全文获取类型
收费全文 | 2872篇 |
免费 | 379篇 |
国内免费 | 2篇 |
专业分类
3253篇 |
出版年
2022年 | 29篇 |
2021年 | 40篇 |
2019年 | 25篇 |
2018年 | 25篇 |
2016年 | 47篇 |
2015年 | 84篇 |
2014年 | 92篇 |
2013年 | 112篇 |
2012年 | 172篇 |
2011年 | 130篇 |
2010年 | 111篇 |
2009年 | 79篇 |
2008年 | 130篇 |
2007年 | 112篇 |
2006年 | 121篇 |
2005年 | 99篇 |
2004年 | 120篇 |
2003年 | 98篇 |
2002年 | 102篇 |
2001年 | 84篇 |
2000年 | 79篇 |
1999年 | 72篇 |
1998年 | 24篇 |
1997年 | 28篇 |
1996年 | 21篇 |
1995年 | 27篇 |
1993年 | 27篇 |
1992年 | 40篇 |
1991年 | 43篇 |
1990年 | 58篇 |
1989年 | 52篇 |
1988年 | 45篇 |
1987年 | 48篇 |
1986年 | 60篇 |
1985年 | 46篇 |
1984年 | 35篇 |
1983年 | 38篇 |
1982年 | 25篇 |
1981年 | 34篇 |
1979年 | 45篇 |
1978年 | 27篇 |
1977年 | 24篇 |
1976年 | 22篇 |
1975年 | 24篇 |
1974年 | 21篇 |
1973年 | 26篇 |
1972年 | 22篇 |
1971年 | 24篇 |
1970年 | 20篇 |
1967年 | 20篇 |
排序方式: 共有3253条查询结果,搜索用时 0 毫秒
71.
72.
Phosphorylation of nuclear and DNA-binding proteins in proliferating and quiescent mammalian cells. 下载免费PDF全文
The dependence of cell proliferation on nuclear protein phosphorylation was studied with exponential-phase and stationary-phase cultures of Chinese-hamster ovary cells. Nuclear proteins were fractionated, according to their DNA-binding affinities, by using sequential extractions of isolated nuclei with increasing concentrations of NaCl. When viable whole cells were labelled with H332PO4, phosphorylation of nuclear proteins was found to be lower in quiescent cells than in proliferating cells. Phosphorylation of nuclear proteins soluble in 0.30M-NaCl (less than 50% of these proteins bind to DNA) was greater than for those proteins soluble in higher salt concentrations (80-100% of these proteins bind to DNA). Cyclic AMP enhanced the phosphorylation of nuclear proteins soluble in 0.3 m-NaCl by 40-50%, and this stimulation was independent of cell growth. Cyclic AMP also increased the phosphorylation of nuclear proteins soluble in 0.6M-NaCl and 2.0M-NaCl by 40-50% in exponential-phase cultures, but not in stationary-phase cultures. Several examples of specific phosphorylation in response to cyclic AMP were observed, including a 35000-mol.wt. protein in the 0.30 M-NaCl-soluble fraction and several proteins larger than 100000 molecular weight within this fraction. A major peptide of molecular weight approx. 31000 extracted with 0.6M-NaCl was also phosphorylated. Its phosphorylation was independent of cyclic AMP in exponential-phase cultures, and it was not phosphorylated in plateau-phase cells. These changes in cell-growth-dependent phosphorylation occurred in the absence of any apparent qualitative changes in the nuclear protein molecular-weight distributions. These data demonstrate that (1) phosphorylation of nuclear proteins is dependent on the culture's proliferative status, (2) both cyclic AMP-dependent and cyclic AMP-independent specific phosphorylation occurs, and (3) the cyclic AMP-dependent growth-independent phosphorylation that occurs does not appear to be a modification of DNA-binding proteins, whereas the cyclic AMP-dependent growth-dependent phosphorylation does involve modification of DNA binding proteins. 相似文献
73.
