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   2147篇
  免费   138篇
  2023年   13篇
  2022年   30篇
  2021年   72篇
  2020年   36篇
  2019年   46篇
  2018年   64篇
  2017年   45篇
  2016年   71篇
  2015年   118篇
  2014年   108篇
  2013年   171篇
  2012年   206篇
  2011年   222篇
  2010年   120篇
  2009年   107篇
  2008年   136篇
  2007年   113篇
  2006年   100篇
  2005年   90篇
  2004年   94篇
  2003年   72篇
  2002年   56篇
  2001年   10篇
  2000年   12篇
  1999年   15篇
  1998年   13篇
  1997年   6篇
  1996年   11篇
  1995年   13篇
  1994年   3篇
  1993年   10篇
  1992年   8篇
  1991年   11篇
  1990年   8篇
  1989年   9篇
  1988年   7篇
  1987年   5篇
  1986年   4篇
  1985年   6篇
  1984年   3篇
  1983年   6篇
  1981年   3篇
  1980年   5篇
  1979年   4篇
  1977年   2篇
  1975年   2篇
  1974年   2篇
  1973年   2篇
  1968年   3篇
  1964年   2篇
排序方式: 共有2285条查询结果,搜索用时 31 毫秒
1.
It has long been debated whether the mind consists of specialized and independently evolving modules, or whether and to what extent a general factor accounts for the variance in performance across different cognitive domains. In this study, we used a hierarchical Bayesian model to re-analyse individual level data collected on seven primate species (chimpanzees, bonobos, orangutans, gorillas, spider monkeys, brown capuchin monkeys and long-tailed macaques) across 17 tasks within four domains (inhibition, memory, transposition and support). Our modelling approach evidenced the existence of both a domain-specific factor and a species factor, each accounting for the same amount (17%) of the observed variance. In contrast, inter-individual differences played a minimal role. These results support the hypothesis that the mind of primates is (at least partially) modular, with domain-specific cognitive skills undergoing different evolutionary pressures in different species in response to specific ecological and social demands.  相似文献   
2.
Tn1935, a 23.5-kb transposon mediating resistance to ampicillin, kanamycin, mercury, spectinomycin, and sulfonamide was isolated from pZM3, an IncFIme virulence plasmid from Salmonella wien. Tn1935 possesses the entire sequence of Tn21 and contains two additional DNA segments of 0.95 and 2.7 kb carrying the ampicillin and kanamycin resistance genes, respectively. The latter is part of a composite element since it is flanked by two IS15-like insertion sequences (IS1936) in direct orientation. IS1936 is about 800 bp long and is closely related to IS15 delta, IS26, IS46, IS140, and IS176. Functional analysis of IS1936-mediated cointegrates shows that both insertion sequences are active and able to form cointegrates at the same frequency. Resolution of the cointegrates requires the presence of the host Rec system. The presence of the composite IS1936-element within Tn1935 supports the hypothesis that multidrug resistance transposons evolved by insertion of antibiotic determinants which are themselves transposable.  相似文献   
3.
Summary Chromosome fragile sites are inducible by aphidicolin in cultured human lymphocytes. To assess the frequency and distribution of these common fragile sites in the general population, a cytogenetic survey was performed on 126 subjects, 59 males and 67 females, whose age ranged from 1 day to 72 years. Common fragile sites, induced by aphidicolin, were widespread and showed a remarkably different sensitivity among individuals; age influenced the overall frequency of fragile sites. Moreover, both age and sex seemed to modulate the expression of specific fragile sites. In our population, the most common fragile sites were: 3p14, 16q23, Xp22, 6q26, 1p31, 4q31, 1p22, 7q22, 2q33, 3q27, 2q31, 7q32, 14q24, 10q22, 5q31, 2q37, 6p21.  相似文献   
4.
