全文获取类型
收费全文 | 3060篇 |
免费 | 191篇 |
专业分类
3251篇 |
出版年
2023年 | 15篇 |
2022年 | 39篇 |
2021年 | 57篇 |
2020年 | 35篇 |
2019年 | 77篇 |
2018年 | 72篇 |
2017年 | 53篇 |
2016年 | 92篇 |
2015年 | 168篇 |
2014年 | 147篇 |
2013年 | 211篇 |
2012年 | 245篇 |
2011年 | 214篇 |
2010年 | 142篇 |
2009年 | 102篇 |
2008年 | 176篇 |
2007年 | 183篇 |
2006年 | 173篇 |
2005年 | 166篇 |
2004年 | 158篇 |
2003年 | 133篇 |
2002年 | 154篇 |
2001年 | 25篇 |
2000年 | 19篇 |
1999年 | 33篇 |
1998年 | 22篇 |
1997年 | 20篇 |
1996年 | 24篇 |
1995年 | 22篇 |
1994年 | 18篇 |
1993年 | 15篇 |
1992年 | 11篇 |
1991年 | 22篇 |
1990年 | 12篇 |
1989年 | 12篇 |
1988年 | 12篇 |
1987年 | 16篇 |
1986年 | 8篇 |
1985年 | 9篇 |
1984年 | 12篇 |
1983年 | 16篇 |
1982年 | 24篇 |
1981年 | 10篇 |
1980年 | 9篇 |
1979年 | 10篇 |
1978年 | 5篇 |
1977年 | 12篇 |
1976年 | 6篇 |
1975年 | 4篇 |
1971年 | 4篇 |
排序方式: 共有3251条查询结果,搜索用时 0 毫秒
31.
32.
Enkephalin-binding systems in human plasma 总被引:1,自引:0,他引:1
Roberta Possenti Valeria De Marco Ornella Cherubini L. Giorgio Roda 《Neurochemical research》1983,8(4):423-432
Three amino acid-containing fractions present in human plasma are shown to bind both leu and met-enkephalin: serum albumin and two species of a much lower molecular weight, in all likelihood polypeptides. The amount of enkephalin associated with serum albumin seems comparatively smaller than that associated with the two low molecular weight systems. These systems jointly are apparently capable of binding a significant part of the circulating enkephalins. The possibility is suggested that the interactions described may play a role in maintaining the integrity of circulating enkephalins. 相似文献
33.
Orsola Tiboni Rita Cantoni Roberta Creti Piero Cammarano Anna Maria Sanangelantoni 《Journal of molecular evolution》1991,33(2):142-151
Summary The gene (fus) coding for elongation factor G (EF-G) of the extremely thermophilic eubacteriumThermotoga maritima was identified and sequenced. The EF-G coding sequence (2046 bp) was found to lie in an operon-like structure between the ribosomal protein S7 gene (rpsG) and the elongation factor Tu (EF-Tu) gene (tuf). TherpsG, fus, andtuf genes follow each other immediately in that order, which corresponds to the order of the homologous genes in thestr operon ofEscherichia coli. The derived amino acid sequence of the EF-G protein (682 residues) was aligned with the homologous sequences of other eubacteria, eukaryotes (hamster), and archaebacteria (Methanococcus vannielii). Unrooted phylogenetic dendrogram, obtained both from the amino acid and the nucleotide sequence alignments, using a variety of methods, lend further support to the notion that the (present) root of the (eu)bacterial tree lies betweenThermotoga and the other bacterial lineages. 相似文献
34.
Roberta Bertelli Fabrizio Ginevri Rosanna Gusmano Gian Marco Ghiggeri 《In vitro cellular & developmental biology. Animal》1991,27(10):799-804
Summary It has been suggested that the generation of toxic radicals plays an important role in toxicity by Adriamycin (ADR) on cancer
cell lines and in vivo. We have examined the role of free radicals in determining toxicity and resistance to ADR of rat glomerular
epithelial cells in culture; this method provides a good model for analyzing the mechanisms responsible for ADR experimental
nephrosis in rats. Three points were established: a) the intra- or extracellular site of ADR toxicity; b) the role of the
superoxide anion and of the hydroxyl radical in determining intra- and-extracellular cytotoxicity; and c) the implication
of oxido-reduction cycling as a potential route for ADR semiquinone transformation. Free ADR was found to induce the same
inhibition of [3H]thymidine incorporation into DNA as ADR bound to an agarose macroporous bed which prevents the intracellular incorporation
of the drug. Specific scavenging of free radical activity by the enzymes catalase and superoxide dismutase, the hydroxyl radical
inhibitors dimethyl sulfoxide and dimethylthiourea (DMTU) and by chelation of intracellular free iron with deferoxamine produced
only a partial restoration of [3H]thymidine incorporation into DNA, which was maximal for DMTU (30% of normal incorporation). DMTU treatment was unsuccessful
in preventing the extracellular cytostatic effect of ADR. Finally, glomerular epithelial cell killing (51Cr-release method) by 5-iminodaunorubicin, an ADR analogue with a modified quinone function that prohibits oxido-reduction
cycling, was higher than unmodified ADR. These results indicate that ADR may exert its cytotoxic effects on glomerular epithelial
cells by interaction at the cell surface, whereas the intracellular compartment, principally DNA, does not seem to be the
target of ADR effects. They also suggest that the free radicals are in part responsible for ADR intracellular cytotoxicity,
but other mechanisms should also be hypothesized. Finally, the participation of the ADR semiquinone radical in oxido-reduction
cycling seems not important for the induction of the cellular damage. 相似文献
35.
