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   29952篇
  免费   2655篇
  国内免费   3471篇
  36078篇
  2024年   102篇
  2023年   448篇
  2022年   976篇
  2021年   1603篇
  2020年   1077篇
  2019年   1348篇
  2018年   1267篇
  2017年   1000篇
  2016年   1245篇
  2015年   1911篇
  2014年   2248篇
  2013年   2410篇
  2012年   2906篇
  2011年   2582篇
  2010年   1736篇
  2009年   1562篇
  2008年   1745篇
  2007年   1490篇
  2006年   1377篇
  2005年   1119篇
  2004年   971篇
  2003年   847篇
  2002年   775篇
  2001年   528篇
  2000年   440篇
  1999年   415篇
  1998年   307篇
  1997年   217篇
  1996年   212篇
  1995年   183篇
  1994年   210篇
  1993年   130篇
  1992年   136篇
  1991年   95篇
  1990年   86篇
  1989年   73篇
  1988年   63篇
  1987年   54篇
  1986年   43篇
  1985年   30篇
  1984年   30篇
  1983年   22篇
  1982年   14篇
  1981年   9篇
  1980年   4篇
  1979年   6篇
  1976年   4篇
  1973年   3篇
  1972年   4篇
  1965年   2篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
871.
目的:建立一种适用于人工miRNA(amiRNA)表达研究的克隆载体。方法:基于实验室构建的短发夹RNA(shRNA)表达载体pshOK-basic,将鼠源miRNA-155的侧翼序列插入合适的酶切位点构建得到amiRNA重组表达载体pOK-basic;应用本载体分别构建靶向萤火虫荧光素酶(luc2)和红色荧光蛋白(mCherry)基因的amiRNA并检测其沉默效果。结果:应用此载体能快速高效地构建amiRNA,靶向luc2和mCherry报告基因的amiRNA能较好地抑制靶基因的表达。结论:构建了一种能高效表达amiRNA的克隆载体,为amiRNA的进一步研究及应用奠定了基础。  相似文献   
872.
记述了中国短翅天牛属1新记录亚种,即柴田短翅天牛Glaphyra shibatai shibatai(Hayashi,1961)。柴田短翅天牛描述:体长8.1 mm宽2.0 mm。身体呈亮黑色。腿节亮棕黑色,后足腿节基部至中部浅黄色。鞘翅中部近中缝处有1对半透明深黄褐色区域。前胸长约1.23倍于宽,最宽广的部位在收缩之前,前足基节窝后方关闭。鞘翅基部宽约与前胸背板长相等,鞘翅长为鞘翅基部宽度的1.20倍,肩角略呈圆形,两侧向端部逐渐收缩,表面具有细而稀疏的小刻点,翅缝在基部3/5处分裂,缝角圆形。前胸腹板具有粗糙的刻点。后胸腹板具有细刻点。腹部刻点比后胸腹板更浅,更细。后足第1跗节1.50倍于2、3节之和。  相似文献   
873.
目的探讨不同周龄和激素水平对长爪沙鼠超数排卵效果的影响,以期确定长爪沙鼠最佳超排周龄和激素使用剂量。方法腹腔注射10 IU PMSG/HCG对4~18周龄8个年龄段的雌性长爪沙鼠进行超数排卵,末次注射16~17 h内对各组动物卵母细胞计数,确定最佳超排周龄后,对该年龄动物以5、10、15 IU3个剂量水平腹腔注射PMSG/HCG,观察各组动物的卵母细胞计数差异。结果与其它周龄组相比,6周龄组长爪沙鼠超数排卵后的卵母细胞数最多,各组间有统计学意义(P〈0.05),而5、10、15IU等3个剂量组的超排效果也有一定的差异,10 IU组数量最高。结论对长爪沙鼠而言,采用10 IU激素注射和6周龄的动物进行超数排卵,获得的卵母细胞数量最多而且超排效果稳定性。  相似文献   
874.
