全文获取类型
收费全文 | 85928篇 |
免费 | 7589篇 |
国内免费 | 46篇 |
专业分类
93563篇 |
出版年
2023年 | 281篇 |
2022年 | 707篇 |
2021年 | 1550篇 |
2020年 | 880篇 |
2019年 | 1108篇 |
2018年 | 1440篇 |
2017年 | 1241篇 |
2016年 | 2136篇 |
2015年 | 3631篇 |
2014年 | 4047篇 |
2013年 | 4815篇 |
2012年 | 6538篇 |
2011年 | 6483篇 |
2010年 | 4172篇 |
2009年 | 3798篇 |
2008年 | 5430篇 |
2007年 | 5433篇 |
2006年 | 5236篇 |
2005年 | 5032篇 |
2004年 | 4892篇 |
2003年 | 4669篇 |
2002年 | 4405篇 |
2001年 | 858篇 |
2000年 | 625篇 |
1999年 | 960篇 |
1998年 | 1230篇 |
1997年 | 821篇 |
1996年 | 742篇 |
1995年 | 653篇 |
1994年 | 617篇 |
1993年 | 666篇 |
1992年 | 554篇 |
1991年 | 523篇 |
1990年 | 462篇 |
1989年 | 398篇 |
1988年 | 422篇 |
1987年 | 345篇 |
1986年 | 318篇 |
1985年 | 399篇 |
1984年 | 519篇 |
1983年 | 403篇 |
1982年 | 504篇 |
1981年 | 487篇 |
1980年 | 417篇 |
1979年 | 296篇 |
1978年 | 326篇 |
1977年 | 283篇 |
1976年 | 263篇 |
1975年 | 208篇 |
1974年 | 244篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
181.
Michael Nee 《Brittonia》1994,46(4):265-269
Talauma boliviana is described as new and illustrated. This species, first collected in 1989, is the only Magnoliaceae known from Bolivia.
It seems to be most closely related toT. sambuensis of northwestern Colombia and eastern Panama. 相似文献
182.
Arthur O. Tucker Michael J. Maciarello Pearl W. Burbage George Sturtz 《Economic botany》1994,48(3):333-336
The essential oils of fresh leaves, twigs, and/or fruits of spicebush cultivated in Oregon (field distilled) and Delaware (laboratory distilled) are examined by GC/MS. The oil of leaves of spicebush is notably high in 6-methyl-5-hepten-2-one (1.94% in Oregon to 34.83 ± 9.69% in Delaware). β-caryophyllene (15.26% in Oregon to 48.44 ± 1.35% in Delaware), and/or (E)-nerolidol (10.20% in Oregon to 12.05 ± 2.02% in Delaware). The oil of the twigs of spicebush is notably high in 1,8-cineole (45.41 ± 0.35% in Delaware), while the oil of the fruits is notably high in a-phellandrene (64.62 ± 0.66% in Delaware) and β-phellandrene (11.23+0.17% in Delaware). 相似文献
183.
The homologous operons for P1 and P7 plasmid partition are autoregulated from dissimilar operator sites 总被引:12,自引:3,他引:9
Finbarr Hayes Lyndsay Radnedge Michael A. Davis Stuart J. Austin 《Molecular microbiology》1994,11(2):249-260
The plasmid-partition regions of the P1 and P7 plasmid prophages in Escherichia coli are homologues which each encode two partition proteins, ParA and ParB. The equivalent PI and P7 proteins are closely related. In each case, the proteins are encoded by an operon that is autoregulated by the ParA and ParB proteins in concert. This regulation is species-specific, as the P1 proteins are unable to repress the P7 par operon and vice versa. The homologous ParA proteins are primarily responsible for repression and bind to regions that overlap the operon promoter in both cases. The DNA-binding domain of the P7 auto-repressor lies in the amino-terminal end of the P7 ParA protein. This region includes a helix-turn-helix motif that has a clear counterpart in the P1 ParA sequence. However, despite the common regulatory mechanism and the similarity of the proteins involved in repression, the promoter-operator sequences of these two operons are very different in sequence and organization. The operator is located downstream of the promoter in P1 and upstream of it in P7, and the two regions show little, if any, homology. How these differences may have arisen from a common ancestral form is discussed. 相似文献
184.
In an attempt to characterize disease producing mutations in the factor VIII gene we screened exons 4, 7, 8, 11, 12 and 16 by PCR-SSCP (polymerase chain reactionsingle strand conformation polymorphism), in 12 randomly selected haemophilia A patients. These exons were chosen because they have been reported to harbour a disproportionately high number of mutations relative to their size. Using this strategy we detected a frame-shifting 5-bp deletion (TACCT, involving nucleotides 519–523), which is predicted to result in a severely truncated factor VIII polypeptide, terminating approximately midway through the conserved A1 domain and resulting in the observed severe phenotype. We also showed that the sequence in the vicinity of the observed deletion is concordant with the modified slipped-mispairing at DNA replication model of Krawczak and Cooper. 相似文献
185.
