全文获取类型
收费全文 | 4578篇 |
免费 | 414篇 |
出版年
2023年 | 16篇 |
2022年 | 28篇 |
2021年 | 77篇 |
2020年 | 47篇 |
2019年 | 83篇 |
2018年 | 94篇 |
2017年 | 71篇 |
2016年 | 138篇 |
2015年 | 211篇 |
2014年 | 211篇 |
2013年 | 265篇 |
2012年 | 345篇 |
2011年 | 311篇 |
2010年 | 213篇 |
2009年 | 212篇 |
2008年 | 290篇 |
2007年 | 295篇 |
2006年 | 287篇 |
2005年 | 288篇 |
2004年 | 238篇 |
2003年 | 230篇 |
2002年 | 243篇 |
2001年 | 50篇 |
2000年 | 25篇 |
1999年 | 44篇 |
1998年 | 59篇 |
1997年 | 39篇 |
1996年 | 38篇 |
1995年 | 40篇 |
1994年 | 27篇 |
1993年 | 25篇 |
1992年 | 25篇 |
1991年 | 17篇 |
1990年 | 22篇 |
1989年 | 22篇 |
1988年 | 23篇 |
1987年 | 15篇 |
1986年 | 19篇 |
1985年 | 18篇 |
1984年 | 32篇 |
1983年 | 27篇 |
1982年 | 24篇 |
1981年 | 35篇 |
1980年 | 24篇 |
1979年 | 18篇 |
1978年 | 18篇 |
1977年 | 19篇 |
1976年 | 14篇 |
1975年 | 17篇 |
1973年 | 22篇 |
排序方式: 共有4992条查询结果,搜索用时 15 毫秒
71.
An E. coli strain carrying a fusion of the malE and lacZ genes is induced for the synthesis of a hybrid protein, consisting of the N-terminal part of the maltose-binding protein and the enzymatically active C-terminal part of β-galactosidase, by addition of maltose to cells. The secretion of the protein is initiated by the signal peptide attached to the N terminus of the maltose-binding protein sequence, but is not completed, presumably because the β-galactosidase moiety of the hybrid protein interferes with the passage of the polypeptide through the cytoplasmic membrane. Thus the protein becomes stuck to the cytoplasmic membrane. Under such conditions, periplasmic proteins, including maltose-binding protein (encoded by the malE gene) and alkaline phosphatase, and the major outer-membrane proteins, including OmpF, OmpA and probably lipoprotein, are synthesized as precursor forms with unprocessed signal sequences. This effect is observed within 15 min after high levels of induction are achieved. The simplest explanation for these results and those of pulse-chase experiments is that specific sites in the cytoplasmic membrane become progressively occupied by the hybrid protein, resulting in an inhibition of normal localization and processing of periplasmic and outer-membrane proteins. These results suggest that most of the periplasmic and outer-membrane proteins share a common step in localization before the polypeptide becomes accessible to the processing enzyme. If this interpretation is correct, we can estimate that an E. coli cell has roughly 2 × 104 such sites in the cytoplasmic membrane. A system is described for detecting the precursor of any exported protein. 相似文献
72.
The frequency characteristics of tetanic and post-tetanic potentiation of the septohippocampal and hippocampal commissural systems were studied in the acute rabbit preparation. Glass micropipettes were employed to stimulate the medial septal (MSR) and contralateral CA1 (cCA1) regions. Extracellular postsynaptic potentials were recorded in the stratum radiatum and stratum oriens layers of dorsal CA1. Low frequencies of stimulation (2–12 Hz) and brief stimulus trains (7 or 16 stimuli) ensured that only short-term effects appeared in the data. With MSR and cCA1 stimulation, tetanic potentiation became pronounced at 4 Hz, and plateaued at 6–8 Hz. Thus potentiation was found to be pronounced within the range of the rabbit hippocampal theta rhythm. No differences were found in the characteristics of potentiation evoked by stimulation of MSR and cCA1. Post-tetanic potentiation lasting 6–12 sec was found. Again, potentiation characteristics did not depend on stimulus site, suggesting a common mechanism for the pathways studied. A two-factor mechanism was proposed to account for the post-tetanic potentiation data. 相似文献
73.