Cytometry and flow cytometry were used to study characteristics of fluorescence of the DNA-DAPI complex in nuclei released from different fresh and formaldehyde-fixed pea ( Pisum sativum L. cv. Lincoln) tissues. The two methods of isolation are compared and discussed as well as their possible use for quantitative analysis of DNA in plant tissues. With fixed tissues it is possible to obtain a number of nuclei sufficient for the flow cytometric analysis, even using small amounts of plant tissue.  相似文献   
5.
Abstract: Previous data indicate that the injection of dopaminergic drugs induces changes in cerebellar 3',5'-guanosine monophosphate (cGMP) content. Accordingly, we have investigated the effects of haloperidol, sulpiride, or apomorphine on cerebellar prostaglandin (PG) concentration, a parameter related to cGMP content. Results obtained show that dopamine receptor blocking agents, such as haloperidol and sulpiride, significantly decrease cerebellar PGE2 and PGF concentrations, while opposite changes are induced by apomorphine, a dopamine receptor agonist.  相似文献   
6.
Abstract: The effects of calcitonin on neurochemical parameters related to the tuberoinfundibular dopaminergic system have been investigated in an attempt to elucidate how calcitonin decreases serum prolactin levels. Intracerebroventricular human or salmon calcitonin injection decreases serum prolactin, medial basal hypothalamic dopamine (DA) and dihydroxyphenylacetic acid (DOPAC) and hypophysial DA and increases hypophysial DOPAC. Results suggest that calcitonin may decrease prolactin secretion via the tuberoinfundibular dopaminergic system.  相似文献   
7.
Between November and December 1987, 201 subjects from three communities of the Cordillera Province, Santa Cruz Department, South-East Bolivia (Camiri, Boyuibe, and Gutierrez) were examined for serum iron levels. Sixty-three (31.3%) were iron deficient. There were no significant differences between males and females (37.2% and 26.9%, respectively) and among the three localities studied. Iron deficiency was highest in the 3- to 9-year-old age group and lowest in the 30- to 39-year-old group. Considering the prevalence in the same localities of infections by hookworms, Trichuris trichiura, Strongyloides stercoralis, and Trichostrongylus, it is suggested that hookworms and possibly other intestinal parasites may be an important cause of the observed iron deficiency.  相似文献   
8.
The galactosyltransferase, GalT-4, which catalyses the biosynthesisin vitro of neolactotetraosylceramide, nLcOse4Cer (Gal1-4GleNAc1-3Gal1-4Glc-Cer) from lactotriaosylceramide, LcOse3Cer (GlcNAc1-3Gal1-4Glc-Cer), and UDP-galactose has been purified 107 500-fold from a mineral oil induced mouse T-lyphoma P-1798, using affinity columns. The purified enzyme is partially stabilized in the presence of phospholipid liposomes. Two closely migrating protein bands of apparent molecular weights 56 kDa and 63 kDa were observed after sodium dodecyl sulfate polyacrylamide gel electrophoresis of highly purified mouse GalT-4. These two protein bands, when subjected to limited proteolysis, resulted in three peptides with identical mobilities indicating amino acid sequence identity between the proteins. Both protein bands from P-1798 gave a positive immunostain when tested with polyclonal antibody against bovine lactose synthetase (UDP-Gal:Glc 4-galactosyltransferase) following Western blot analysis on nitrocellulose paper. The enzyme has a pH optimum between 6.5 and 7.0 and like all other galactosyltransferases, GalT-4 has absolute requirements for divalent cation (Mn2+). TheK m values for the substrate LcOse3Cer and donor UDP-galactose are 110 and 250 µm, respectively. Substrate competition studies with LcOse3Cer and either asialo-agalacto-1-acid glycoprotein orN-acetylglucosamine revealed that these reactions might be catalysed by the same protein. The only other glycolipid which showed acceptor activity toward the purified GalT-4 was iLcOse5Cer (GlcNAc1-1-3Gal1-4Lc3), the precursor for polylactosamine antigens. However, competition studies with these two active substrates using the most purified enzyme fraction, revealed that these two reactions