Andrew Clarke J. Cynan Ellis-Evans Mark W. Sanders Lesley J. Holmes 《Hydrobiologia》1989,172(1):183-191
The copepod Pseudoboeckella poppei (Daday) (Calanoida, Centropagidae) was sampled from Sombre and Heywood Lakes on Signy Island, Antarctica (60° S, 45° W) between January 1984 and March 1985. Sombre Lake is clear and oligotrophic with little phytoplankton and a bottom sediment low in organic content. By contrast Heywood Lake is turbid and mesotrophic; a substantial phytoplankton develops in summer and the bottom sediments are comparatively rich in organics. Both lakes freeze over for much of the year, forcing the copepods to adopt a benthic feeding strategy over winter. Adult Pseudoboeckella feed on phytoplankton when this is available, but also on detritus, diatoms and short algal filaments stirred up from the sediment. In Heywood Lake, male copepods show a smooth seasonal trend in lipid content with lipid being synthesised in early summer and utilised in late summer and winter. The summer increase in lipid content is associated with an increase in dry weight. Female lipid contents show evidence of two peaks of egg production. In Sombre Lake both male and female copepods increase in size during summer and show a wider range of lipid contents than in Heywood Lake; it is likely that this is due to the poorer winter feeding conditions which necessitate the synthesis of a much larger store of reserves during the summer. In contrast to marine calanoid copepods, lipid stores are exclusively triacylglycerol with no trace of wax ester. 相似文献
74.
Biosynthesis of fucose containing lacto-series glycolipids has been studied in human colonic adenocarcinoma Colo 205 cells. Transfer of fucose in both alpha 1----3 linkage to type 2 chain acceptors and alpha 1----4 linkage to type 1 chain acceptors was demonstrated with a Triton X-100 solubilized membrane fraction. The enzyme was found to be highly active over a broad pH range between 6.0 and 7.5. Kinetics of the transfer reactions were studied and indicated that the enzyme had an apparent Km for GDPfucose of 53 and 49 microM with acceptors nLc4 and Lc4, respectively. The apparent Km values for acceptors Lc4, nLc4, and IV3NeuAcnLc4 were determined to be 42, 18, and 26 microM, respectively. Transfer of fucose to the type 1 chain acceptor Lc4 alone and in the presence of increasing concentrations of the type 2 chain acceptor IV3NeuAcnLc4 or Gb3 suggested that both type 1 and 2 acceptors were alternate acceptors for a single enzyme. This was further established by the finding that IV3NeuAcnLc4 behaved as a competitive inhibitor of fucose transfer with respect to Lc4. Conditions were defined for preparative scale in vitro synthesis of fucosylated products of nLc6 catalyzed by the Colo 205 cell enzyme. Yields of the monofucosyl derivative of 2.5 mg (46%) and 1 mg (17%) of the difucosyl derivative were obtained from 5 mg of original nLc6. The structures of these biosynthetic products were carefully studied by 1H NMR, +FAB-MS, and methylation analysis. These studies revealed extremely high purity products composed of III3FucnLc6 and III3V3Fuc2nLc6. The significance of the nature of these products and enzymatic properties is discussed. 相似文献
75.
Milen Georgiev Vasil Georgiev Plamen Penchev Daniela Antonova Atanas Pavlov Mladenka Ilieva Simeon Popov 《Engineering in Life Science》2010,10(2):148-157
Cell suspension cultures of Lavandula vera (Lamiaceae), Nicotiana tabacum (Solanaceae), and Helianthus annuus (Asteraceae) were cultivated in three different ways: in shake flasks both as free suspensions and in two‐phase systems (in the presence of Amberlite XAD‐4 resin as a second phase), as well as in 3‐L stirred tank reactor, and their volatile metabolic profiles were studied using GC‐MS. A number of compounds, some of them having allelochemical and biological activities, were identified in all the three cell suspension cultures under study. Also the presence of some compounds, unusual for the intact plants, was observed. It was found that the cultivation mode strongly influences the production and the transport (secretion into the culture medium) of the low‐molecular‐mass volatile metabolites. Principal component analyses of 12 common hydrocarbons showed discrimination between the different cultivation modes (shake flasks and two‐phase systems cultivation) by first principal component (PC1) and second principal component (PC2). 相似文献
76.
