全文获取类型
收费全文 | 34197篇 |
免费 | 3164篇 |
国内免费 | 2篇 |
专业分类
37363篇 |
出版年
2023年 | 126篇 |
2022年 | 200篇 |
2021年 | 448篇 |
2020年 | 254篇 |
2019年 | 358篇 |
2018年 | 635篇 |
2017年 | 615篇 |
2016年 | 897篇 |
2015年 | 1120篇 |
2014年 | 1253篇 |
2013年 | 1714篇 |
2012年 | 2558篇 |
2011年 | 2540篇 |
2010年 | 1484篇 |
2009年 | 1217篇 |
2008年 | 2111篇 |
2007年 | 2127篇 |
2006年 | 2003篇 |
2005年 | 1879篇 |
2004年 | 1750篇 |
2003年 | 1712篇 |
2002年 | 1644篇 |
2001年 | 917篇 |
2000年 | 979篇 |
1999年 | 573篇 |
1998年 | 396篇 |
1997年 | 297篇 |
1996年 | 308篇 |
1995年 | 293篇 |
1994年 | 257篇 |
1993年 | 254篇 |
1992年 | 254篇 |
1991年 | 231篇 |
1990年 | 228篇 |
1989年 | 190篇 |
1988年 | 193篇 |
1987年 | 179篇 |
1986年 | 180篇 |
1985年 | 226篇 |
1984年 | 236篇 |
1983年 | 176篇 |
1982年 | 239篇 |
1981年 | 210篇 |
1980年 | 173篇 |
1979年 | 170篇 |
1978年 | 118篇 |
1977年 | 122篇 |
1976年 | 105篇 |
1975年 | 105篇 |
1974年 | 118篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
91.
92.
Anne M. Gallagher Catherine T. Kelly William M. Fogarty 《Applied microbiology and biotechnology》1991,35(4):455-460
Summary The ascosporogenous yeast Lipomyces tetrasporus produced an unusual extracellular carbohydrase. It was purified to homogeneity using ammonium sulphate precipitation and DEAE Bio-gel A ion-exchange chromatography. While retaining highest activity on low-molecular-weight saccharides such as maltose and nigerose, it displays considerable activity towards polymeric substrates including soluble starch. It is particularly unusual in that it also hydrolyses dextran and has a very high affinity for this substrate. The enzyme has an exo-lytic mode of action with the only hydrolysis product, glucose, being released in the -anomeric form. Optimum activity occurs at pH 4.5 and at 50°C. It is a glycoprotein, and has an M
r value of 150 000 (sodium dodecyl sulphate-polyacrylamide gel electrophoresis) — 183 000 (fast protein liquid chromatography) and a pI of 6.0.
Offprint requests to: C. T. Kelly 相似文献
93.
Motility and orientation has been studied in the unicellular photosynthetic flagellate, Euglena gracilis, using real time image analysis capable of tracking up to 200 cells simultaneously in the slow rotating centrifuge microscope (NIZEMI) which allows one to observe the cells' swimming behavior during centrifugation accelerations between 1 g and 5 g. At 1 g the cells show a weak negative gravitaxis, which increases significantly at higher accelerations up to about 3 g. Though most cells were capable of swimming even against an acceleration of 4.5 g, the degree of gravitaxis decreased and some of the cells were passively moved downward by the acceleration force; this is true for most cells at 5 g. The velocity of cells swimming against 1 g is about 10% lower than that of cells swimming in other directions. The velocity decreases even more drastically in cells swimming against higher acceleration forces than those at 1 g. The degree of gravitactic orientation drastically decreases after short exposure to artificial UV radiation which indicates that gravitaxis may be due to an active physiological perception rather than a physical effect such as an asymmetry of the center of gravity within the cell.
Offprint requests to: D.-P. Häder 相似文献
94.
Paul Salers L'Houcine Ouafik Pierre Giraud Anne Dutour Jean-Yves Maltese Charles Oliver 《Molecular and cellular biochemistry》1991,106(1):15-24
Thyrotropin-R eleasing hormone (TRH)-degrading pyroglutamyl peptidase I(PGP I) and prolyl endopeptidase (PE) activities have been demonstrated in rat insulinoma RINm 5F cell line. These two enzymes catalyze the conversion of TRH to Histydyl-Proline-Diketopiperazine and to acid TRH respectively.After cell fractionation, we found all the PGP I and PE activities in the cytosolic fraction. The membranebound PGP II activity is not detectable in the RINm 5F cells. Further investigations on these two cytosolic enzymes show that pyroglutamyl- and proline-containing peptides are inhibitors of each TRH-degrading enzyme.Gelfiltration chromatography on Sephadex G100 shows that PGP I and PE activity have an apparent molecular mass of about 18 kDa and 57 kDa, respectively. Kinetic analysis with TRH as substrate, gives a Km of 44 µM and 235 µM, and a Vmax of 1.49 and 8.80 pmoUmin/µg protein for PGP I and PE, respectively. Immunoreactive TRH, His-Pro-Diketopiperazine and acid TRH levels in the cell line extracts are 2.2 ± 0.9, 22.5 ± 11.1 and 28.7 ± 14.6pg/106 cells, respectively. When cells have been incubated for 2 to 72 hours with a P. E. inhibitor (Z-Gly-Pro-CHN2) at 5 × 10–7M, both cell PGP I and PE activities are inhibited. No change in the cellular content of immunoreactive TRH, His-Pro-Diketopiperazine and acid TRH have been observed in treated cells.These data suggest that TRH is not degraded by cytosolic, unspecific PGP I and PE enzymes in RINm 5F. The finding that these cells contain 10 and 13 times more His-Pro-Diketopiperazine and acid TRH than TRH may be an indirect evidence for the existence of another precursor than TRH for these two peptides or of the possibility that TRH can be degraded by other peptidases.Abbreviations TRH
Thyrotropin-Releasing Hormone or Thyroliberin
- His-Pro-DKP
Histidyl-ProlineDiketopiperazine
- TRH-OH
acid TRH or deamidated TRH
- LH-RH
Luteinizing Hormone-Releasing Hormone
- Z-Gly-Pro-CHN2
N-benzyloxycarboxyl-Gly-Pro-diazomethylketone
- PGP
Pyroglutamyl Peptidase, PGP I (EC 3.4.19.3) and PGP II (EC 3.4.19.-)
- PE
Prolyl Endopeptidase or post-proline cleaving enzyme (EC 3.4.21.26) 相似文献
95.
