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   3937篇
  免费   179篇
  国内免费   359篇
  2023年   55篇
  2022年   58篇
  2021年   71篇
  2020年   70篇
  2019年   102篇
  2018年   73篇
  2017年   61篇
  2016年   55篇
  2015年   79篇
  2014年   121篇
  2013年   195篇
  2012年   113篇
  2011年   120篇
  2010年   94篇
  2009年   177篇
  2008年   199篇
  2007年   224篇
  2006年   209篇
  2005年   221篇
  2004年   223篇
  2003年   169篇
  2002年   161篇
  2001年   137篇
  2000年   151篇
  1999年   115篇
  1998年   103篇
  1997年   87篇
  1996年   91篇
  1995年   95篇
  1994年   85篇
  1993年   75篇
  1992年   97篇
  1991年   79篇
  1990年   49篇
  1989年   55篇
  1988年   49篇
  1987年   58篇
  1986年   37篇
  1985年   48篇
  1984年   46篇
  1983年   15篇
  1982年   25篇
  1981年   22篇
  1980年   24篇
  1979年   26篇
  1978年   20篇
  1977年   6篇
  1976年   11篇
  1974年   6篇
  1973年   5篇
排序方式: 共有4475条查询结果,搜索用时 31 毫秒
1.
Few prospective studies support the use of anticoagulation during the acute phase of ischemic stroke, though observational data suggest a role in certain populations. Depending on the mechanism of stroke, systemic anticoagulation may prevent recurrent cerebral infarction, but concomitantly carries a risk of hemorrhagic transformation. In this article, we describe a case where anticoagulation shows promise for ischemic stroke and review the evidence that has discredited its use in some circumstances while showing its potential in others.  相似文献   
2.
The restriction deficient mutant 8195 of Acholeplasma laidlawii strain JA1 was transformed by the promiscuous streptococcal plasmid vector pNZ18 at a frequency of 4 x 10(-4)/cfu. The plasmid was maintained without structural rearrangements but was lost in the absence of a selection pressure, i.e. kanamycin or neomycin. Transformed primary colonies were easily recognized due to a different colony morphology. Replica filter plating, previously not obtained with mycoplasmas, was achieved using pNZ18 as a marker by incubating the replica filters with the cell side down on the new agar plates. These findings should greatly facilitate the genetic and functional analysis of A. laidlawii.  相似文献   
3.
A propiconazole-resistant Trichoderma harzianum strain with high phylloplane survival capability was transformed with the E. coli hygromycin B phosphotransferase gene (hph), coding for hygromycin B resistance. Four transformants were analysed for survival ability on the phylloplane of tomato plants grown under glasshouse conditions in comparison with their prototype and a yellow, hygromycin B-sensitive mutant. Over 2 weeks, the four transformants showed higher survival rates in comparison with the wild-type strain. The yellow mutant TF3/973 did not significantly differ in survival from the transformants. Both hygromycin B resistance and mitotic stability of transformants were evaluated during growth in vitro and after reisolation from tomato phylloplane. Hybridization patterns with the complete plasmid indicated that all four transformants were mitotically stable after several rounds of vegetative growth without selective pressure and during 2 weeks on tomato plants. None of the transformants had lost the ability to grow in the presence of both propiconazole and hygromycin B after growth under the same conditions. The results are discussed in relation to risk assessment of the release of transgenic fungi.  相似文献   
4.
Factors influencing the fate and impact of hybrids between crop plants and their related species operate from the early zygote, through to plant establishment in different habitats, to their ability to form self-sustaining populations. Many of the classes of genes being introduced by modern methods of genetic modification are similar to those manipulated by conventional plant breeding. In assessing the impact of transgenes in hybrids between crops and related species, therefore, it is important to be informed about the consequences of hybridization between conventionally bred varieties and their relatives. Some transgenes will have novel effects (e.g. production of pharmaceutical substances or certain fatty acids) on plants, and are likely to need specific assessment studies to determine their impact on hybrids. This will be particularly important if there is the possibility of these transgenes becoming established in wild populations. Some recommendations for further research are outlined.  相似文献   
5.
The asymptotic equivalence of nonparametric tests and parametric tests based on rank-transformed data (CONOVER and IMAN , 1981) can be extended to the case of censoring. This paper presents generalized rank transformations for analyses of censored data, of interval-censored data and of survival data with uncertain causes of death. A Monte Carlo study and an analysis of leukemia remission times demonstrate excellent agreement of suggested procedures with GEHAN 'S (1965) and PRENTICE 'S (1978) tests.  相似文献   
6.
Long noncoding RNA HOX antisense intergenic RNA (HOTAIR) is overexpressed in many types of cancers, and substantial evidence has suggested a link between cancers and HOTAIR. In the present study, we reviewed the structure and the corresponding biologic function of HOTAIR to clarify its molecular mechanism in cancer progression. HOTAIR promotes proliferation, invasion, and migration, and inhibits apoptosis in cancer cells. HOTAIR also participates in the pathogenesis and progression of cancer by regulating inflammation and immune signaling. These findings suggested that HOTAIR is a novel biomarker in human cancers.  相似文献   
7.
A highly efficient method of regenerating fertile, phenotypically normal plants from shoot apex cultures of T. aestivum was developed. The hypodermal layer (L2) of the vegetative apex containing germ line precursor cells could be located with bright field microscopy and targeted for microinjection. Fluorescently labelled dextrans were used as markers to develop a microinjection procedure which did not disrupt nuclear or cytoplasmic structure. This procedure was used to inject plasmid DNA into L2 cells. Capillary microinjection did not shear the plasmid DNA and delivery of DNA was confirmed by polymerase chain reaction analysis of DNA isolated from injected apices. The significance of these findings in relation to the development of cereal transformation systems will be discussed.  相似文献   
8.
9.
A pCTVHyg plasmid was constructed in a unicellular green alga Chlamydomonas reinhardtii Dang. by using the hygromycin phosphotransferase gene (hpt) as a selectable marker and the Escherichia coli transposon Tn5 under the early SV40 viral gene promoter. CW-15 mutant cells devoid of cell walls were transformed by electroporation in an electric field of 1 kV/cm and a pulse duration of 2 ms. A suspension density of 106 cell/ml and the mid-logarithmic growth phase were the optimum conditions for transformation, producing up to 103 hygromycin-resistant (HygR) clones per 106 HygR recipient cells. Exogenous DNA integrated in the nuclear genome of C. reinhardtii was steadily inherited in subsequent generations within at least a 8-month period; however, the HygR trait manifestation was not stable. The comparative analysis of frequencies in codon usage in hpt and in the nuclear genes of C. reinhardtii significantly excluded the possibility that the bias in codon usage was the primary factor affecting foreign gene expression. The advantages of using theCW-15 mutant and the described selection system are discussed in the context of heterologous transformation of C. reinhardtii.  相似文献   
10.
Comment on: Rokavec M, et al. Mol Cell 2012; 45:777-89.  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