全文获取类型
收费全文 | 4175篇 |
免费 | 403篇 |
国内免费 | 4篇 |
专业分类
4582篇 |
出版年
2021年 | 46篇 |
2020年 | 31篇 |
2019年 | 48篇 |
2018年 | 47篇 |
2017年 | 56篇 |
2016年 | 58篇 |
2015年 | 119篇 |
2014年 | 122篇 |
2013年 | 142篇 |
2012年 | 216篇 |
2011年 | 241篇 |
2010年 | 157篇 |
2009年 | 151篇 |
2008年 | 253篇 |
2007年 | 216篇 |
2006年 | 213篇 |
2005年 | 259篇 |
2004年 | 232篇 |
2003年 | 235篇 |
2002年 | 225篇 |
2001年 | 81篇 |
2000年 | 57篇 |
1999年 | 71篇 |
1998年 | 73篇 |
1997年 | 64篇 |
1996年 | 41篇 |
1995年 | 53篇 |
1994年 | 36篇 |
1993年 | 43篇 |
1992年 | 58篇 |
1991年 | 40篇 |
1990年 | 50篇 |
1989年 | 56篇 |
1988年 | 45篇 |
1987年 | 45篇 |
1986年 | 46篇 |
1985年 | 43篇 |
1984年 | 37篇 |
1983年 | 44篇 |
1982年 | 41篇 |
1981年 | 45篇 |
1980年 | 33篇 |
1979年 | 26篇 |
1978年 | 20篇 |
1977年 | 35篇 |
1976年 | 20篇 |
1975年 | 19篇 |
1974年 | 25篇 |
1971年 | 21篇 |
1968年 | 19篇 |
排序方式: 共有4582条查询结果,搜索用时 15 毫秒
71.
Ecogeographical rules that associate climate with organismal form and function can reveal patterns of climatic adaptation. Two rules link animal coloration with climate: Gloger's rule (darker coloration where wet and warm), and Bogert's rule (darker coloration where cold). Whereas Gloger's rule was proposed for endotherms, and Bogert's rule for ectotherms, both rules may apply more broadly, despite their seemingly opposing effects. Here, we test this contradiction on a global scale across passerine birds. Consistent with Gloger's rule, birds were darker in wetter areas and, following Bogert's rule, lighter where warm, although birds became lighter again at very low temperatures. Rainfall and temperature had antagonistic or additive effects depending on their pattern of covariation, and this predicted whether birds followed the rules. We integrate both rules into a general framework to explain heterogeneity in climatic effects on coloration, which has implications to understand patterns of diversification, climatic adaptation and climate change impacts. 相似文献
72.
P. M. Selkirk M. Skotnicki K. D. Adam M. B. Connett T. Dale T. W. Joe J. Armstrong 《Polar Biology》1997,18(5):344-350
Sixty-six isolates of the moss Sarconeurum glaciale were collected from sites in continental Antarctica at Ross Island, southern Victoria Land and the Vestfold Hills. Genetic
variation within and among the populations was estimated using isozymes and random amplified polymorphic DNA (RAPD) technology.
Isozyme results only reproducibly showed variation between the populations with one enzyme; RAPDs indicated significantly
higher levels of genetic variability within and among the Vestfold Hills samples than in the Ross Sea region samples. A dendrogram
produced from the RAPD bands suggested that the Ross Island and southern Victoria Land samples form one population, and those
from the Vestfold Hills form a separate and more variable population, possibly resulting from separate colonisation events
on the continent.
Received: 15 March 1996 / Accepted: 1 May 1997 相似文献
73.
