全文获取类型
收费全文 | 21811篇 |
免费 | 2465篇 |
国内免费 | 5篇 |
专业分类
24281篇 |
出版年
2021年 | 362篇 |
2020年 | 185篇 |
2019年 | 230篇 |
2018年 | 284篇 |
2017年 | 277篇 |
2016年 | 431篇 |
2015年 | 695篇 |
2014年 | 781篇 |
2013年 | 1011篇 |
2012年 | 1281篇 |
2011年 | 1242篇 |
2010年 | 818篇 |
2009年 | 716篇 |
2008年 | 972篇 |
2007年 | 1061篇 |
2006年 | 975篇 |
2005年 | 886篇 |
2004年 | 850篇 |
2003年 | 810篇 |
2002年 | 713篇 |
2001年 | 522篇 |
2000年 | 544篇 |
1999年 | 474篇 |
1998年 | 304篇 |
1997年 | 227篇 |
1996年 | 220篇 |
1995年 | 200篇 |
1994年 | 185篇 |
1993年 | 209篇 |
1992年 | 390篇 |
1991年 | 301篇 |
1990年 | 342篇 |
1989年 | 310篇 |
1988年 | 321篇 |
1987年 | 327篇 |
1986年 | 258篇 |
1985年 | 284篇 |
1984年 | 236篇 |
1983年 | 239篇 |
1982年 | 213篇 |
1981年 | 186篇 |
1980年 | 177篇 |
1979年 | 245篇 |
1978年 | 213篇 |
1977年 | 189篇 |
1976年 | 187篇 |
1975年 | 176篇 |
1974年 | 184篇 |
1973年 | 189篇 |
1972年 | 165篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
41.
When expressed in Escherichia coli, a truncated form of phytochrome (oat PHYA AP3 residues 464-1129) self associates to form a series of products ranging in size from monomers to aggregates of greater than 20 subunits. When these same phytochrome sequences are coexpressed with the chaperonins GroEL and GroES, the truncated phytochrome migrates as a native-like dimer in size exclusion chromatography and no higher-order aggregates were detected. GroEL and GroES inhibition of phytochrome aggregation in E. coli presumably occurs via the suppression of folding pathways leading to incorrectly folded phytochrome. 相似文献
42.
A. T. Axon G. D. Bell R. H. Jones M. A. Quine R. F. McCloy 《BMJ (Clinical research ed.)》1995,310(6983):853-856
Upper gastrointestinal endoscopy is a valuable diagnostic tool, but for an endoscopy service to be effective it is essential that it is not overloaded with inappropriately referred patients. A joint working party in Britain has considered the available literature on indications for endoscopy, assessed standard practice through a questionnaire, and audited randomly selected cases using an independent panel of experts and an American database system. They used these data to produce guidelines on the appropriate and inappropriate indications for referral for endoscopy, although they emphasise that under certain circumstances there may be reasons to deviate from the advice given. The need for endoscopy is most difficult to judge in patients with dyspepsia, and this aspect is discussed in detail. Early endoscopy will often prove more cost effective than delaying until the indications are clearer. 相似文献
43.
44.
Cytokine production in a model of wound healing: the appearance of MIP-1, MIP-2, cachectin/TNF and IL-1 总被引:9,自引:1,他引:8
T J Fahey B Sherry K J Tracey S van Deventer W G Jones J P Minei S Morgello G T Shires A Cerami 《Cytokine》1990,2(2):92-99
Macrophages are essential for normal wound repair and many of their effects on healing wounds are likely to be mediated by the secretion of cytokines. This study examines the appearance of messenger RNA (mRNA) for cachectin/tumor necrosis factor (TNF), IL 1, and macrophage inflammatory proteins 1 and 2 (MIP-1 and MIP-2), as well as the mature peptides, in a model of wound healing using wound chambers. RNA for all four cytokines can be detected in wound inflammatory cells by polymerase chain reaction amplification throughout the first 7 days. Cachectin/TNF and IL 1 protein levels peaked on the first day after wound chamber implantation, and MIP-1 and MIP-2 were detected only on day 3. The data suggest that these cytokines participate in the early inflammatory response to wounding. 相似文献
45.
46.
Kateryna Raykova Melina V. Jones Hwa Huang Paul F. Hoffman Michael Levy 《Journal of visualized experiments : JoVE》2015,(99)
The rat optic nerve is a useful model for stem cell regeneration research. Direct injection into the rat optic nerve allows delivery into the central nervous system in a minimally-invasive surgery without bone removal. This technique describes an approach to visualization and direct injection of the optic nerve following minor fascial dissection from the orbital ridge, using a conjunctival traction suture to gently pull the eye down and out. Representative examples of an injected optic nerve show successful injection of dyed beads. 相似文献
47.
How do adapting populations navigate the tensions between the costs of gene expression and the benefits of gene products to optimize the levels of many genes at once? Here we combined independently-arising beneficial mutations that altered enzyme levels in the central metabolism of Methylobacterium extorquens to uncover the fitness landscape defined by gene expression levels. We found strong antagonism and sign epistasis between these beneficial mutations. Mutations with the largest individual benefit interacted the most antagonistically with other mutations, a trend we also uncovered through analyses of datasets from other model systems. However, these beneficial mutations interacted multiplicatively (i.e., no epistasis) at the level of enzyme expression. By generating a model that predicts fitness from enzyme levels we could explain the observed sign epistasis as a result of overshooting the optimum defined by a balance between enzyme catalysis benefits and fitness costs. Knowledge of the phenotypic landscape also illuminated that, although the fitness peak was phenotypically far from the ancestral state, it was not genetically distant. Single beneficial mutations jumped straight toward the global optimum rather than being constrained to change the expression phenotypes in the correlated fashion expected by the genetic architecture. Given that adaptation in nature often results from optimizing gene expression, these conclusions can be widely applicable to other organisms and selective conditions. Poor interactions between individually beneficial alleles affecting gene expression may thus compromise the benefit of sex during adaptation and promote genetic differentiation. 相似文献
48.
49.
Summary Previous work has shown that the monoclonal antibody 22/18 identifies progenitor cells (blastemal cells) which depend on the nerve for their division in the early stages of limb regeneration in the newt,Notophthalmus viridescens. This antibody also reacts with cultured cells derived from the newt limb, and the intensity of immunoreactivity appears related to cell density and differentiation into myotubes. We report here that the monoclonal antibody 22/18 recognizes a polypeptide (22/18 antigen) which is intracellular and filamentous. Double staining of cells with 22/18 monoclonal antibody and antibodies against various cytoskeletal components indicates that the epitope is expressed on an intermediate filament component. Although this antibody is specific for blastemal cells in cryostat sections of the regenerating limb, its reactivity on immunoblots is not confined to this tissue. The 22/18 antigen is differentially affected by aldehyde fixatives distinguished by the spacing of their reactive groups. While formaldehyde fixation impairs detection of the antigen, ethylene glycol-bis[succinic acid n-hydroxysuccinimide ester] reveals the antigen in sections of normal and regenerating limbs in a distribution that is consistent with the one obtained from immunoblots. We suggest that the 22/18 monoclonal antibody detects a change in protein conformation, probably related to changes in the physiological state of the cell, that occurs transiently during regeneration and possibly during development. 相似文献
50.
The cold-shock response — a hot topic 总被引:4,自引:2,他引:2