全文获取类型
收费全文 | 8742篇 |
免费 | 801篇 |
国内免费 | 3篇 |
出版年
2022年 | 78篇 |
2021年 | 134篇 |
2020年 | 88篇 |
2019年 | 106篇 |
2018年 | 132篇 |
2017年 | 124篇 |
2016年 | 193篇 |
2015年 | 280篇 |
2014年 | 342篇 |
2013年 | 456篇 |
2012年 | 457篇 |
2011年 | 512篇 |
2010年 | 360篇 |
2009年 | 324篇 |
2008年 | 458篇 |
2007年 | 430篇 |
2006年 | 367篇 |
2005年 | 369篇 |
2004年 | 355篇 |
2003年 | 329篇 |
2002年 | 342篇 |
2001年 | 159篇 |
2000年 | 201篇 |
1999年 | 152篇 |
1998年 | 113篇 |
1997年 | 92篇 |
1996年 | 81篇 |
1995年 | 81篇 |
1994年 | 77篇 |
1992年 | 122篇 |
1991年 | 98篇 |
1990年 | 115篇 |
1989年 | 91篇 |
1988年 | 128篇 |
1987年 | 91篇 |
1986年 | 83篇 |
1985年 | 114篇 |
1984年 | 82篇 |
1983年 | 81篇 |
1982年 | 94篇 |
1981年 | 58篇 |
1980年 | 61篇 |
1979年 | 67篇 |
1978年 | 68篇 |
1977年 | 76篇 |
1976年 | 71篇 |
1975年 | 68篇 |
1974年 | 73篇 |
1973年 | 56篇 |
1972年 | 62篇 |
排序方式: 共有9546条查询结果,搜索用时 31 毫秒
31.
Oligonucleotide probes for the detection of TEM-1 and TEM-2 beta-lactamase genes and their transposons 总被引:5,自引:0,他引:5
We describe the use of molecular probes to detect the TEM-type beta-lactamase genes. As a general probe, we prepared a 656 base pair restriction fragment, entirely within the TEM structural gene. This probe was specific for the TEM family, hybridizing only with TEM-1 and TEM-2. The TEM-1 and TEM-2 beta-lactamases differ by only one amino acid. We synthesized two oligonucleotides whose central bases correspond to this difference. The use of these oligonucleotides enables us to discriminate between TEM-1 and TEM-2 genes. Using oligonucleotides homologous to parts of Tn3, we also monitored the presence of TnA-like transposons in bacteria harboring different beta-lactamase genes. Only the TEM-1 and TEM-2 genes were found to be on transposons with terminal sequences identical to those of Tn3. All hybridization experiments were performed with both dot-blot and colony-hybridization techniques, and the suitability of these two methods for epidemiological studies is compared. 相似文献
32.
33.
Summary Two annual species of Bromus, an invader (B. hordeaceus, ex B. mollis) and a non-invader (B. intermedius), were grown for 28 days in growth chambers, at 5 and 100 M NO
3
-
in flowing nutrient solution. No differences between the two species were observed at either NO
3
-
level, in terms of relative growth rate (RGR) or its components, dry matter partitioning, specific NO
3
-
absorption rate, nitrogen concentration, and other characteristics of NO
3
-
uptake and photosynthesis. The effects of decreasing NO
3
-
concentration in the solution were mainly to decrease the NO
3
-
concentration in the plants through decreased absorption rate, and to decrease the leaf area ratio through increased specific leaf mass and decreased leaf mass ratio. Organic nitrogen concentration varied little between the two treatments, which may be the reason why photosynthetic rates were not altered. Consequently, RGR was only slightly decreased in the 5-M treatment compared to the 100-M treatment. This is in contrast with other species, where growth is reduced at much higher NO
3
-
concentrations. These discrepancies may be related to differences in RGR, since a log-linear relationship was found between RGR and the NO
3
-
concentration at which growth is first reduced. In addition, a strong linear relationship was found between the RGR of these species and their maximum absorption rate for nitrate, suggesting that the growth of species with low maximum RGR may be partly regulated by nutrient uptake. 相似文献
34.
On Day 3 of the estrous cycle (estrus = Day 0), dairy heifers were given either 10 mg i.m. FSH-P (FSH-P primed; n = 9) or a saline vehicle (saline primed; n = 9). On Day 10, all heifers were superovulated with FSH-P (total = 27.7 mg i.m.) in declining doses over 5 d. Heifers were inseminated artificially at estrus. From Day 2 until estrus, the number and size of follicles >2 mm were monitored daily by ultrasonography. The mean (+/- SEM) number of corpora lutea (CL) (6.2 +/- 1.5 vs 10.7 +/- 0.9; P<0.05) and the mean number of recovered embryos and unfertilized ova (3.6 +/- 1.7 vs 8.4 +/- 2.2; P<0.05) were lower in FSH-P-primed than in saline-primed heifers. Prior to initiation of superovulation, follicles >10 mm appeared on Days 6 to 7 in saline-primed heifers but only on Days 8 to 10 in FSH-P-primed heifers (P<0.05). Also, until Day 10, the mean number of follicles 4 to 6 mm and 7 to 10 mm was higher (P<0.05) in FSH-P-primed than in saline-primed heifers. After initiation of the superovulatory treatment (Day 10 to estrus), saline-primed heifers had a greater and faster increase in the mean number of follicles >10 mm (P<0.02) than FSH-P-primed heifers did. Depletion in the number of follicles 2 to 3 mm (P<0.001) between Day 10 and estrus and in the number of follicles 4 to 6 mm (P<0.05) between Day 12 and estrus occurred in both groups of heifers. Decreased superovulatory response and embryo recovery in FSH-P-primed heifers may have been due to the presence of large follicles (>10 mm) prior to the initiation of the superovulatory treatment which reduced the ability of small follicles to grow into larger size classes during superovulatory treatment. 相似文献
35.
