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   9878篇
  免费   986篇
  国内免费   388篇
  11252篇
  2024年   5篇
  2023年   54篇
  2022年   147篇
  2021年   266篇
  2020年   230篇
  2019年   290篇
  2018年   335篇
  2017年   259篇
  2016年   380篇
  2015年   558篇
  2014年   641篇
  2013年   686篇
  2012年   914篇
  2011年   768篇
  2010年   556篇
  2009年   501篇
  2008年   645篇
  2007年   581篇
  2006年   521篇
  2005年   492篇
  2004年   508篇
  2003年   447篇
  2002年   395篇
  2001年   222篇
  2000年   170篇
  1999年   162篇
  1998年   96篇
  1997年   59篇
  1996年   41篇
  1995年   33篇
  1994年   36篇
  1993年   15篇
  1992年   28篇
  1991年   24篇
  1990年   20篇
  1989年   15篇
  1988年   17篇
  1987年   11篇
  1986年   12篇
  1985年   14篇
  1983年   7篇
  1982年   6篇
  1979年   9篇
  1978年   7篇
  1977年   6篇
  1975年   5篇
  1974年   6篇
  1973年   6篇
  1969年   7篇
  1967年   5篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
41.
This study was undertaken to investigate the development of immature oocytes after their fusion with male somatic cells expressing red fluorescence protein (RFP). RFP‐expressing cells were fused with immature oocytes, matured in vitro and then parthenogenetically activated. Somatic nuclei showed spindle formation, 1st polar body extrusion after in vitro maturation and protruded the 2nd polar body after parthenogenetic activation. RFP was expressed in the resultant embryos; two‐cell stage and blastocysts. Chromosomal analysis showed aneuploidy in 81.82% of the resulting blastocysts while 18.18% of the resulting blastocysts were diploid. Among eight RFP‐expressing blastocysts, Xist mRNAs was detected in six while Sry mRNA was detected in only one blastocyst. We propose “prematuration somatic cell fusion” as an approach to generate embryos using somatic cells instead of spermatozoa. The current approach, if improved, would assist production of embryos for couples where the male partner is sterile, however, genetic and chromosomal analysis of the resultant embryos are required before transfer to the mothers.  相似文献   
42.
江西九连山自然保护区昆虫区系分析   总被引:10,自引:1,他引:10  
据对有较详细分布资料的1312种昆虫的分析,该区的昆虫区组成以东洋界成员为主体计817种,占总数的62.27%,说明本区昆虫属于东洋界范畴,按中国动物区划分析表明,华中,华南,西南三区的昆虫亲缘关系最为密切,阐述了182种昆虫在我国东部地区分布的南北限。  相似文献   
43.
ABSTRACT: BACKGROUND: Careful hand hygiene (HH) is the single most important factor in preventing the transmission of infections to patients, but compliance is difficult to achieve and maintain. A lack of understanding of the processes involved in changing staff behaviour may contribute to the failure to achieve success. The purpose of this study was to identify nurses' and administrators' perceived barriers and facilitators to current HH practices and the implementation of a new electronic monitoring technology for HH. METHODS: Ten key informant interviews (three administrators and seven nurses) were conducted to explore barriers and facilitators related to HH and the impact of the new technology on outcomes. The semi structured interviews were based on the Theoretical Domains Framework by Michie et al. and conducted prior to intervention implementation. Data were explored using an inductive qualitative analysis approach. Data between administrators and nurses were compared. RESULTS: In 9 of the 12 domains, nurses and administrators differed in their responses. Administrators believed that nurses have insufficient knowledge and skills to perform HH, whereas the nurses were confident they had the required knowledge and skills. Nurses focused on immediate consequences, whereas administrators highlighted long-term outcomes of the system. Nurses concentrated foremost on their personal safety and their families' safety as a source of motivation to perform HH, whereas administrators identified professional commitment, incentives, and goal setting. Administrators stated that the staff do not have the decision processes in place to judge whether HH is necessary or not. They also highlighted the positive aspects of teams as a social influence, whereas nurses were not interested in group conformity or being compared to others. Nurses described the importance of individual feedback and self-monitoring in order to increase their performance, whereas administrators reported different views. CONCLUSIONS: This study highlights the benefits of using a structured approach based on psychological theory to inform an implementation plan for a behavior change intervention. This work is an essential step towards systematically identifying factors affecting nurses' behaviour associated with HH.  相似文献   
44.
Hericium erinaceus is a well known edible and medicinal mushroom used in East-Asia. Recently, H. erinaceus has attracted a lot of attention owing to its antitumor, immuno-modulatory, and cytotoxic effect. It has been postulated that the fruiting body of H. erinaceus contains a polysaccharide that is similar to β-D-glucan, which is known to have antitumor activity against Sarcoma 180. However, optimized liquid culture conditions for enhanced polysaccharide productivity have yet to be developed, which is a necessary step for industrial applications. Therefore, the aim of this study was to determine the optimal liquid culture conditions for maximum polysaccharide production. In shake flask cultures, the optimal concentration of ascorbic acid was found to be 2.0 g/L, which prevented the broth from changing color from yellow to black. The optimal culture conditions were determined to be 23°C, 200 rpm, and a 10% inoculum size, at an uncontrolled initial pH. In addition, the modified medium contained 20 g/L glucose, 10 g/L yeast extract, and 2.0 g/L ascorbic acid. The maximum mycelial biomass and exo-polysaccharide (EPS) production in the modified medium containing uracil was 13.43 and 1.26 g/L, respectively.  相似文献   
45.
