotoin and of its metabolites found in urine from patients treated with therapeutic dozes of ethotoin. 相似文献
88.
J B Lowe J F Kukowska-Latallo R P Nair R D Larsen R M Marks B A Macher R J Kelly L K Ernst 《The Journal of biological chemistry》1991,266(26):17467-17477
We have used the human Lewis blood group fucosyltransferase cDNA and cross-hybridization procedures to isolate a human gene that encodes a distinct fucosyltransferase. Its DNA sequence predicts a type II transmembrane protein whose sequence is identical to 133 of 231 amino acids at corresponding positions within the catalytic domain of the Lewis fucosyltransferase. When expressed by transfection in cultured cell lines, this gene determines expression of a fucosyltransferase capable of efficiently utilizing N-acetyllactosamine to form the Lewis x determinant (Gal beta 1----4[Fuc alpha 1----3]GlcNAc). By contrast, biochemical and flow cytometry analyses suggest that the enzyme cannot efficiently utilize the type II acceptor NeuNAc alpha 2----3Gal beta 1----4GlcNAc, to form the sialyl Lewis x determinant. In Chinese hamster ovary cells, however, the enzyme can determine expression of the alpha 2----3-sialylated, alpha 1----3-fucosylated structure known as VIM-2, a putative oligosaccharide ligand for ELAM-1. Cell adhesion assays using VIM-2-positive, sialyl Lewis x-negative transfected Chinese hamster ovary cells indicate that surface expression of the VIM-2 determinant is not sufficient to confer ELAM-1-dependent adhesive properties upon the cells. These results demonstrate that substantial structural similarities can exist between mammalian glycosyltransferases with closely related enzymatic properties, thus facilitating isolation of their cognate genes by cross-hybridization methods. The results further suggest that cell surface expression of the VIM-2 determinant is not necessarily sufficient to mediate ELAM-1-dependent cell adhesion. 相似文献
89.
Identification and characterization of a new member of the prolactin family, placental lactogen-I variant 总被引:1,自引:0,他引:1
S Deb T N Faria K F Roby D Larsen S C Kwok F Talamantes M J Soares 《The Journal of biological chemistry》1991,266(3):1605-1610
This report describes the identification and characterization of a new member of the placental prolactin (PRL) family, termed placental lactogen-I variant (PL-Iv). PL-Iv was isolated from medium conditioned by late gestation placental explants. Rat PL-Iv was found to be closely related to rat PL-I. Amino-terminal sequence analysis indicated that PL-Iv shared approximately 88% sequence identity with the amino terminus of PL-I. PL-Iv proteins cross-reacted with antiserum to recombinant mouse PL-I and PL-Iv mRNA hybridized with a PL-I cDNA. Multiple PL-I and PL-Iv species were present in placental cytosol. Despite the structural similarities between PL-I and PL-Iv, distinct differences were also evident. Antibodies generated to the amino-terminal 19 amino acids of PL-Iv specifically recognized PL-Iv, while failing to recognize PL-I. Secreted PL-Iv had an affinity for concanavalin A, whereas secreted PL-I lacked affinity for the lectin. PL-I was predominantly secreted as a 36-40-kDa species and PL-Iv was predominantly secreted as a 33-kDa species. Furthermore, PL-I and PL-Iv were synthesized at different times during gestation and by different cell types. PL-I was synthesized by trophoblast giant cells during the first half of gestation, while PL-Iv was predominantly synthesized by spongiotrophoblast cells during the later stages of gestation. PL-Iv was shown to stimulate the proliferation of rat Nb2 lymphoma cells, an in vitro measure of lactogenic activity. In summary, PL-Iv shares structural similarities with PL-I; however, it shows other structural differences in addition to unique cell- and temporal-specific patterns of expression in the rat chorioallantoic placenta. 相似文献
90.
Production and partial characterization of monoclonal antibodies specific for the gamonts of Eimeria tenella. 总被引:1,自引:0,他引:1
Thirteen hybridomas secreting monoclonal antibodies (Mabs) specific for the sexual stages (gamonts) of Eimeria tenella were produced by fusing spleen cells of gamont-immunized RBF/Dn mice with FOX-NY myeloma cells. A Mab subisotype profile revealed 1 IgG2a and 12 IgG1. All Mabs demonstrated a similar binding pattern when incubated with parasitic gamonts as determined by the indirect fluorescent antibody test. Ascitic fluid containing Mab (GD9 (IgG1) was produced and used to immunize chicks passively per os. There was a 34% decrease (P less than 0.05) in oocyst output from immunized chicks when compared to control chicks. Passively immunized chicks also had reduced cecal lesion scores when compared to control chicks. These results suggest that Mab GD9 partially inhibited the fertilization process of E. tenella. 相似文献