The evolution of the mouse immunoglobulin heavy chain variable region (Igh-V) locus was investigated by the comprehensive analysis of variable region (Vh) gene family content and restriction fragment polymorphism in the genusMus. The examination of naturalMus domesticus populations suggests an important role for recombination in the generation of the considerable restriction fragment polymorphism
found at theIgh-V locus. Although the sizes of individualVh gene families vary widely both within and between differentMus species, evolutionary trends ofVh gene family copy number are revealed by the analysis of homologues of mouseVh gene families inRattus andPeromyscus. Processes of duplication, deletion, and sequence divergence all contribute to the evolution ofVh gene copy number. CertainVh gene families have expanded or contracted differently in the various muroid lineages examined. Collectively, these findings
suggest that the evolution of individualVh family size is not driven by strong selective pressure but is relatively neutral, and that gene flow, rather than selection,
serves to maintain the high level of restriction fragment polymorphism seen inM. domesticus. 相似文献
36.
Denis Roy Marco Lagimonire Marie-Jose Hardy Jean-Franois Bourassa Pierre Mourot 《Journal of biotechnology》1989,10(3-4):227-240
Factors affecting the viability and infectivity of an ectomycorrhizal fungus during moderate concentration by cross-flow filtration were determined. Mycelial suspensions were concentrated with three commercial membrane filters (Prostak Millipore Co., M14 Tech-Sep Co. and Ceraflo Norton Co.) under aseptic conditions. Medium components may reduce the filtration rate due to their low solubility. An antifoam agent did not reduce the average flux rates as much as did the malt extract. Clear unobstructed channels (I.D. 6mm) of the tubular modules (Tech-Sep) gave the best results both in terms of performance (filtration rate) and cell viability. Shear stresses caused by pumping and flow through narrow retentate channels were probably responsible for lowering viability and infectivity. There was no linear relationship between permeate fluxes and cell concentration. There is an optimum pore size both in terms of performance (filtration rate) and cell viability. Physical blockage of large pores by hyphae could explain lower permeate flux rates than those obtained with lower pore sizes membranes. 相似文献
37.
38.
N Jamin P Roy F Fridlansky M Delepierre E Milgrom B P Roques J P Mornon 《European journal of biochemistry》1989,183(1):219-226
Two-dimensional NMR methods have been used to assign aromatic and methyl group resonances in the 1H-NMR spectrum of oxidized uteroglobin. Assignments to specific amino acids are based on X-ray-determined structures of two crystal forms (C222(1) and P2(1] and on an energy-minimized X-ray structure of the C222(1) form of uteroglobin. These preliminary assignments are sufficient to probe the interaction of oxidized uteroglobin with progesterone in solution. The protein global structure is unmodified but some direct or indirect conformational changes are induced in the H1H4(H1'H4') pockets and close to Phe28 by progesterone. 相似文献
39.
The site of substrate and fructose 2,6-bisphosphate binding to rabbit liver fructose-1,6-bisphosphatase 总被引:1,自引:0,他引:1
The binding site(s) in rabbit liver fructose-1,6-bisphosphatase for the active site binding ligand, fructose 6-phosphate, and the inhibitor, fructose 2,6-bisphosphate, have been investigated by using nuclear magnetic resonance spectroscopy. The distance from a nitroxide spin label to the bound ligands and the distance from the structural metal site to the bound ligands are about the same within experimental error. These data indicate that the two ligands probably bind at the active site in the rabbit liver enzyme. 相似文献
40.
The cytoplasmic androgen-binding (CAB) protein of the male rat liver has been implicated to play a role in the androgen-dependent regulation of alpha 2u-globulin synthesis. The liver of the adult male rat contains about 50 fmol of specific high-affinity androgen-binding activity per milligram of total cytosolic protein. Photoaffinity labeling with [3H]R-1881 followed by SDS-polyacrylamide gel electrophoresis and autoradiography shows that the CAB is a 31-kilodalton protein. By means of DEAE-cellulose chromatography and preparative SDS-polyacrylamide gel electrophoresis, we have purified the CAB protein to electrophoretic homogeneity and have raised polyclonal rabbit antiserum that is monospecific to this protein. In the sucrose density gradient, the antiserum reacted with the androgen-binding component of the male liver cytosol prelabeled with tritiated dihydrotestosterone. Western blot analysis of the liver cytosol showed that the antiserum recognizes only the 31-kDa androgen-binding component. Such immunoblotting also showed that unlike the young adult, the androgen-insensitive states during prepuberty and senescence are associated with a marked reduction in the hepatic concentration of the immunoreactive CAB protein. No immuno-chemical cross-reactivity between CAB and another androgen-binding component of Mr 29K (which is associated with androgen insensitivity during prepuberty and senescence) was observed. The latter finding favors the possibility that 31- and 29-kDa androgen-binding components may have distinct sequence structure. 相似文献