海洋环境来源的淀粉酶AmyP对生玉米 淀粉的降解特性   总被引:1,自引:0,他引:1  
来自海洋宏基因组文库的 α-淀粉酶(AmyP)属于最新建立的糖苷水解酶亚家族GH1337。AmyP 是一个生淀粉降解酶,能有效降解玉米生淀粉。在最适反应条件 pH 7.5和 40 °C 下,生玉米淀粉的比活达到 39.6 ± 1.4 U/mg。酶解反应动力学显示 AmyP 可以非常快速的降解生玉米淀粉。对 1%的生玉米淀粉仅需要 30 min;4%和 8%的生玉米淀粉只需 3 h。DTT 可以显著提高 AmyP 对生玉米淀粉的降解活性,1% DTT 促使活性增加 1倍。根据电镜观察和产物分析,认为 AmyP 是以内腐蚀的模式降解生玉米淀粉颗粒,释放出葡萄糖、麦芽糖和麦芽三糖作为终产物。  相似文献   
875.
目的:探讨光电阅读机在大样本现场调查项目质量控制和数据录入中的作用.方法:使用光电阅读机读入《子宫颈癌危险因素调查表》信息并进行表格质量评估,核查调查表中调查对象编码缺失、重复及进行读入信息一致性比对.结果:在录入的70160份数据中,调查对象编码缺失率为0.87%,调查对象编码重复率为0.79%,信息一致率为99.47%.3个项目点连续三个月质量控制,表格不合格率略有下降.结论:在大人群现场调查项目中通过光电阅读机进行质量控制及数据录入,能快速准确的将纸质信息转化为电子信息,并对电子信息进行核查,能迅速将调查表评估结果和建议反馈给调查表填写人员,及时指导调查员改进调查项目的编写方法和调查表内容的填写方法,从而提高了调查表信息采集质量,促进项目科研数据真实可信.  相似文献   
876.
目的:探讨人白蛋白对体外培养的HK-2细胞凋亡的作用。方法:本实验研究对象为HK-2细胞株,将培养的HK-2细胞与20g/L的人白蛋白共同孵育0、4、6、8小时后,用Hoechst33258染色检测细胞凋亡。不同浓度(0g/L、5 g/L、10 g/L、20g/L和30g/L)的人白蛋白与体外培养的HK-2分别共同孵育0、4、6、8小时后,流式细胞仪检测细胞凋亡。结果:Hoechst33258染色结果显示:培养基对照组未见明显细胞凋亡;20g/L白蛋白与HK-2细胞共同孵育4、6、8小时,与对照组比较,均可见HK-2细胞荧光强度增加,有着典型凋亡形态的细胞增多,且随着人白蛋白与HK-2细胞作用时间的延长,细胞凋亡的程度和数目也增多。流式细胞仪检测结果显示:与对照组比较,HK-2细胞的凋亡率随着人白蛋白与HK-2细胞作用的浓度和时间增加而显著性增高,细胞凋亡率在8h组为(9.15±0.15%),在30g/L组为(9.35±0.46%),均为最高。结论:人白蛋白以时间和剂量依赖方式诱导肾小管细胞凋亡,以30g/L作用浓度和8小时作用时间的人白蛋白诱导HK-2细胞凋亡的作用最显著。  相似文献   
877.
This study aims to construct a robust prognostic model for adult adrenocortical carcinoma (ACC) by large-scale multiomics analysis and real-world data. The RPPA data, gene expression profiles and clinical information of adult ACC patients were obtained from The Cancer Proteome Atlas (TCPA), Gene Expression Omnibus (GEO) and The Cancer Genome Atlas (TCGA). Integrated prognosis-related proteins (IPRPs) model was constructed. Immunohistochemistry was used to validate the prognostic value of the IPRPs model in Fudan University Shanghai Cancer Center (FUSCC) cohort. 76 ACC cases from TCGA and 22 ACC cases from GSE10927 in NCBI’s GEO database with full data for clinical information and gene expression were utilized to validate the effectiveness of the IPRPs model. Higher FASN (P = .039), FIBRONECTIN (P < .001), TFRC (P < .001), TSC1 (P < .001) expression indicated significantly worse overall survival for adult ACC patients. Risk assessment suggested significantly a strong predictive capacity of IPRPs model for poor overall survival (P < .05). IPRPs model showed a little stronger ability for predicting prognosis than Ki-67 protein in FUSCC cohort (P = .003, HR = 3.947; P = .005, HR = 3.787). In external validation of IPRPs model using gene expression data, IPRPs model showed strong ability for predicting prognosis in TCGA cohort (P = .005, HR = 3.061) and it exhibited best ability for predicting prognosis in GSE10927 cohort (P = .0898, HR = 2.318). This research constructed IPRPs model for predicting adult ACC patients’ prognosis using proteomic data, gene expression data and real-world data and this prognostic model showed stronger predictive value than other biomarkers (Ki-67, Beta-catenin, etc) in multi-cohorts.  相似文献   
878.
