全文获取类型
收费全文 | 8764篇 |
免费 | 826篇 |
国内免费 | 92篇 |
专业分类
9682篇 |
出版年
2023年 | 37篇 |
2022年 | 135篇 |
2021年 | 183篇 |
2020年 | 108篇 |
2019年 | 136篇 |
2018年 | 149篇 |
2017年 | 139篇 |
2016年 | 249篇 |
2015年 | 447篇 |
2014年 | 426篇 |
2013年 | 523篇 |
2012年 | 682篇 |
2011年 | 629篇 |
2010年 | 383篇 |
2009年 | 345篇 |
2008年 | 462篇 |
2007年 | 416篇 |
2006年 | 429篇 |
2005年 | 373篇 |
2004年 | 328篇 |
2003年 | 307篇 |
2002年 | 254篇 |
2001年 | 236篇 |
2000年 | 229篇 |
1999年 | 183篇 |
1998年 | 81篇 |
1997年 | 58篇 |
1996年 | 69篇 |
1995年 | 57篇 |
1994年 | 46篇 |
1993年 | 59篇 |
1992年 | 124篇 |
1991年 | 96篇 |
1990年 | 101篇 |
1989年 | 108篇 |
1988年 | 77篇 |
1987年 | 83篇 |
1986年 | 88篇 |
1985年 | 90篇 |
1984年 | 61篇 |
1983年 | 62篇 |
1982年 | 50篇 |
1981年 | 49篇 |
1980年 | 46篇 |
1979年 | 76篇 |
1978年 | 61篇 |
1977年 | 45篇 |
1976年 | 47篇 |
1974年 | 43篇 |
1973年 | 41篇 |
排序方式: 共有9682条查询结果,搜索用时 15 毫秒
61.
62.
Sharrock RA Rubinstein S Chan M Leighton T 《Molecular & general genetics : MGG》1984,194(1-2):260-264
Summary A collection of intergenic suppressors of the Bacillus subtilis spoOF221 mutation has been isolated. One of these suppressors, rvtA, has been mapped between lys-1 and aroD. The rvtA suppressor restores spoOF sporulation to wild type levels and substantially improves the sporulation efficiencies of spoOB and spoOE strains. The rvtA gene does not affect the Spo phenotype of spoOH, spoOJ or spoOK mutants. The rvtA gene also prevents the induction by aliphatic alcohols of SpoO phenocopies in wild type B. subtilis cells. 相似文献
63.
Electron spin relaxation of CuA and cytochrome a in cytochrome c oxidase. Comparison to heme, copper, and sulfur radical complexes 总被引:2,自引:0,他引:2
The method of continuous saturation has been used to measure the electron spin relaxation parameter T1T2 at temperatures between 10 and 50 K for a variety of S = 1/2 species including: CuA and cytochrome a of cytochrome c oxidase, the type 1 copper in several blue copper proteins, the type 2 copper in laccase, inorganic Cu(II) complexes, sulfur radicals, and low spin heme proteins. The temperature dependence and the magnitude of T1T2 for all of the species examined are accounted for by assuming that the Van Vleck Raman process dominates the electron spin-lattice relaxation. Over the entire temperature range examined, the relaxation of the type 1 coppers in six to seven times faster than that of type 2 copper, inorganic copper, and sulfur radicals, in spite of the similar g-anisotropies of these species. This result may indicate that the coupling of the phonon bath to the spin center is more effective in type 1 coppers than in the other complexes studied. The relaxation of CuA of cytochrome oxidase exhibits an unusual temperature dependence relative to the other copper complexes studied, suggesting that the protein environment of this center is different from that of the other copper centers studied and/or that CuA is influenced by a magnetic dipolar interaction with another, faster-relaxing paramagnetic site in the enzyme. A comparison of the saturation characteristics of the CuA EPR signal in native and partially reduced CO complexes of the enzyme also suggests the existence of such an interaction. The implications of these results with respect to the disposition of the metal centers in cytochrome oxidase are discussed. 相似文献
64.
P T Chan 《Nucleic acids research》1986,14(14):5940
65.
The sequence of the nucleotides at the alpha-sarcin cleavage site in rat 28 S ribosomal ribonucleic acid 总被引:7,自引:0,他引:7
The sequence of the 521 nucleotides at the 3' end of a rat 28 S rRNA gene was determined. The region encompasses the site of cleavage of 28 S rRNA by the cytotoxin alpha-sarcin. The toxin hydrolyzes a phosphodiester bond on the 3' side of a guanine residue 393 nucleotides from the 3' end. The alpha-sarcin domain is composed of a purine-rich sequence of 14 highly conserved nucleotides. 相似文献
66.
A family of Saccharomyces cerevisiae repetitive autonomously replicating sequences that have very similar genomic environments 总被引:20,自引:0,他引:20
We have characterized a family of moderately repetitive autonomously replicating sequences (ARSs) in Saccharomyces cerevisiae. Restriction mapping, deletion studies and hybridization studies suggest that these ARSs, which are probably less than 350 base-pairs in size, share one common feature: each is located close to, but not within, a repetitive sequence (131) of approximately 10(3) to approximately 1.5 X 10(3) base-pairs in length. These ARSs can be divided into two classes (X and Y) by their sequence homology and genomic environments. Each of the class X ARSs is embedded within a repetitive sequence (X) of variable length (approximately 0.3 X 10(3) to approximately 3.75 X 10(3) base-pairs); each of the class Y ARSs is embedded within a highly conserved repetitive sequence (Y) of approximately 5.2 X 10(3) base-pairs in length. Both of these sequences are located directly adjacent to the 131 sequence. 相似文献
67.
