全文获取类型
收费全文 | 105402篇 |
免费 | 4232篇 |
国内免费 | 63篇 |
出版年
2018年 | 1134篇 |
2017年 | 1313篇 |
2016年 | 3138篇 |
2015年 | 6697篇 |
2014年 | 6336篇 |
2013年 | 6349篇 |
2012年 | 5644篇 |
2011年 | 2953篇 |
2010年 | 2813篇 |
2009年 | 2613篇 |
2008年 | 1305篇 |
2007年 | 1291篇 |
2006年 | 1293篇 |
2005年 | 7350篇 |
2004年 | 6151篇 |
2003年 | 4318篇 |
2002年 | 1854篇 |
2001年 | 1761篇 |
2000年 | 983篇 |
1999年 | 2092篇 |
1998年 | 648篇 |
1997年 | 508篇 |
1992年 | 2498篇 |
1991年 | 2490篇 |
1990年 | 2593篇 |
1989年 | 2513篇 |
1988年 | 2372篇 |
1987年 | 2235篇 |
1986年 | 1989篇 |
1985年 | 2026篇 |
1984年 | 1487篇 |
1983年 | 1142篇 |
1982年 | 741篇 |
1981年 | 689篇 |
1980年 | 610篇 |
1979年 | 1398篇 |
1978年 | 1020篇 |
1977年 | 885篇 |
1976年 | 840篇 |
1975年 | 1103篇 |
1974年 | 1232篇 |
1973年 | 1215篇 |
1972年 | 1112篇 |
1971年 | 1109篇 |
1970年 | 939篇 |
1969年 | 957篇 |
1968年 | 827篇 |
1967年 | 835篇 |
1966年 | 664篇 |
1965年 | 492篇 |
排序方式: 共有10000条查询结果,搜索用时 78 毫秒
41.
Chromosome transfer and R-prime formation by an RP4::mini-Mu derivative in Escherichia coli, Salmonella typhimurium, Klebsiella pneumoniae, and Proteus mirabilis 总被引:34,自引:0,他引:34
We have introduced into the wide host range conjugative plasmid RP4, a mini-Mu derivative which was known to be able to transpose spontaneously in E. coli K-12, and to induce in such a host several kinds of chromosomal rearrangements including replicon fusions. Unlike RP4, RP4::mini-Mu can mediate the transfer of the host chromosome to a recipient bacterium and generate R primes at high frequencies (10?4 for the transfer of a given marker, 10?5 for the formation of R primes carrying a given marker). Two such RP4::mini-Mu plasmids were introduced into one Salmonella typhimurium strain, one Klebsiella pneumoniae strain, and one Proteus mirabilis strain. Each of these three strains were mated with an E. coli K-12 recipient and transconjugants carrying R primes were recovered in all three cases at frequencies ranging from 5 × 10?6 to 10?7. 相似文献
42.
Cells of Entamoeba histolytica accumulated K+ and extruded Na+ compared to the concentrations of those ions present in the growth medium. Pinocytic activity, measured by the uptake of horseradish peroxidase of 125I-polyvinylpyrrolidone, was high (up to 0.3 ml/ml cells per h). Upon addition of cytochalasin B, at a concentration (20 microM) that completely blocked pinocytosis, cells lost up to 40% of their Na+ content within 90 min; K+ content was not affected or increased slightly compared to control cells without the inhibitor. Cation loss was associated with cell shrinkage. The dose-response curves for the effects of cytochalasin B on pinocytosis and Na+ content were identical. These data provide direct evidence that pinocytosis is an important component of the homeostatic system for Na+. 相似文献
43.
44.
45.
The effect of UV light (lambda = 254 nm) on calf thymus DNP at low ionic strengths was studied. It was found that at the irradiation doses used the protein in the DNA-protein complex increases as the irradiation dose rises. Thermal treatment and acid hydrolysis resulted in a predominant release of histones H3 and H1 from the complex. Data from liquid high performance chromatography, amino acid analysis, thin-layer chromatography point to the induction by UV-light of a thymine-lysine bond, whose formation involves DNA thymines and histone lysine residues, predominantly H3 and H1 fractions. 相似文献
46.