Peter G.W. Plagemann Richard Marz Robert M. Wohlhueter Jon C. Graff John M. Zylka 《生物化学与生物物理学报:生物膜》1981,647(1):49-62
6-Mercaptopurine and 6-thioguanine strongly inhibited the zero-trans entry of hypoxanthine into Novikoff rat hepatoma cells which lacked hypoxanthine/guanine phosphoribosyltransferase, whereas 8-azaguanine had no significant effect. 6-Mercaptopurine was transported by the hypoxanthine carrier with about the same efficiency as its natural substrates (Michaelis-Menten constant = 372 ± 23 μM; maximum velocity = 30 ± 0.7 pmol/μl cell H2O per s). 8-Azaguanine entry into the cells, on the other hand, showed no sign of saturability and was not significantly affected by substrates of the hypoxanthine/guanine carrier. The rate of entry of 8-azaguanine at 10–100 μM amounted to only about 5% of that of hypoxanthine transport and was related to its lipid solubility in the same manner as observed for various substances whose permeation through the plasma membrane is believed to be non-mediated. Only the non-ionized form of 8-azaguanine (pKa = 6.6) permeated the cell membrane.Studies with wild type Novikoff cells showed that permeation into the cell was the main rate-determining step in the conversion of extracellular 8-azaguanine to intracellular aza-GTP and its incorporation into nucleic acids. In contrast, 6-mercaptopurine was rapidly transported into cells and phosphoribosylated; the main rate-determining step in its incorporation into nucleic acids was the further conversion of 6-mercaptopurine riboside 5'-monophosphate. 相似文献
74.
Robert L. Wykle Craig H. Miller Jon C. Lewis Jeffrey D. Schmitt Jennie A. Smith Jefferson R. Surles Claude Piantadosi Joseph T. OFlaherty 《Biochemical and biophysical research communications》1981,100(4):1651-1658
1-O-Hexadecyl-2-O-acetyl--glycero-3-phosphocholine (platelet activating factor) stimulated the degranulation of rabbit platelets and human neutrophils, whereas the enantiomer, 3-O-hexadecyl-2-O-acetyl--glycero-1-phosphocholine, was inactive. The analogs compared had the following relative potencies in degranulating platelets and neutrophils: 1-O-hexadecyl-2-O-acetyl--glycero-3-phosphocholine > 1-O-hexadecyl-2-O-ethyl--glycero-3-phosphocholine >-1-O-octadecyl-2-O-ethylglycero-3-phosphocholine = 1-O-hexadecyl-2-O-methyl--glycero-3-phosphocholine >-1-O-dodecyl-2-O-ethyl-glycero-3-phosphocholine. The deacetylated compound, 1-O-hexadecyl-2-lyso--glycero-3-phosphocholine, and 1-O-hexadecyl-2,2-dimethylpropanediol-3-phosphocholine were inactive. The active analogs selectively desensitized the response to each other in the neutrophils. It is suggested that these compounds may activate cells through interaction with a stereospecific receptor. 相似文献
75.
Jon F. Fobes 《Biochemical genetics》1980,18(3-4):401-421
Five isozymic loci were localized in the tomato (Lycopersicon esculentum) genome by trisomic analysis. Results revealed the following locations: Aps-1 on chromosome 6, Est-1 and Prx-2 on chromosome 2, Prx-4 on chromosome 10, and Prx-7 on chromosome 3. Three genes—Aps-1, Prx-2, and Prx-4—showed an arithmetic increase in allozyme concentration in direct proportion to the increase of gene dosage in respective primary trisomics. In contrast, no increase in relative Est-1 isozyme concentration was observed for any primary trisomic type. The phenotypes of the Aps-1, Prx-2, and Est-1 genes showed a pattern of banding intensity proportional to the allelic ratio (+/+/a vs. + /a/a) in primary trisomics; zymotypes of these differential trisomic heterozygotes appeared as converse images of each other.This research was performed under the auspices of NSF Grants BMS75-03024 and DEB77-02248 to C. M. Rick. 相似文献
76.