Michael V. Holmes Tina Shah Christine Vickery Liam Smeeth Aroon D. Hingorani Juan P. Casas 《PloS one》2009,4(12)
Background
Studies of the genetic basis of drug response could help clarify mechanisms of drug action/metabolism, and facilitate development of genotype-based predictive tests of efficacy or toxicity (pharmacogenetics).Objectives
We conducted a systematic review and field synopsis of pharmacogenetic studies to quantify the scope and quality of available evidence in this field in order to inform future research.Data Sources
Original research articles were identified in Medline, reference lists from 24 meta-analyses/systematic reviews/review articles and U.S. Food and Drug Administration website of approved pharmacogenetic tests.Study Eligibility Criteria, Participants, and Intervention Criteria
We included any study in which either intended or adverse response to drug therapy was examined in relation to genetic variation in the germline or cancer cells in humans.Study Appraisal and Synthesis Methods
Study characteristics and data reported in abstracts were recorded. We further analysed full text from a random 10% subset of articles spanning the different subclasses of study.Results
From 102,264 Medline hits and 1,641 articles from other sources, we identified 1,668 primary research articles (1987 to 2007, inclusive). A high proportion of remaining articles were reviews/commentaries (ratio of reviews to primary research approximately 25∶1). The majority of studies (81.8%) were set in Europe and North America focussing on cancer, cardiovascular disease and neurology/psychiatry. There was predominantly a candidate gene approach using common alleles, which despite small sample sizes (median 93 [IQR 40–222]) with no trend to an increase over time, generated a high proportion (74.5%) of nominally significant (p<0.05) reported associations suggesting the possibility of significance-chasing bias. Despite 136 examples of gene/drug interventions being the subject of ≥4 studies, only 31 meta-analyses were identified. The majority (69.4%) of end-points were continuous and likely surrogate rather than hard (binary) clinical end-points.Conclusions and Implications of Key Findings
The high expectation but limited translation of pharmacogenetic research thus far may be explained by the preponderance of reviews over primary research, small sample sizes, a mainly candidate gene approach, surrogate markers, an excess of nominally positive to truly positive associations and paucity of meta-analyses. Recommendations based on these findings should inform future study design to help realise the goal of personalised medicines.Systematic Review Registration Number
Not Registered 相似文献77.
Choonkeun Kim Catarina Milheiri?o Susana Gardete Mark A. Holmes Matt T. G. Holden Hermínia de Lencastre Alexander Tomasz 《The Journal of biological chemistry》2012,287(44):36854-36863
Methicillin-resistant Staphylococcus aureus (MRSA) strains show strain-to-strain variation in resistance level, in genetic background, and also in the structure of the chromosomal cassette (SCCmec) that carries the resistance gene mecA. In contrast, strain-to-strain variation in the sequence of the mecA determinant was found to be much more limited among MRSA isolates examined so far. The first exception to this came with the recent identification of MRSA strain LGA251, which carries a new homolog of this gene together with regulatory elements mecI/mecR that also have novel, highly divergent structures. After cloning and purification in Escherichia coli, PBP2ALGA, the protein product of the new mecA homolog, showed aberrant mobility in SDS-PAGE, structural instability and loss of activity at 37 °C, and a higher relative affinity for oxacillin as compared with cefoxitin. The mecA homolog free of its regulatory elements was cloned into a plasmid and introduced into the background of the β-lactam-susceptible S. aureus strain COL-S. In this background, the mecA homolog expressed a high-level resistance to cefoxitin (MIC = 400 μg/ml) and a somewhat lower resistance to oxacillin (minimal inhibitory concentration = 200 μg/ml). Similar to PBP2A, the protein homolog PBP2ALGA was able to replace the essential function of the S. aureus PBP2 for growth. In contrast to PBP2A, PBP2ALGA did not depend on the transglycosylase activity of the native PBP2 for expression of high level resistance to oxacillin, suggesting that the PBP2A homolog may preferentially cooperate with a monofunctional transglycosylase as the alternative source of transglycosylase activity. 相似文献
78.
79.
80.
Michael J. Bayly Gareth D. Holmes Paul I. Forster David J. Cantrill Pauline Y. Ladiges 《PloS one》2013,8(8)