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   30279篇
  免费   1673篇
  国内免费   2665篇
  34617篇
  2023年   501篇
  2022年   709篇
  2021年   906篇
  2020年   781篇
  2019年   1062篇
  2018年   988篇
  2017年   728篇
  2016年   718篇
  2015年   885篇
  2014年   1807篇
  2013年   2026篇
  2012年   1280篇
  2011年   1761篇
  2010年   1377篇
  2009年   1479篇
  2008年   1601篇
  2007年   1597篇
  2006年   1439篇
  2005年   1264篇
  2004年   1166篇
  2003年   1036篇
  2002年   829篇
  2001年   640篇
  2000年   584篇
  1999年   565篇
  1998年   502篇
  1997年   452篇
  1996年   434篇
  1995年   443篇
  1994年   414篇
  1993年   367篇
  1992年   368篇
  1991年   305篇
  1990年   258篇
  1989年   240篇
  1988年   185篇
  1987年   197篇
  1986年   148篇
  1985年   280篇
  1984年   309篇
  1983年   240篇
  1982年   290篇
  1981年   231篇
  1980年   242篇
  1979年   188篇
  1978年   169篇
  1977年   138篇
  1976年   155篇
  1975年   103篇
  1974年   96篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
61.
Backgroundobesity is a major risk factor for many metabolic diseases such as diabetes and cardiometabolic diseases. This study aimed to evaluate the association of plasma and urinary barium concentrations, CYP19A1 gene polymorphisms, and their interaction with central obesity in a rural Chinese population.Methodsrestricted cubic spline model was used to explore the dose-response relationship between barium and the risk of developing central obesity and waist circumference; logistic regression model was used to assess the association between barium, CYP19A1 gene polymorphisms and their interaction with central obesity.Resultsthe results of the restricted cubic spline model showed that plasma barium concentration was linearly associated with the risk of developing central obesity and non-linearly associated with waist circumference. Logistic regression analysis showed that participants with Q4 plasma barium concentration exhibited a higher risk of central obesity compared to participants with Q1 barium concentration; participants carrying the rs10046-AA gene exhibited a lower risk of central obesity than those carrying the rs10046-G(GG+GA) gene; participants carrying the rs10046-GA genotype showed 1.754 times higher risk of central obesity than those carrying rs10046-GG+AA genotype. There was a significant interaction between plasma barium and CYP19A1 gene polymorphism on central obesity.Conclusionthe development of central obesity was associated with plasma barium and CYP19A1.  相似文献   
62.
Plants of two potato clones which, in preliminary greenhouse assessments, showed resistance to multiplication and accumulation of potato leafroll virus (PLRV) were graft or aphid inoculated with the virus and grown in the greenhouse; plants of a moderately susceptible cultivar were used for comparison in all experiments. A high concentration of aphid‐borne inoculum was used to ensure strong infection pressure. Clone M62759 appeared to be highly resistant to PLRV infection, whereas clone PS1706 was more susceptible. Both clones expressed a high level of resistance to virus multiplication, when primary or secondary infection was assayed by enzyme‐linked immunosorbent assay. Moreover, PLRV was detected in only few or none of the progeny plants of clone M62759, which thus strongly inhibited virus transport to tubers. The study on PLRV translocation from aphid‐inoculated shoots to uninoculated shoots sprouted from the same tubers showed that no specific mechanisms are likely to impair PLRV movement through the tubers of the resistant genotypes. These results indicate that three valuable components of the resistance to PLRV are probably closely linked in the genotype, a combination that seems to occur rather rarely in potato clones. Nevertheless, selecting potato genotypes for the complex resistance to PLRV may prove to be a worthwhile part of breeding programmes, provided that the genetic mechanisms governing particular types of resistance are better recognized.  相似文献   
63.