Malgorzata Schmidt Desirée Du Sart Paul Kalitsis Margaret Leversha Sue Dale Leslie Sheffield Daniela Toniolo 《Human genetics》1991,86(5):519-521
Summary We have analysed two duplications of the X chromosome in male patients using chromosome replication and DNA methylation patterns as determinants of the functional status of the duplicated segments. In both cases, the large duplicated regions, Xq12-q22 and Xq26.3-qter, were not inactivated. A review of previously reported male cases revealed that these duplications were also not subject to inactivation. Taken together, the examined duplications cover almost the entire X chromosome except the pericentromeric region and Xq25–26. Thus, most regions of the X chromosome can be present in two functional copies without lethal consequences. 相似文献
74.
Polymorphic repetitive DNA sequences in Mycobacterium tuberculosis detected with a gene probe from a Mycobacterium fortuitum plasmid 总被引:14,自引:0,他引:14
Clinical isolates of Mycobacterium tuberculosis were shown by Southern blotting to contain DNA sequences hybridizing to a probe derived from a Mycobacterium fortuitum plasmid. Two such M. tuberculosis DNA fragments, isolated from a gene library, were used as probes to show restriction fragment length polymorphism in M. tuberculosis strains by detecting a repetitive sequence apparently located at different points on the chromosome. This could indicate the presence of a transposable element in M. tuberculosis which is partly homologous to a region of the M. fortuitum plasmid. The probes described can be used to fingerprint M. tuberculosis isolates, and in addition are capable of distinguishing M. tuberculosis from Mycobacterium bovis and BCG. 相似文献
75.
Jiandong Zheng Xixiang Huo Yang Huai Lin Xiao Hui Jiang John Klena Carolyn M. Greene Xuesen Xing Jigui Huang Shali Liu Youxing Peng Hui Yang Jun Luo Zhibin Peng Linlin Liu Maoyi Chen Hui Chen Yuzhi Zhang Danqin Huang Xuhua Guan Luzhao Feng Faxian Zhan Dale J. Hu Jay K. Varma Hongjie Yu 《PloS one》2016,11(3)
76.
Srinivasan Vijayaraghavan Dale D. Hoskins 《Molecular reproduction and development》1988,20(3):343-352
Secretions from the mammalian epididymis contain proteins that bind to developing sperm and are presumed to play a role in sperm maturation. The biochemical functions in sperm of most of these proteins are not known. In this report we describe the presence of a low molecular weight compound in bovine caudal epididymal luminal fluid (CF) that has a potent stimulatory effect on calcium (45Ca2+) uptake in immature caput epididymal spermatozoa. The studies were initially undertaken to characterize the effect of the protein caltrin, present in bovine seminal plasma (BSP), on calcium uptake into caput spermatozoa. Caltrin is known to block calcium influx into mature bovine sperm. Unexpectedly, the kinetics of calcium uptake into caput sperm showed a biphasic response when treated with BSP, namely, a stimulation of uptake at 1 to 5 min and inhibition of uptake after this time. Since caudal sperm do not show this biphasic response, we reasoned that BSP contained a factor derived from CF that must interact with developing sperm before the binding of caltrin to sperm can prevent further calcium uptake. We first demonstrated that preincubation of caput sperm with CF eliminated the biphasic calcium uptake effect induced in caput sperm by BSP and that caudal fluid alone had a potent stimulatory effect on calcium uptake in caput sperm. Half-maximal stimulation (fivefold over control) occurred at a caudal fluid protein concentration of 0.27 mg/ml. Partial purification of the factor indicates that it is of low molecular weight (MW ~ 1,000), but further chemical characterization has not been carried out and its epididymal site of origin is not known. The results indicate that the regulation of intracellular calcium levels in sperm differs in immature and mature bovine sperm in that an epididymal factor promotes calcium uptake during epididymal maturation, and the seminal fluid protein caltrin prevents it at ejaculation. 相似文献
77.