Sex change in teleost fishes is commonly regulated by social factors. In species that exhibit protogynous sex change, such as the orange-spotted grouper Epinephelus coioides, when the dominant males are removed from the social group, the most dominant female initiates sex change. The aim of this study was to determine the regulatory mechanisms of socially controlled sex change in E. coioides. We investigated the seasonal variation in social behaviours and sex change throughout the reproductive cycle of E. coioides, and defined the behaviour pattern of this fish during the establishment of a dominance hierarchy. The social behaviours and sex change in this fish were affected by season, and only occurred during the prebreeding season and breeding season. Therefore, a series of sensory isolation experiments was conducted during the breeding season to determine the role of physical, visual and olfactory cues in mediating socially controlled sex change. The results demonstrated that physical interactions between individuals in the social groups were crucial for the initiation and completion of sex change, whereas visual and olfactory cues alone were insufficient in stimulating sex change in dominant females. In addition, we propose that the steroid hormones 11-ketotestosterone and cortisol are involved in regulating the initiation of socially controlled sex change.  相似文献   
879.
Increasing clinical evidence shows that acute kidney injury (AKI) is a common and severe complication in critically ill COVID-19 patients. The older age, the severity of COVID-19 infection, the ethnicity, and the history of smoking, diabetes, hypertension, and cardiovascular disease are the risk factor for AKI in COVID-19 patients. Of them, inflammation may be a key player in the pathogenesis of AKI in patients with COVID-19. It is highly possible that SARS-COV-2 infection may trigger the activation of multiple inflammatory pathways including angiotensin II, cytokine storm such as interleukin-6 (IL-6), C-reactive protein (CRP), TGF-β signaling, complement activation, and lung-kidney crosstalk to cause AKI. Thus, treatments by targeting these inflammatory molecules and pathways with a monoclonal antibody against IL-6 (Tocilizumab), C3 inhibitor AMY-101, anti-C5 antibody, anti-TGF-β OT-101, and the use of CRRT in critically ill patients may represent as novel and specific therapies for AKI in COVID-19 patients.  相似文献   
880.
The emergence of Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) has resulted in a pandemic causing significant damage to public health and the economy. Efforts to understand the mechanisms of Coronavirus Disease 2019 (COVID-19) have been hampered by the lack of robust mouse models. To overcome this barrier, we used a reverse genetic system to generate a mouse-adapted strain of SARS-CoV-2. Incorporating key mutations found in SARS-CoV-2 variants, this model recapitulates critical elements of human infection including viral replication in the lung, immune cell infiltration, and significant in vivo disease. Importantly, mouse adaptation of SARS-CoV-2 does not impair replication in human airway cells and maintains antigenicity similar to human SARS-CoV-2 strains. Coupled with the incorporation of mutations found in variants of concern, CMA3p20 offers several advantages over other mouse-adapted SARS-CoV-2 strains. Using this model, we demonstrate that SARS-CoV-2–infected mice are protected from lethal challenge with the original Severe Acute Respiratory Syndrome Coronavirus (SARS-CoV), suggesting immunity from heterologous Coronavirus (CoV) strains. Together, the results highlight the use of this mouse model for further study of SARS-CoV-2 infection and disease.

Studying cross-protection from different coronaviruses is important to inform the research for a universal vaccine. This study uses a mouse-adapted SARS-CoV-2 strain to show that it confers protection from SARS-CoV challenge, suggesting possible immunity from heterologous challenge following natural infection.  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