Comparative development of Cryptosporidium parvum (Apicomplexa) in 11 continuous host cell lines 总被引:5,自引:0,他引:5
Abstract Using standardized media, incubation, and parasite inoculating procedures, we compared development of Crytosporidium parvum between Madin-Darby bovine kidney (MDBK) cells and 10 additional host cell lines available through the American Type Culture Collection. Parasite development was assessed by counting parasite numbers atop monlayers in 25 random oil fields 68 h post-infection using Nomarski interference-contrast optics. Results revealed that the human ileocecal adenocarcinoma (HCT-8) cell line supported nearly twice the number of parasite developmental stages than MDBK cells or any of the other host cell types. 相似文献
186.
Relationship between osmoprotection and the structure and intracellular accumulation of betaines by Escherichia coli 总被引:2,自引:0,他引:2
Barbara A. Peddie Michael Lever Colin M. Hayman Kelly Randall Stephen T. Chambers 《FEMS microbiology letters》1994,120(1-2):125-131
Abstract Naturally occuring betaines, especially glycine betaine and proline betaine, were accumulated by Escherichia coli from urine. In synthetic hyperosmotic medium, with an homologous series of added betaines, (CH3 )3 N+ -(CH2 ) n -COO− , osmoprotective activity and intracellular accumulation decreased monotonically as n increased from 1 to 5. In contrast, α -substituted glycine betaines were accumulated in a similar manner to glycine betaine, but with different osmoprotective activities. Arsenobetaine, with a quaternary arsonium group, was also accumulated but amino acids which can become negatively charged in a chemically basic environment were not. 相似文献
187.
188.
Martin M. Lee Francis H.Y. Green W.Michael Schoel Samuel Schürch 《生物化学与生物物理学报:疾病的分子基础》1994,1226(2):151-162
Cell-substrate adhesion was quantified for two cultured mesothelioma cell lines (epitheliomatus and sarcomatous) on glass, fibronectin and laminin substrates. Interference reflection microscopy (IRM) was used to image the adhesion patterns of cells and a grey level analysis was employed to quantify adhesion. Sarcomatous cells demonstrated marked adhesion to glass and fibronectin-coated substrates but not to laminin-coated substrate, with the greatest adhesion occurring on the fibronectin-coated surface. This adhesion was accompanied by cytoplasmic spreading. By contrast, epitheliomatous cells showed little tendency to adhere to any of the substrates and only showed significant spreading when in contact with the laminin substrate (P < 0.01). A bioassay was used to determine the metastatic potential of each of the cell lines. Via the intravenous route, the sarcomatous cells killed the host rats in 24.7 ± 1.5 (S.D.) days compared to 27.3 ± 0.9 (S.D.) days for the epitheliomatous cells (P < 0.01). After subcutaneous inoculation of tumour cells, the sarcomatous cells killed the host rats in 54.7 ± 0.7 (S.D.) days compared to 48.5 ± 0.5 (S.D.) days for the epitheliomatous cells (P < 0.01). We conclude that the results of the metastasis bioassays were consistent with the predicted behavior of these cell lines based on their ability to adhere to substrates in the in vitro adhesion assays. 相似文献
189.
190.
Jacques Magdalou Sylvie Fournel-Gigleux Michael Pritchard Grard Siest 《Biology of the cell / under the auspices of the European Cell Biology Organization》1993,77(1):13-16
Summary— Peroxisome proliferators, despite their chemically unrelated structures, share the common property of being able to stimulate the glucuronidation of bilirubin in rodents and, probably, also in man. The aryloxycarboxylic acids (clofibric acid, fenofibrate, bezafibrate, ciprofibrate), tiadenol and probucol, all of which have hypolipidemic properties, as well as the fatty acid-like perfluorodecanoic acid all enhanced the expression of the UDP-glucuronosyltransferase (UGT) form involved in the conjugation of the pigment. This induction is manifested by an increase in the mRNA species encoding the protein with a subsequent increase in the neosynthesis of the corresponding protein in the endoplasmic reticulum. The induction process is concomitant with that of cytochrome P-450-IVA1 and cytosolic epoxide hydrolase, which, like bilirubin UGT, are mainly involved in the metabolism of endogenous substrates. With a series of carboxylic acids related to clofibric acid, it was possible to demonstrate that induction was mediated via specific interactions based on the physicochemical properties of the inducers. Until now, the molecular basis of induction of bilirubin UGT is not known. The peroxisome proliferators that possess a carboxyl group are good substrates of UGT, especially in man. The acylglucuronides formed are known for their instability and reactivity which could contribute to the toxicity encountered in some patients treated with the drugs. There is convincing evidence that UGT bilirubin does not catalyze the glucuronidation of these substances even if the two types of substrate form acylglucuronides. 相似文献