Organization of DNA sequences and replication origins at yeast telomeres 总被引:50,自引:0,他引:50
We have shown that the DNA sequences adjacent to the telomeres of Saccharomyces cerevisiae chromosomes are highly conserved and contain a high density of replication origins. The salient features of these telomeres can be summarized as follows. There are three moderately repetitive elements present at the telomeres: the 131 sequence (1 to 1.5 kb), the highly conserved Y sequence (5.2 kb), and the less conserved X sequence (0.3 to 3.75 kb). There is a high density of replication origins spaced about 6.7 kb apart at the telomeres. These replication origins are part of the X or the Y sequences. Some of the 131-Y repetitive units are tandemly arranged. The terminal sequence T (about 0.33 to 0.6 kb) is different from the 131, X, or Y sequences and is heterogeneous in length. The order of these sequences from the telomeric end towards the centromere is T-(Y-131)n-X-, where n ranges from 1 to no more than 4. Although these telomeric sequences are conserved among S. cerevisiae strains, they show striking divergence in certain closely related yeast species. 相似文献
68.
Cognitive schema were used to explain health and illness behaviors among Chinese students. University students in Hong Kong were asked to attribute causes and suggest solutions to five health/mental health problems: Weakness/Fatigue, Tension/ Anxiety, Sleep Difficulty, Hollow/Emptiness, and Headache. The patterns of endorsement on the causal and solution categories used for the five problems were compared using a new asymptotic chi-squared test. The response patterns were found to be significantly different across the five problems. Each problem was attributed to multiple causes including psychological, social, situational, somatic, and existential factors. The intended solutions were often related to the nature of the causal attributions especially when the problems were mild. In lay help-seeking, the Hong Kong students would attempt a variety of self-help measures. However, for professional consultation, the medical doctor would be the primary care professional the students would turn to for most of the problems except in the case of Hollow/Emptiness. 相似文献
69.
Complexation and phase transfer of nucleotides by gramicidin S 总被引:1,自引:0,他引:1
Gramicidin S (GrS), an amphiphilic cyclosymmetric decapeptide produced by Bacillus brevis G-B and Nagano, binds nucleotides in water to yield a complex which partitions into organic solvents. The observed phase-transfer efficiencies at a given pH increase in the order AMP less than ADP less than ATP. The lipophilic complexes have well-defined stoichiometries, which were determined to be 1:1 for ADP-GrS at pH 7 and ATP-GrS at pH 3 and 1:2 for ATP-GrS at pH 7. The interaction is primarily ionic, involving coordination of the ornithine N delta H3+ groups of the peptide and the phosphoryl groups of the nucleotide, with little contribution from the nucleoside moiety. Exchange of organic and inorganic phosphates was also found to be mediated by GrS. The nucleotide complexes are sparingly soluble in water and self-associate extensively in CHCl3, most likely by cross-beta-aggregation, to yield large, ribbonlike aggregates which give rise to broad NMR resonances. Structures for the 1:1 and 1:2 complexes are proposed. In the latter, two GrS molecules envelop the nucleotide, orienting their apolar faces externally in opposite directions, while the lateral faces retain considerable polar character and direct aggregation in organic media. The 1:1 complex possesses a single apolar face and is less lipophilic. Binding constants were estimated by simulation of the extraction data. For the 1:1 complexes, K1:1 congruent to 4 X 10(4) M-1 for either ADP or ATP. Phase transfer of the ATP complex at pH 7 could be modeled either by stochastically independent binding to two noninteracting sites on the nucleotide with K1 approximately K2 approximately K1:1 or by a sequential process with K1 approximately K1:1 and K2/K1 less than 100. It is concluded that the apparent selectivity of GrS for ATP over ADP is a consequence of the greater lipophilicity and tendency to aggregate of the 1:2 complex, rather than an intrinsically higher binding affinity for triphosphates. GrS is, to our knowledge, the first peptide known to possess phase-transfer activity toward nucleotides; this is, in addition, the first molecular recognition process in which GrS is demonstrated to participate in vitro at physiologically active concentrations. 相似文献
70.
Purification and properties of a plasminogen activator from cultured rat prostate adenocarcinoma cells 总被引:1,自引:0,他引:1
Zymographic analysis of the supernates from confluent cultures of a rat prostate adenocarcinoma cell line, PA-III, revealed the existence of two molecular forms of specific plasminogen activators, one of molecular weight of approximately 80 000 and another of approximate molecular weight of 45 000, in sodium dodecyl sulfate. The low molecular weight form has been purified 364-fold in 66% yield from the culture medium by a combination of gel filtration on Sephacryl S-200 and affinity chromatography on Sepharose 4B-benzamidine. The purified material possessed a specific activity of 192 000 urokinase CTA units mg-1. This enzyme displayed activity toward human Glu1-plasminogen, characterized by a Km of 1.7 +/- 0.2 microM and a Vmax of 0.53 +/- 0.1 pmol of plasmin min-1 unit-1. A synthetic chromogenic substrate, H-D-Ile-Pro-Arg-p-nitroanilide (S-2288), was found for the activator. The enzyme possessed a Km of 0.33 mM and a kcat of 55 s-1 for S-2288. The activator was found to be a serine protease, inhibited by diisopropyl fluorophosphate (iPr2PF). At a concentration of 1 mM iPr2PF, and 30 nM enzyme, the half-time of this inhibition was 3.8 min. The 45 000 molecular weight enzyme was found to be inhibited by rabbit antibodies to human urokinase, thus characterizing the activator as a member of the urokinase class. The 80 000 molecular weight enzyme was not neutralized by anti-human urokinase but was neutralized by rabbit anti-human melanoma activator, likely allowing it to be classified as the tissue activator type. 相似文献