might be catalysed by two different proteins since the experimental values were closer to the theoretical values calculated for two enzymes. Interestingly however, it seems that the GalT-4 from P-1798 has an absolute requirement for anN-acetylglucosamine residue in the substrate since the lyso-derivative (GlcNH21-3Gal1-4Glc-sphingosine) of the acceptor glycolipid LcOse3Cer is completely inactive as substrate while theK m andV max of the reacetylated substrate (GlcNac1-3Gal1-4Glc-acetylsphingosine) was comparable with LcOse3Cer. Autoradiography of the radioactive product formed by purified P-1798 GalT-4 confirmed the presence of nLcOse4Cer, as the product cochromatographed with authentic glycolipid. The monoclonal antibody IB-2, specific for nLcOse4Cer, also produced a positive immunostained band on TLC as well as giving a positive ELISA when tested with radioactive product obtained using a highly purified enzyme from mouse P-1798 T-lymphoma.Abbreviations EDTA ethylenediamine tetraacetate - ME -mercaptoethanol - PEG polyethylene glycol - PBS phosphate buffered saline - Suc sucrose - Mn2+ manganese - Gal galactose - GlcNAc N-acetylglucosamine - UDP-Gal Uridine diphosphate galactose - Ab antibody - SDS sodium dodecyl sulphate - PAGE polyacrylamide gel electrophoresis - ECB embryonic chicken brain - Cer ceramide - nLc4 or NlcOse4Cer Gal1-4GleNac1-3Gal1-4Glc-Cer, neoLactotetraosylceramide - Lc3 or LcOse3Cer GlcNac1-3Gal1-4Glc-Cer, lactotriaosylceramide - iLc5 iLcOse5Cer, GlcNAc1-3nLcOse4Cer - nLc6 nLcOse6Cer, Gal1-4iLcOse5Cer - SAGal1AGP asialo-agalacto1-acid glycoprotein - TLC thin layer chromatography  相似文献   
9.
The morphogenic effect of different light wavelengths on in vitro rooting of Prunus insititia GF655-2 in relation to the presence of napthaleneacetic acid (NAA) in the culture medium was investigated. Results of experiments in which plantlets were rooted in NAA enriched medium showed that the presence of auxin induced rooting even in the dark after an initial lag period. Illumination of the cultures with Red light was as effective in promoting rooting as treatment with 0.5 M NAA; Red was more active in stimulating rooting in the short term than was NAA. The pattern of root formation resulting from the addition of NAA appeared to dominate development under White, Blue and Far Red treatments. Although it was possible to correlate the rooting response to the phytochrome photoequilibrium induced by the light treatments used, there arises a possible interference of specific Blue absorbing photoreceptors.Abbreviations B Blue - FR Far Red - HIR High Irradiance Response - Pfr active (far-red absorbing) form of phytochrome - Ptot total phytochrome - R Red - W White - NAA -naphtaleneacetic acid - BA benzyladenine - IAA indole 3-acetic acid  相似文献   
10.
Chronic Lymphocytic Leukaemia (CLL) is the most common adult B-cell leukaemia and despite improvement in patients' outcome, following the use of targeted therapies, it remains incurable. CLL supportive microenvironment plays a key role in both CLL progression and drug resistance through signals that can be sensed by the main components of the focal adhesion complex, such as FAK and PYK2 kinases. Dysregulations of both kinases have been observed in several metastatic cancers, but their role in haematological malignancies is still poorly defined. We characterized FAK and PYK2 expression and observed that PYK2 expression is higher in leukaemic B cells and its overexpression significantly correlates with their malignant transformation. When targeting both FAK and PYK2 with the specific inhibitor defactinib, we observed a dose–response effect on CLL cells viability and survival. In vivo treatment of a CLL mouse model showed a decrease of the leukaemic clone in all the lymphoid organs along with a significant reduction of macrophages and of the spleen weight and size. Our results first define a possible prognostic value for PYK2 in CLL, and show that both FAK and PYK2 might become putative targets for both CLL and its microenvironment (e.g. macrophages), thus paving the way to an innovative therapeutic strategy.  相似文献   
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