Absorption and imagery locate immune responses in the body   总被引:2,自引:0,他引:2  
Imagery instructions specifying mucosal immunity should alter mucosal immunoglobulin A (m-IgA) levels in high absorbers, whose intent concentration evokes intense physiological responses. After screening for health status, 121 high or low absorbers were randomly assigned to either Relaxation Alone (R), Relaxation with Mucosal Immune Imagery (RI), or Vigilance Task control (VT). Before and after one 60-min intervention, subjects reported theory-relevant psychological variables and provided 5ml whole saliva, which was immediately frozen and assayed lateren masse with enzyme-linked immunoabsorbence (ELISA). MANOVA analysis of psychological variables replicated past research. ANOVA on residualized m-IgA found Time × Absorption interaction and Condition main effects. High more than low absorbers responded to relaxation with mucosal immune imagery by producing higher m-IgA. High absorbers appear able to locate where their immune systems will respond. Individual differences like absorption level need to be emphasized in diagnosis and treatment responsiveness.National Institutes of HealthM. Banks (Jasnoski) Gregerson, Department of Psychology, The George Washington University, changed to The Family Therapy Institute; Ingram M. Roberts, The George Washington University Medical Center, changed to Department of Medicine, Bridgeport Hospital; and Michael M. Amiri, The George Washington University Medical Center, changed to the Department of Neuroscience, NINDS Branch, National Institutes of Health. This research supported by an intra-mural BioMedical Research Grant from The George Washington University, was presented at the 1992 Annual Meeting of the Eastern Psychological Association, Boston, Massachusetts. Special thanks are extended to the following students who assisted instrumentally at various stages: undergraduates Lina Alathari, S. Theodor King, Beth Lieberman, Parisa Lotfi, Anita McClenon, and Karen Siscoe, and graduate student Mariken Hasert.  相似文献   
64.
65.
Affinity tag AG consisting of immunoglobulin G (lgG)-binding domains of protein A from Staphylococcus aureus (EDABC) and those of protein G from Streptococcus strain G148 (C2C3) were used to facilitate immobilization of beta-galactosidase (betagal) from Escherichia coli. Poly(methylmethacrylate/N-isopropylacrylamide/methacrylic acid) [P(MMA/NIPAM/MAA)] and poly(styrene/N-isopropylacrylamide/methacrylic acid) [P(St/NIPAM/MAA)] latex particles, which show thermosensitivity, were used as support materals to prepare affinity adsorbents. Human gamma-globulin (HgammaGb), whose major fraction is lgG, was used as an affinity ligand and was covalently immobilized onto the both latex particles by the carbodiimide method under various conditions. A fusion protein, AGbetagal, was immobilized at pH 7.3 by the specific binding of affinity tag to these affinity adsorbents. The amount of adsorbed AGbetagal per unit amount of immobilized HgammaGb, namely, efficiency of ligand utilization, was strongly affected by the type of latex particles and pH value for HgammaGb immobilization. The efficiency of ligand utilization was maximum in the affinity adsorbents prepared at pH 6.0 to 7.0, and that in the HgammaGb-P(MMA/NIPAM/MAA) latex particles was high. This result could be explained by the conformation and orientation of immobilized HgammaGb molecules. Immobilized AGbetagal retained approximately 75% of its activity in solution and the binding is stable enough to allow repeated use. These results clearly demonstrate that combination of the affinity tag AG and the affinity adsorbents, based on the thermosensitive latex particles, offers a simple and widely applicable method for preparation of immobilized enzyme with high activity. (c) 1995 John Wiley & Sons, Inc.  相似文献   
66.
DNA shuffling is a practical process for directed molecular evolution which uses recombination to dramatically accelerate the rate at which one can evolve genes. Single and multigene traits that require many mutations for improved phenotypes can be evolved rapidly. DNA shuffling technology has been significantly enhanced in the past year, extending its range of applications to small molecule pharmaceuticals, pharmaceutical proteins, gene therapy vehicles and transgenes, vaccines and evolved viruses for vaccines, and laboratory animal models.  相似文献   
67.
β-Lactamase II has two metal-binding sites. The electronic spectra of Cd(II)- and Co(II)-substituted β-lactamase II have been investigated. It is suggested that a thiol ligand is involved in metal binding at the first site. The stoichiometric dissociation constants for Co(II) binding to β-lactamase II were estimated to be 0.13 and 2.66 mM (pH 6.0, 4°C, 1 M NaCl) by equilibrium dialysis. Competition between Zn(II) and Co(II) for the first metal binding site suggests a value of 0.7 μM (pH 6.0, 30°C, 1 M NaCl) for the dissociation constant o Zn(II).The electronic spectra of the Co(II) enzyme lead to the suggestion that the coordination geometries around the metal ions in the first and second sites are related to those of a distorted tetrahedron and octahedron, respectively.  相似文献   
68.
69.
Sackett GP  Erwin J 《Theriogenology》1981,15(5):505-511
In an effort to determine empirically the proper age for separation of pigtailed macaque (Macaca nemestrina ) infants from their dams in a large domestic breeding colony, we conducted a retrospective survey of 1,592 infants. Survivorship was highest for infants not separated from their dams at all and for those separated within the first four months of life. Survivorship was poor for animals separated during or after the fifth month. Thus, not weaning or weaning early are the most acceptable management strategies in terms of mortality risk. This result is paradoxical with regard to nutritional considerations, but appears to be related to a sensitive period in social development. The relative advantages and disadvantages of various weaning ages are discussed.  相似文献   
70.
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