Characterization of a frequent polymorphism in the coding sequence of the Tp53 gene in colonic cancer patients and a control population 总被引:2,自引:0,他引:2
Sylviane Olschwang Pierre Laurent-Puig Anne Vassal Rémy-J. Salmon Gilles Thomas 《Human genetics》1991,86(4):369-370
Summary We describe a simple method for characterizing a frequent polymorphism (that subsitutes an arginine for a proline) in the coding sequence of the Tp53 gene in patients with colonic cancer and in a control population. We could find no evidence that this polymorphism is associated with a marked predisposition to colorectal cancer. 相似文献
96.
97.
Ronald J. Hill Margaret R. Mott Fujiko Watt Theodora Fifis P. Anne Underwood 《Chromosoma》1986,94(6):441-448
An antigen making a major contribution to the immune response to Drosophila melanogaster chromatin resides primarily on a nonhistone charge-class family of proteins of Mr 74,000. Immunofluorescence detects this antigen at interbands, puffs and diffuse bands of D. melanogaster salivary chromosomes isolated without exposure to acid fixatives, and on nucleoplasmic ribonucleoprotein droplets. In the electron microscope, gold labelling reveals the binding of monoclonal antibodies specific for the antigen at chromosomal loci generally bearing putative ribonucleoprotein (RNP) particles. However, the locus 3C 11–12 is remarkable in that it bears putative RNP particles but is virtually unlabelled, suggesting protein specificity at different active loci. 相似文献
98.
The O-methyl substituents of aromatic compounds constitute a C1 growth substrate for a number of taxonomically diverse anaerobic acetogens. In this study, strain TH-001, an O-demethylating obligate anaerobe, was chosen to represent this physiological group, and the carbon flow when cells were grown on O-methyl substituents as a C1 substrate was determined by 14C radiotracer techniques. O-[methyl-14C]vanillate (4-hydroxy-3-methoxy-benzoate) was used as the labeled C1 substrate. The data showed that for every O-methyl carbon converted to [14C]acetate, two were oxidized to 14CO2. Quantitation of the carbon recovered in the two products, acetate and CO2, indicated that acetate was formed in part by the fixation of unlabeled CO2. The specific activity of 14C in acetate was 70% of that in the O-methyl substrate, suggesting that only one carbon of acetate was derived from the O-methyl group. Thus, it is postulated that the carboxyl carbon of the product acetate is derived from CO2 and the methyl carbon is derived from the O-methyl substituent of vanillate. The metabolism of O-[methyl-14C]vanillate by strain TH-001 can be described as follows: 314CH3OC7H5O3 + CO2 + 4H2O → 14CH3COOH + 214CO2 + 10H+ + 10e- + 3HOC7H5O3. 相似文献
99.
Christian Boucher Anne Martinel Patrick Barberis Genevieve Alloing Claudine Zischek 《Molecular & general genetics : MGG》1986,205(2):270-275
Summary A class of avirulent mutants of the plant pathogenic bacterium Pseudomonas solanacearum, strain GMI1000, resistant to acridine orange (Acrr), harbour a deletion of over 85 kb in their genome. This deletion affects, a1,000 kb megaplasmid which has previously been shown to be present in most of the strains of this species. In addition at least 11 out of 13 independent Tn5 insertions, leading to loss of virulence, are located on the megaplasmid. Nine of them are present in the region which is deleted from the Acrr mutants. These results suggest that the majority of virulence genes identified so far are plasmid borne. 相似文献
100.
Ten type 1 fimbriate strains of Enterobacteriaceae were examined in an in vitro adhesion assay with HEp2 epithelial cells. The range of HEp2 cell adhesiveness, which was characteristic for each strain, was affected by motility, type 1 fimbriation and production of mannose sensitive haemagglutinin. Nevertheless, not all type 1 fimbriate strains adhered well in this model. The findings are discussed with regard to the possibility that different type 1 fimbriate enterobacteria, though all are mannose sensitive, recognize different mannose-containing receptors present or available on the surfaces of the HEp2 cells. 相似文献