全文获取类型
收费全文 | 29245篇 |
免费 | 3710篇 |
国内免费 | 29篇 |
学科分类
生物科学 | 32984篇 |
出版年
2016年 | 259篇 |
2015年 | 319篇 |
2014年 | 408篇 |
2013年 | 680篇 |
2012年 | 646篇 |
2011年 | 699篇 |
2010年 | 492篇 |
2009年 | 411篇 |
2008年 | 606篇 |
2007年 | 658篇 |
2006年 | 630篇 |
2005年 | 627篇 |
2004年 | 630篇 |
2003年 | 648篇 |
2002年 | 657篇 |
2001年 | 1859篇 |
2000年 | 1833篇 |
1999年 | 1436篇 |
1998年 | 477篇 |
1997年 | 477篇 |
1996年 | 482篇 |
1995年 | 424篇 |
1994年 | 428篇 |
1993年 | 422篇 |
1992年 | 1071篇 |
1991年 | 1086篇 |
1990年 | 1065篇 |
1989年 | 1029篇 |
1988年 | 962篇 |
1987年 | 901篇 |
1986年 | 770篇 |
1985年 | 748篇 |
1984年 | 588篇 |
1983年 | 508篇 |
1982年 | 369篇 |
1981年 | 343篇 |
1980年 | 331篇 |
1979年 | 561篇 |
1978年 | 449篇 |
1977年 | 412篇 |
1976年 | 340篇 |
1975年 | 456篇 |
1974年 | 470篇 |
1973年 | 416篇 |
1972年 | 394篇 |
1971年 | 310篇 |
1970年 | 258篇 |
1969年 | 241篇 |
1968年 | 225篇 |
1967年 | 205篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Limited proteolysis of glutamine synthetase from Escherichia coli has been studied under nondenaturing conditions (pH 7.6, 20 degrees C). Trypsin cleaves the polypeptide chain of glutamine synthetase into two principal fragments, Mr = about 32,000 and 18,000. The covalently bound AMP group is attached to the larger fragment and its presence does not affect cleavage. Although the cleaved polypeptide chain does not dissociate under nondenaturing conditions, catalytic activity is lost. Chymotrypsin and Staphylococcus aureus protease produce similar cleavages in glutamine synthetase. The substrate L-glutamate retards tryptic as well as chymotryptic digestion. Tryptic digestion is also retarded by some of the feedback inhibitors of glutamine synthetase including CTP, L-alanine, L-serine, L-histidine, and glucosamine 6-phosphate. An implication of these findings is that there is a region of the glutamine synthetase polypeptide chain that is particularly susceptible to proteolysis. Either the glutamate and inhibitor sites are formed partly by this suceptible peptide or the binding of glutamate and some inhibitors induces conformational changes within the E. coli glutamine synthetase molecule in the region of the susceptible peptide. 相似文献
992.
M E Frankel R B Effros P C Doherty W U Gerhard 《Journal of immunology (Baltimore, Md. : 1950)》1979,123(5):2438-2440
A particular monoclonal antibody that binds to the influenza virus HA molecule inhibits HA-specific thymus-derived lymphocytes mediating cytotoxicity in the context of H-2Dd but not of H-2Kd. Another monoclonal antibody blocks both sets of HA-specific effector T cells. This observation, together with related findings from other laboratories, is considered to support the idea that T cell recognition is directed against some association of viral and H-2 glycoproteins, as proposed in the original formulation of the "altered self" concept. 相似文献
993.
We describe a new method for quantitatively assaying the omega subunit of Escherichia coli RNA polymerase. The assay is based on the ability of RNA polymerase holoenzyme to catalyze the continuous synthesis of the dinucleotide pApU on a poly[d(A-T)] . poly[d(A-T)] template when supplied with AMP and UTP as substrates. Core enzyme, lacking omega subunit, catalyzed this reaction at a rate less than 1% that of holoenzyme. The omega subunit was not released from the enzyme/DNA complex during dinucleotide synthesis. Using this assay, a titration of a fixed concentration of core enzyme was observed with increasing concentrations of added omega subunit. Below a 1:1 omega:core ratio the measured activity increased linearly with omega concentration, whereas above a 1:1 ratio the activity remained constant. An immediate application of the assay is in determining the concentration of active omega, or equivalently of active holoenzyme, in any RNA polymerase preparation. 相似文献
994.
R L Bellisario G F Maley D U Guarino F Maley 《The Journal of biological chemistry》1979,254(4):1296-1300
The 102 amino acid residues of CNBr 4, the largest of 5 cyanogen bromide peptides from the Lactobacillus casei thymidylate synthetase were completely sequenced by means of limited tryptic, tryptic, chymotryptic, and staphylococcal protease peptides. CNBr 4 contains both of the cysteines in an enzyme subunit, with the 5-fluorodeoxyuridylate-reactive cysteine at residue 198 and the other at residue 244. 相似文献
995.