Thiosulfate Oxidation and Tetrathionate Reduction by Intact Cells of Marine Pseudomonad Strain 16B 总被引:3,自引:1,他引:2
下载免费PDF全文
![点击此处可从《Applied microbiology》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Jon H. Tuttle 《Applied microbiology》1980,39(6):1159-1166
Levels of thiosulfate-oxidizing enzyme (TSO) and tetrathionate reductase (TTR) were measured in washed cell suspensions of a heterotrophic marine thiosulfate-oxidizing bacterium, strain 16B. TSO activity remained virtually constant in aerobically and anaerobically grown cells and was unaffected by the presence or absence of thiosulfate and tetrathionate in the growth medium. TTR was also present in cells grown aerobically and anaerobically, but its activity was threefold greater in cells cultured in media containing tetrathionate or thiosulfate. Tetrathionate appears to be the inducer of increased TTR activity in both aerobically and anaerobically grown cells. TTR (constitutive or induced) and TSO have different pH and temperature optima. Both TTR activities were unaffected by 10 mM KCN, which reversed oxygen inhibition of tetrathionate reduction. TSO was partially inhibited by 5 μM KCN and completely inhibited by 90 μM KCN. These findings and results of experiments to determine the influence of several inorganic electron donors and acceptors on TSO and TTR activities suggest that constitutive TSO and TTR represent reverse activities of the same enzyme, whereas inducible TTR is a separate enzyme used by strain 16B only for anaerobic respiration of tetrathionate. The bacterium appears well adapted to growth in environments characterized by low oxygen tension, dilute organic carbon concentrations, and the presence of a variety of reduced, inorganic sulfur compounds. 相似文献
77.
Colonic lesions in experimental swine dysentery were studied electron microscopically. Changes indicative of stasis were commonly observed in the microcirculatory vessels of lamina propria. Early lesions in epithelial cells included sparse, short and irregular microvilli, swollen and degenerated mitochondria, and swelling and vesiculation of endoplasmatic reticulum. Numerous large spirochaetes were observed in these locations: a) in the crypts, b) free (i.e. not membrane bound) in cytoplasm of damaged epithelial cells, and c) in cavities, around vessels of lamina propria. It is suggested that stasis, and resultant disturbances in microcirculation in early developmental stages of swine dysentery, may play a pathogenetic role in the development of the necrotic colonic lesions. Finally, it is discussed whether a mechanism related to Sanarelli-Shwartzman reaction is implicated in the development of colonic lesions in swine dysentery. 相似文献
78.
79.
Relative amounts of nuclear DNA were determined in root tip cells of seven species of Astereae: Aster hydrophilus Greene, A. oblongifolius Nutt., A. riparius H.B.K., Machaeranthera boltoniae (Greene) Turner and Home, M. brevilingulata (Sch-Bip.) Turner and Home, M. parviflora Gray, and M. tenuis (S. Wats.) Turner and Home. The results show that A. hydrophilus and M. brevilingulata, with a chromosome number of n = 9, have less nuclear DNA than other closely related species which are either n = 4 or n = 5. Cytological analyses of meiosis in the intergeneric hybrid M. parviflora X A. hydrophilus showed cells with two or more small chromosomes of the latter species pairing with single large chromosomes of the former. Pachytene cells of the hybrids M. parviflora X A. hydrophilus, M. parviflora X A. riparius, and M. boltoniae X M. tenuis showed some unpaired chromosome segments. The significance of these results to chromosome evolution in the tribe Astereae is discussed. 相似文献
80.
A bioluminescence chemical oceanography research cruise (Varifront III) through the Sea of Cortez from November through December 1981 provided an opportunity to investigate plankton associated with a brilliant and extensive display of surface water bioluminescence at the north end of Balleñas Channel. New observations of bioluminescence were made on larval stages of the euphausiid Nyctiphanes simplex Hansen (Calyptopis II, Furcilia I, II, and III, and juveniles) and Euphausia eximia Hansen (Calyptopis I), the Calanoida copepods Centropages furcatus Dana, Paracalanus indiens Wolfenden, Acrocalanus longicornis Giesbrecht, the Cyclopoida copepods Corycaeus (Corycaeus) speciousus Dana, Corycaeus (Onychocorycaeus) latus Dana, and several dinoflagellates Ceratium breve Ostenfeld and Schmidt, Ceratium horridum Gran, and Ceratium gibberum Gourret. These observations indicate the increasing importance of some of the smaller copepods and larval euphausiids contributing to surface bioluminescence. 相似文献