Using two-dimensional thin-layer chromatography, the effect of serotonin on the yield of thymine dimers and on cleavage of the N-glycosidic bond in the DNA irradiated with ultraviolet (UV) light and X-ray was studied. Bound serotonin was shown to reduce the synthesis of UV-induced thymine dimers but had no effect on the number of X-ray-induced breaks in the N-glycoside bonds in thymidine residues. The data obtained are discussed in terms of the mechanisms of serotonin involvement in the photoprotection of yeast cells from the lethal action of UV and X-ray irradiations. 相似文献
47.
J Markussen I Diers A Engesgaard M T Hansen P Hougaard L Langkjaer K Norris U Ribel A R S?rensen E S?rensen 《Protein engineering》1987,1(3):215-223
It has previously been found that insulins, to which positive charge has been added by substitutions in position B30, thus raising the isoelectric point towards pH 7, had a prolonged action when injected as slightly acidic solutions because such derivatives crystallize very readily upon neutralization. Positive charge has now been added by substituting the B13 and A17 glutamic acid residues with glutamines and B27 threonine with lysine or arginine. These substitutions were introduced by site-specific mutagenesis in a gene coding for a single-chain insulin precursor. By tryptic transpeptidation the single-chain precursors were transformed to the double-chain insulin structure, concomitantly with incorporation of residue B30. Thus insulins combining B13 glutamine, A17 glutamine and B27 lysine or arginine with B30 threonine, threonine amide or lysine amide were synthesized. The time course of blood glucose lowering effect and the absorption were studied after subcutaneous injection in rabbits and pigs. The prolonged action of B30-substituted insulins was markedly enhanced by B27 lysine or arginine substitutions and by B13 glutamine. The B27 residue is located on the surface of the hexamer, so a basic residue in this position presumably promotes the packing of hexamers at neutral pH. The B13 residues cluster in the centre of the hexamer. When the electrostatic repulsive forces from six glutamic acid residues are abolished by substitution with glutamine, a stabilization of the hexamer can be envisaged.(ABSTRACT TRUNCATED AT 250 WORDS) 相似文献
48.
C Wynants G Van Binst H R Loosli 《International journal of peptide and protein research》1985,25(6):622-627
The conformational properties of the somatostatin analogue 201-456 (1) have been studied by high field n.m.r. in DMSO. This analogue is the base structure of nine derivates synthesized by Bauer et al. and shows a very low biological activity, although derived structures such as SMS 201-995 (2) are very potent. Our study has shown an important difference between the most stable conformation of the two compounds: although the beta turn type II' structure at the Phe3-Trp4-Lys5 level is present in both analogues, an important conformational change appears at the cystine bridge. In SMS 201-995 the beta turn/beta sheet conformation is stabilized by the additional amino-acids D-Phe1 and Thr8 (ol) through intramolecular H-bonds. 相似文献
49.
I P Grigor'ev A A Neokesari?ski? 《Biulleten' eksperimental'no? biologii i meditsiny》1986,102(9):288-289
The effects of different concentrations of L-ascorbic acid (Asc) on Na+-dependent binding of 3H-GABA and 3H-DL-glutamic acid to rat brain cortical synaptosomes were studied. Asc, at a concentration nearly equal to brain extracellular one (3 X 10(-4) M), had no effect on specific and nonspecific 5H-GABA binding. At higher concentrations (10(-3) M) Asc strongly inhibited, and at lower concentrations (10(-6) M) considerably stimulated 3H-GABA binding. At a concentration of 10(-5)-10(-3) M Asc tended to decrease 3H-DL-glutamic acid binding. 相似文献
50.
D G Astaf'ev I V Maianskaia A K Akatov A N Maianski? 《Zhurnal mikrobiologii, epidemiologii, i immunobiologii》1986,(7):48-51
In investigations based on the use of a highly sensitive test system permitting the detection of normal human antibodies to S. aureus peptidoglycan, the antigenic relationships between the peptidoglycans of S. aureus and other representatives of the genus Staphylococcus have been studied. Among other staphylococcal species, S. simulans, S. xylosus, S. hyicus, S. cohnii, S. hyicus s. s. chromogenes have been found to possess peptidoglycans most closely related to S. aureus peptidoglycans, while S. warneri and S. epidermidis peptidoglycans have proved to be least closely related to it. 相似文献