Members of the Pumilio (Pum) family of RNA-binding proteins act as translational repressors and are required for germ cell development and asymmetric division. We identified the chicken Pum1 and Pum2 genes and analyzed their expression patterns in various tissues. Comparative sequence analysis of the Pum1 and Pum2 proteins from the drosophila, chicken, mouse, and human revealed a high degree of evolutionary conservation in terms of the levels of homology of the peptide sequences and the structure of Pumilio homology domain (PUM-HD), C-terminal RNA-binding domain, with similar spacing between the adjacent Pum eight tandem repeats. In addition, phylogenetic patterns of pumilio family showed that Pum 1 and 2 of chicken are more closely related to those of mouse and human than other species and Pum1 is more conserved than Pum2. Using real-time RT-PCR, the expression levels of the Pum1 and Pum2 genes were found to be highest in hatched female gonads, and high-level expression of Pum2 was detected in 12-day and hatched gonads among the various chicken embryonic tissues tested. In adult tissues, the expression levels of Pum1 and Pum2 were expressed at higher levels in the testis and muscle than in any other tissue. The characteristics of the tissue-specific expression of Pum genes suggest that Pum1 and Pum2 have effects crucially in particular stage during development of chicken gonads depending on sexual maturation.  相似文献   
46.
Cervical cancer is a serious threat to women’s health and is the third most common malignancy in women worldwide. Recent studies indicate that the long non-coding RNA CCAT1 plays a role in the malignant behavior of many tumors. However, the role of CCAT1 in cervical cancer is still unknown. Our aim is to evaluate the expression and investigate the regulatory role and potential mechanism of CCAT1 in cervical cancer. CCAT1 expression was measured by qRT-PCR. In addition, CCK-8 assays, colony formation assays, qRT-PCR assays, Transwell assays and xenograft experiments were performed to determine the role of CCAT1 in the proliferation and invasion in cervical cancer cells. The expression of CCAT1 in the cervical cancer tissues was higher than in the adjacent normal tissues. Overexpressing CCAT1 promoted cervical cancer cell proliferation, colony formation, and invasion in vitro. Elevated CCAT1 suppressed miR-181a expression, which was accompanied by an increased expression of MMP14 and HB-EGF. In contrast, knocking down CCAT1 resulted in increased expression of miR-181a, along with decreased expression of MMP14 and HB-EGF. Thus, CCAT1 is a key oncogenic lncRNA associated with cervical cancer and plays a role in promoting cervical cancer cell proliferation and invasion by regulating the miR-181a-5p/MMP14 axis.  相似文献   
47.
Mites are one of the serious pests of turfgrass. Our survey of turfgrass fields from 2013 to 2015 in Korea showed that the occurrence of leaf chlorotic symptom has gradually extended to at least 60% of the examined golf courses. We identified the zoysiae mite Aceria zoysiae in most damaged zoysiagrasses. Artificial infestation of A. zoysiae into zoysiagrasses in pots resulted in symptoms of chlorosis and marginal rolling of the leaves within 3 weeks. We firstly determined the nucleotide sequence of mitochondrial cytochrome c oxidase subunit I (COI) and internal transcribed spacer 2 (ITS2) of the nuclear ribosomal DNA region of A. zoysiae. The variations in COI and ITS2 between A. zoysiae and other species of the genus were 20.9%–43.0% and 7.5%–67.3%, respectively, suggesting significant genetic divergence within the genus. Our study provides valuable information for the rapid diagnosis and infestation monitoring of A. zoysiae in turfgrass fields.  相似文献   
48.
The solution conformation of O-beta-D-galactopyranosyl-(1----4)-O-beta-D-xylopyranosyl-(1----0)-L-ser ine (GXS), a carbohydrate-protein linkage region fragment from connective tissue proteoglycans, was investigated by two-dimensional NMR spectroscopy and molecular modeling calculations. Specifically, the 1H and 13C resonances were assigned by 2D-COSY and by 1H-13C heteronuclear correlation spectroscopy methods. 2D-NOESY was used to generate distance constraints between the galactose and xylose and between the xylose and serine residues. The 1H vicinal coupling constants for the sugars and the serine were also determined. A general molecular modeling methodology suitable for complex carbohydrates was developed. This methodology employed molecular dynamics and energy minimization procedures together with the application of inter-residue spatial constraints across the linkages derived from 2D-NOESY. The first step in this methodology is the generation of a wide variety of starting conformations that span the (phi, psi) space for each linkage. In the present study, nine such conformations were constructed for each linkage using the torsion angles phi and psi corresponding to the gauche+, gauche-, and trans configurations across each of the two bonds constituting the linkage. These conformations were subjected to a combined molecular dynamics/energy minimization refinement using the NOESY derived constraints as pseudoenergy functions. Families of conformations for the whole molecule were then constructed from the structures derived for each linkage. Characterization of GXS using this methodology identified a single family of conformations that are consistent with the solution phase NMR data on this molecule.  相似文献   
49.
50.
本文研究了分布在细叶益母草(Leonurussibiricus) 叶表面三种腺毛的发育过程,在此基础上,对2细胞头状腺毛、4细胞头状腺毛和8细胞盾状腺毛的多样性特征进行了讨论  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