Provost AC Péquignot MO Sainton KM Gadin S Sallé S Marchant D Hales DB Abitbol M 《Comptes rendus biologies》2003,326(9):841-851
The class-B type-I scavenger receptor (SR-BI) plays a key role in cholesterol homeostasis; it mediates the selective uptake of lipoprotein cholesterol to steroidogenic tissues. We show by RT-PCR, western blot, in situ hybridization and immunohistochemistry analysis that SR-BI is highly expressed in different neuro-retinal and non-neuronal cells types on rat eye. Immunohistochemistry of the steroidogenic acute regulatory protein (StAR) involved in neurosteroid production showed the same expression pattern than SR-BI in rat eye. Our results may suggest a key role of these genes in the ocular cholesterol metabolism for membranes biosynthesis and neurosteroidogenesis. 相似文献
78.
Jensen MD Cardin S Edgerton D Cherrington A 《American journal of physiology. Endocrinology and metabolism》2003,284(6):E1140-E1148
These studies were conducted to assess the relationship between visceral adipose tissue free fatty acid (FFA) release and splanchnic FFA release. Steady-state splanchnic bed palmitate ([9,10-(3)H]palmitate) kinetics were determined from 14 sampling intervals from eight dogs with chronic indwelling arterial, portal vein, and hepatic vein catheters. We tested a model designed to predict the proportion of FFAs delivered to the liver from visceral fat by use of hepatic vein data. The model predicted that 15 +/- 2% of hepatic palmitate delivery originated from visceral lipolysis, which was greater (P = 0.004) than the 11 +/- 2% actually observed. There was a good relationship (r(2) = 0.63) between the predicted and observed hepatic palmitate delivery values, but the model overestimated visceral FFA release more at lower than at higher palmitate concentrations. The discrepancy could be due to differential uptake of FFAs arriving from the arterial vs. the portal vein or to release of FFAs in the hepatic circulatory bed. Splanchnic FFA release measured using hepatic vein samples was strongly related to visceral adipose tissue FFA release into the portal vein. This finding suggests that splanchnic FFA release is a good indicator of visceral adipose tissue lipolysis. 相似文献
79.
Silvia Penuela Alexander W Lohman Wesley Lai Laszlo Gyenis David W Litchfield Brant E Isakson Dale W Laird 《Channels (Austin, Tex.)》2014,8(2):124-130
The pannexin family of channel-forming proteins is composed of 3 distinct but related members called Panx1, Panx2, and Panx3. Pannexins have been implicated in many physiological processes as well as pathological conditions, primarily through their function as ATP release channels. However, it is currently unclear if all pannexins are subject to similar or different post-translational modifications as most studies have focused primarily on Panx1. Using in vitro biochemical assays performed on ectopically expressed pannexins in HEK-293T cells, we confirmed that all 3 pannexins are N-glycosylated to different degrees, but they are not modified by sialylation or O-linked glycosylation in a manner that changes their apparent molecular weight. Using cell-free caspase assays, we also discovered that similar to Panx1, the C-terminus of Panx2 is a substrate for caspase cleavage. Panx3, on the other hand, is not subject to caspase digestion but an in vitro biotin switch assay revealed that it was S-nitrosylated by nitric oxide donors. Taken together, our findings uncover novel and diverse pannexin post-translational modifications suggesting that they may be differentially regulated for distinct or overlapping cellular and physiological functions. 相似文献
80.
Wang GT Wang S Gentles R Sowin T Maring CJ Kempf DJ Kati WM Stoll V Stewart KD Laver G 《Bioorganic & medicinal chemistry letters》2005,15(1):125-128
(+/-)-(2R,3R,5R)-[2-(1'-S-acetamido-3'-methyl)butyl-3-methoxycarbonyl]tetrahydrofuran-5-carboxylic acid (9) and (+/-)-(2R,3R,5R)-[2-(1'-S-acetamido-3'-methyl)butyl-3-(4'-imidazolyl)]tetrahydrofuran 5-carboxylic acid (14) were synthesized as inhibitors of influenza neuraminidase (NA). Both compounds 9 and 14 inhibit influenza NA A with an IC(50) of about 0.5 microM and NA B with an IC(50) of 1.0 microM. 相似文献