首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   5677篇
  免费   632篇
  2021年   84篇
  2020年   49篇
  2019年   53篇
  2018年   74篇
  2017年   67篇
  2016年   102篇
  2015年   159篇
  2014年   212篇
  2013年   248篇
  2012年   361篇
  2011年   284篇
  2010年   194篇
  2009年   185篇
  2008年   227篇
  2007年   247篇
  2006年   219篇
  2005年   202篇
  2004年   213篇
  2003年   187篇
  2002年   182篇
  2001年   179篇
  2000年   187篇
  1999年   162篇
  1998年   75篇
  1997年   71篇
  1996年   58篇
  1995年   61篇
  1994年   67篇
  1993年   43篇
  1992年   127篇
  1991年   100篇
  1990年   94篇
  1989年   98篇
  1988年   92篇
  1987年   88篇
  1986年   94篇
  1985年   65篇
  1984年   61篇
  1983年   56篇
  1982年   69篇
  1981年   51篇
  1980年   36篇
  1979年   64篇
  1978年   69篇
  1977年   47篇
  1976年   51篇
  1975年   60篇
  1974年   66篇
  1973年   42篇
  1972年   41篇
排序方式: 共有6309条查询结果,搜索用时 15 毫秒
991.
The HSSP database of protein structure-sequence alignments.   总被引:4,自引:0,他引:4       下载免费PDF全文
HSSP is a derived database merging structural (3-D) and sequence (1-D) information. For each protein of known 3-D structure from the Protein Data Bank (PDB), the database has a multiple sequence alignment of all available homologues and a sequence profile characteristic of the family. The list of homologues is the result of a database search in SwissProt using a position-weighted dynamic programming method for sequence profile alignment (MaxHom). The database is updated frequently. The listed homologues are very likely to have the same 3-D structure as the PDB protein to which they have been aligned. As a result, the database is not only a database of aligned sequence families, but also a database of implied secondary and tertiary structures covering 29% of all SwissProt-stored sequences.  相似文献   
992.
Two novel cDNAs, DNAS1L2 and DNAS1L3, are predicted to encode proteins of 299 and 305 amino acids with 56 and 46% residue identity (71 and 63% similarity), respectively, to deoxyribonuclease I (DNase I). DNAS1L2 is located on a 16p13.3 cosmid, while DNAS1L3 maps to 3p14.3–p21.1 by fluorescencein situhybridization and by PCR analysis of a radiation hybrid panel. Northern analysis revealed DNAS1L3 expression nearly exclusively in liver, while DNAS1L2 expression was detected in brain by RT-PCR. The previously defined DNL1L or DNAS1L1 is expressed highest in heart and skeletal muscle, while DNase I is expressed in the pancreas, parotid gland, and kidney. Thus, to date, four DNase I-like genes that show different tissue expression patterns are known. A comparison of DNAS1L1, DNAS1L2, and DNAS1L3 with the well-characterized DNase I suggests that the DNAS1L proteins are unlikely to be glycosylated or bind actin; however, catalytic and calcium- and DNA-binding residues are conserved, and potentially cleavable signal peptides are present among all these proteins. This analysis also identifies regions of high conservation among these proteins with no currently assigned function.  相似文献   
993.
Marmosets (genus Callithrix) are a diverse group of platyrrhine primates with 13-15 purported taxa, many of them considered endangered. Morphological analyses constitute most of the basis for recognition of these forms as distinct taxa. The purpose of this study was to provide a molecular view, based on mitochondrial control region sequences, of the evolutionary history of the marmosets, concomitant with a molecular phylogenetic perspective on species diversity within the group. An additional purpose was to provide the first comparative examination of a complete New World monkey control region sequence with those of other mammals. The phylogenetic analyses provide convincing support for a split between the Atlantic forest and Amazonian marmosets, with the inclusion of the pygmy marmoset (Cebuella pygmaea) at the base of the Amazonian clade. The earliest branch of the Atlantic forest group was C. aurita. In the Amazonian group, the analyses do not support the recognition of C. humeralifer and the recently described C mauesi as distinct taxa. They do, however, support a clear distinction between C. argentata and a strongly supported mixed clade of C. humeralifer and C. mauesi. In the Atlantic forest group, the phylogenetic tree suggests mixing between C. penicillata, C. kuhli, and possibly C. jacchus. Most of the sequence features characteristic of other mammal control regions were also evident in marmosets, with the exception that conserved sequence blocks (CSBs) 2 and 3 were not clearly identifiable. Tandem repeat units often associated with heteroplasmy in a variety of other mammals were not evident in the marmoset sequences.   相似文献   
994.
What is the relevance of Europeanist ethnography for anthropological theory generally? Considering a region usually regarded as the source rather than an object of anthropology and colonialism alike, seven anthropologists reflexively address, inter alia, the implications of studying spaces already deeply explored by other disciplines, the potential of economic history to defamiliarize Eurocentric models and of recent events to illuminate such concepts as state and market, the meaning of "West" as a specific locus of power and reification, the limits of the "local" as the focus of ethnography, and the tensions among politically and culturally disparate entities within emergent ideologies of cultural unity.  相似文献   
995.