Corticotropin releasing factor (CRF) was recently isolated from ovine hypothalami by its ability to stimulate adrenocorticotropin (ACTH) and β-endorphin release from dispersed rat pituitary cells. Intramuscular injection of synthetic ovine CRF conugated to bovine thyroglobulin with 1-ethyl-3(3-dimethylaminopropyl) carbodiimide and emulsified with Freund's complete adjuvant into a random bred New Zealand white rabbit resulted in antiserum production to CRF associated with adrenal atrophy. A decrease in the level of plasma coticosteroids was associated with an increase in mean total binding of 125I-N-Tyr-CRF. Upon sacrifice, a decrease in pituitary content of ACTH and a decrease in adrenal weight and content of corticosteroids was observed in the rabbit producing antiserum to CRF. Adrenal atrophy was histologically verified with an observed decrease in the adrenal cortical zone not reflected in the zona glomerulosa. Individual cells were relatively larger either with more abundant pale cytoplasm or with distinctly vacuolated cytoplasm. The results presented here are consistent with a physiologically necessary role for this CRF or peptides with similar structures in the hypothalamic-pituitary-adrenal axis. 相似文献
36.
Two new adenosine analogs, 2-(2-bromoethyl) adenosine monophosphate and 3-(2-bromoethyl) adenosine monophosphate, were synthesized, purified by semipreparative high-pressure liquid chromatography, and completely characterized. A new synthesis of 5-(2-bromoethyl) adenosine monophosphate is presented which facilitates the preparation of radioactive reagent with label either in the ethyl group or the purine ring of the nucleotide derivative. The reactive moiety of these derivatives, a bromoalkyl group, has the ability to react with the nucleophilic side chains of several amino acids. The second-order, pH-independent rate constants for reaction with the side chains of the amino acids cysteine, lysine, histidine, and tyrosine were determined as 3×10–4, 6×10–6, 3×10–7, and <1×10–7 M–1 sec–1, respectively. These data could be use in estimating the rate enhancement observed in modification of a protein by these affinity-labeling reagents. 5-(S-(2-hydroxyethyl)cysteine) adenosine monophosphate, the derivative expected from exhaustive digestion of protein in which a cysteinyl residue is modified by 5-(2-bromoethyl) adenosine monophosphate, and S-2-hydroxyethyl)cysteine, the derivative anticipated upon acid hydrolysis of such a modified protein, were synthesized, characterized, and their elution positions from an amino acid analyzer determined. These bromoethyl AMP derivatives are potential affinity labels for enzymes that bind 2-, 3-, or 5-nucleotides such as TPN, coenzyme A, or ADP, respectively. 相似文献
37.
38.
Liver tissue grafts between seven H-2 mutants and their parental strains have been studied. Each of these mutants was originally identified by reciprocal mutant—parental strain skin graft rejection. However, liver grafts among mutants and parental standard strains are not uniformly rejected. Liver graft rejection also fails to correlate with mutant—parental stimulation in CML and MLC. In addition, the immune reaction pattern of female mutant animals against grafts of male liver differs from the reaction pattern found in parental standard strains. Several explanations for the differences between immune response to liver and skin grafts are proposed, including different T cell subsets involved in recognition, availability of antigenic sites to immunocompetent cells, and structural differences between mutant and parental H-2 antigens.
Abbreviations used in this paper: bml, 2, 3, 4,14; dml; fm2=mutants of strains C57BL/6, B10.D2 and B10.M respectively; B6=C57BL/6 相似文献
39.
In the dynamics of the survival of chromosomal polymorphism selection may be operating at the genic level, at the chromosomal level or at the supergene level. Tests designed to distinguish between these levels were run on Drosophila pseudoobscura. There was no evidence for heterosis, a necessary requirement for gene-determined chromosomal polymorphism. A strong chromosmal selection was observed. No evidence was found for the presence within one locality of more than a single superallele for each supergene (= gene order). These results are compared to those found by others. 相似文献
40.