M K Hoffmann S Koenig R S Mittler H F Oettgen P Ralph C Galanos U Hammerling 《Journal of immunology (Baltimore, Md. : 1950)》1979,122(2):497-502
Peritoneal macrophages of the mouse produce, in response to cell wall components of Gram-negative bacteria (lipopolysaccharide and lipoproteins), a factor that causes antigen-stimulated B cells of differentiate into antibody-producing cells. Unlike lipopolysaccharide, this factor is not mitogenic for B cells. Production of the macrophage factor does not depend on participation of T cells or other accessory cells since it is readily produced by several cloned macrophage cell lines as well as by peritoneal macrophages of athymic nude mice. The factor is active only in conjunction with antigen. T cells, although apparently not necessary, amplify its effect. The factor induces phenotypic differentiation of B cell precursors as selectively as thymopoietin induces differentiation of prothymocytes. 相似文献
996.
This communication reports on the tolerogenic properties of carboxymethyl cellulose (CMC) as a nonimmunogenic carrier for 2.4 dinitrophenyl (DNP) and the benzylpenicilloyl determinant (BPO). Either normal or primed mice, given an optimal dose of 250 micrograms per animal of DNP CMC, when challenged with an immunogenic form of the hapten as early as 30 min or as late as 21 days thereafter were completely and specifically unresponsive to it. Experimental evidence suggests that this unresponsiveness is not due to suppressor cells. Furthermore, DNP CMC induces tolerance in vivo but fails to do so in vitro under conditions at which other tolerogenic carbohydrate hapten conjugates such as DNP-dextran do. This together with comparative studies of tolerance induction kinetics by DNP CMC and DNP-dextran in vivo led us to conclude that molecular properties other than the epitope density must be attributed to CMC's tolerogenic potential. CMC may also be used as a tolerogenic carrier for BPO with respect to IgE antibody production. Thus, normal or primed mice injected with the BPO CMC conjugate were found specifically unresponsive to a challenge with an immunogenic form of penicillin. 相似文献
997.
U Aebi R van den Broek P R Smith B ten Heggeler J Dubochet V V Mesyanzhinov A Tsugita J Kistler 《Journal of molecular biology》1979,130(3):255-272
An analysis has been made of the composition and structure of the two types of sheets assembled from material from dissociated bacteriophage T2 (Poglazov &; Mesyhanzhinov, 1967) and T4 capsids. Serological techniques have been used to show that both types of sheet are assembled from proteolytic fragment of P231, the major capsid constituent. The two types of sheets have been found to interconvert depending on the concentration of Mg2+ ions in the buffer. Computer modelling experiments show that the “hexagonal” and “rectangular” morphologies observed in the negative stain are due to in-register and staggered associations, respectively, of a single basic hexagonal lattice. Analysis by polyacrylamide gel electrophoresis of samples of sheets and dissociated capsids, together with previous results from immune electron microscopy (Kistler et al., 1978), suggest that hexamers of the proteolytic fragment are derived conservatively from capsomers of the phage head.The value of this proteolytic P23 fragment has been twofold: (1) it has proved to be a useful peptide in the ongoing primary sequence determination of P23 and (2) antibodies raised against it have been employed to follow the fate of P23 antigenic sites during various steps of phage capsid maturation (Kistler et al., 1978). 相似文献
998.
B U Ramírez 《Journal of neurobiology》1979,10(3):239-245
Cytochalasin-B, a drug known to interfere with axoplasmic transport, evoked fibrillary potentials in the geniohyoid muscle when applied to its motor nerve. Despite this denervation-like effect, neuromuscular transmission remained normal. Some contractile characteristics of the muscle were studied. It was found that contraction time, isometric twitch tension, and half-relaxation time were not altered by the drug treatment. The present findings show that neurogenic molecular factors conveyed by axoplasmic transport to the nerve terminal are involved in the regulation of some muscle membrane characteristics but do not modify the muscle contractile features. 相似文献
999.
Requirement for the C-terminal region of middle T-antigen in cellular transformation by polyoma virus 总被引:10,自引:1,他引:9 下载免费PDF全文
Deletions of polyoma virus DNA around the region that codes for the C-terminus of the viral middle T-antigen were created using a transforming fragment (BamH I/EcoR I) of viral DNA cloned in the plasmid vector pAT153. These species were recloned and assayed for their ability to transform Rat-1 cells in culture. Our results showed that whereas the DNA sequence between the presumed translational termination codon for the viral middle T-antigen and the single viral EcoR I site could be removed with no apparent effect on transformation, the removal of the termination codon itself or any amino acid coding sequences of this protein caused a drastic decrease in the transforming ability of the DNA. Transfection of Rat-1 cells with plasmids that contained viral DNA with deletions which corresponded to the last fourteen or more amino acids of the middle T-antigen never gave rise to cellular transformation. 相似文献
1000.