Microbial footprints of Pseudomonas aeruginosa MDC attached for 1 h to clean or silanized glass were analyzed with fluorescently labeled lectin probes. Footprint composition varied, depending on cell physiology and substratum surface chemistry. This suggests that substratum physicochemistry affected the structure of cell surfaces of adsorbed organisms.  相似文献   
996.
Perlecan is a highly conserved heparan sulfate proteoglycan in cartilage and basement membranes. We identified chick perlecan and a 90 KD perlecan fragment in vivo using a newly generated monoclonal antibody. Chick perlecan is, like its human and mouse homologue, a hybrid heparan sulfate/chondroitin sulfate proteoglycan with a core protein of 400 KD. Analysis of the 90 KD fragment by Matrix Assisted Laser Desorption Ionization-Time of Flight Mass Spectrometry (MALDI-TOF MS) and Capillary LC nano Electrospray Ionization tandem MS (LC nano ESI MS/MS) showed that it belonged to domain IV of the perlecan core protein. We found that full-length perlecan and its domain IV fragment are abundant in embryonic vitreous body and serum. Their expression in vitreous and serum is greatly down-regulated shortly after hatching of the chick. We speculate that the abundance of perlecan in the embryonic circulation and vitreous reflects the ongoing formation of new BMs in the expanding vascular system and the growing retina. In addition, we found that perlecan as a substrate does not support, rather inhibits neurite outgrowth.  相似文献   
997.
The peptide sequence DSYG(893-896) of the sheep sodium pump alpha 1 subunit is highly conserved among all K(+)-transporting P-type ATPases. To obtain information about its function, single mutations were introduced and the mutants were expressed in yeast and analysed for enzymatic activity, ion recognition, and alpha/beta subunit interactions. Mutants of Ser894 or Tyr895 were all active. Conservative phenylalanine and tryptophan mutants of Tyr895 displayed properties that were similar to the properties of the wild-type enzyme. Replacement of the same amino acid by cysteine, however, produced heat-sensitive enzymes, indicating that the aromatic group contributes to the stability of the enzyme. Mutants of the neighbouring Ser894 recognized K(+) with altered apparent affinities. Thus, the Ser894-->Asp mutant displayed a threefold higher apparent affinity for K(+) (EC(50) = 1.4 +/- 0.06 mm) than the wild-type enzyme (EC(50) = 3.8 +/- 0.33 mm). In contrast, the mutant Ser894-->Ile had an almost sixfold lower apparent affinity for K(+) (EC(50) = 21.95 +/- 1.41 mm). Mutation of Asp893 or Gly896 produced inactive proteins. When an anti-beta 1 subunit immunoglobulin was used to co-immunoprecipitate the alpha 1 subunit, neither the Gly896-->Arg nor the Gly896-->Ile mutant could be visualized by subsequent probing with an anti-alpha 1 subunit immunoglobulin. On the other hand, co-immunoprecipitation was obtained with the inactive Asp893-->Arg and Asp893-->Glu mutants. Thus, it might be that Asp893 is involved in enzyme conformational transitions required for ATP hydrolysis and/or ion translocation. The results obtained here demonstrate the importance of the highly conserved peptide DSYG(893-896) for the function of alpha/beta heterodimeric P-type ATPases.  相似文献   
998.
Respiratory allergies represent a failure to generate nonpathogenic responses to innocuous foreign materials. Herein we assessed the role of the sensitizing dose of allergen in this response/nonresponse paradigm, sensitizing BALB/c mice with 5 ng-2 microg of OVA-alum and assessing their responses to repeated OVA aerosol challenge. Mice sensitized with < or = 25 ng of OVA-alum did not develop atopic antibodies, airway hyperresponsiveness (AHR), eosinophilia, or pulmonary Th2 responses, but the 25-ng group animals did develop significant IgA responses. The mice sensitized with 100 ng of OVA-alum developed AHR in the absence of detectable allergic disease, while the mice sensitized with 250 ng-2 microg of OVA/alum developed full-spectrum allergic disease (i.e., eosinophilia, IgE, IgG1, pulmonary Th2 cytokine responses, and AHR). These data indicate that limiting doses of allergen can differentially induce IgA or AHR in the absence of atopic disease in mice.  相似文献   
999.
Pathological alterations in dendrites and axons (i.e., neuritic pathologies) occur in the normal aging brain as well as in brains from elders with mild cognitive impairment and neurodegenerative dementia. These alterations may correlate with clinical measures of cognitive abilities, but the contribution of neuropil threads (NTs), which constitute 85-90% of cortical tau pathology, has not been clear because of the lack of quantitative methodologies. We combined quantitative fractionation and image analysis to devise a strategy for measuring the burden of tau-rich NTs in the entorhinal and perirhinal cortex of brains from elders with and without cognitive impairment, including dementia due to Alzheimer's disease (AD). On the basis of data presented here using this novel strategy, we conclude that this quantitative imaging technique will facilitate efforts to determine the behavioral correlations of neuritic lesions in AD and other brain disorders.  相似文献   
1000.
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号