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   3857篇
  免费   251篇
  国内免费   300篇
  2023年   48篇
  2022年   69篇
  2021年   70篇
  2020年   51篇
  2019年   90篇
  2018年   87篇
  2017年   101篇
  2016年   106篇
  2015年   94篇
  2014年   134篇
  2013年   147篇
  2012年   86篇
  2011年   157篇
  2010年   120篇
  2009年   244篇
  2008年   293篇
  2007年   239篇
  2006年   215篇
  2005年   189篇
  2004年   147篇
  2003年   126篇
  2002年   100篇
  2001年   85篇
  2000年   85篇
  1999年   101篇
  1998年   77篇
  1997年   91篇
  1996年   84篇
  1995年   72篇
  1994年   71篇
  1993年   68篇
  1992年   72篇
  1991年   60篇
  1990年   58篇
  1989年   55篇
  1988年   58篇
  1987年   69篇
  1986年   37篇
  1985年   38篇
  1984年   23篇
  1982年   22篇
  1981年   28篇
  1980年   22篇
  1979年   23篇
  1978年   19篇
  1977年   19篇
  1976年   24篇
  1975年   12篇
  1973年   14篇
  1972年   12篇
排序方式: 共有4408条查询结果,搜索用时 19 毫秒
51.
IntroductionInsufficient physical activity is a known risk factor for various co-morbidities, including cancer. Globally, its prevalence has increased markedly over the past decades. The aim of this study was to estimate the proportion and number of cancers that were attributable to insufficient physical activity in France in 2015.MethodsPopulation attributable fractions (PAFs) and numbers of cancer cases attributable to insufficient physical activity (<30 min daily of moderate-to-vigorous physical activity) were estimated by age, sex and cancer site. Assuming a 10-year lag-period, PAFs were calculated using physical activity prevalence from a cross-sectional French population survey and cancer-specific relative risks.ResultsAbout half of all French adults were found to be insufficiently physically active, with great variation by age and sex. In 2015, an estimated 2973 cancer cases diagnosed in French adults aged 30y+ were attributable to insufficient physical activity, corresponding to 0.8% of all cancer cases (0.2% in men and 1.6% in women). This comprised 3.8% of all postmenopausal breast cancers (1620 cases), 3.6% of all colon cancers (902 cases) and 6.0% of all cancers of the corpus uteri (450 cases). If at least half of the recommended physical activity level was achieved, 1095 cancer cases could have been avoided.ConclusionInsufficient physical activity is associated to about 3000 cancer cases in France, a country with comparatively low but increasing prevalence of this risk factor. This result is important for setting priorities in cancer prevention programmes aiming to increase physical activity in France and Europe in general.  相似文献   
52.
BackgroundSociodemographic and spatial disparities in incidence and mortality burden of colorectal cancer (CRC) are important to consider in the implementation of population screening, in order to achieve expected benefit and not increase health inequities. Analytic methods should be adapted to provide rational support for targeted interventions.MethodsCRC incidence rates by tumor stage (I-IV) and location (colon vs. rectum) were analyzed for the time period 2008–2016 within a screening-relevant age interval of 55–74 years for the population of South and West Sweden, where screening is planned for. The study population was stratified by sex, country of birth, educational level (for Swedish-born citizens) and residential area. We also estimated disparities in excess mortality from CRC across groups of patients accordant to relevant population groups.ResultsThe analyses were based on 8961 patients with a first CRC diagnosis. There were marked socioeconomic gradients in the stage II-IV CRC incidence rates among Swedish-born men and women. Compared to men with high educational level, the incidence rate ratios (IRRs) of stage II, III, and IV CRC in men with low educational level were 1.38 (95% confidence interval 1.18, 1.62), 1.09 (0.95, 1.26), and 1.18 (1.02, 1.37), respectively. In women, the corresponding figures were 1.26 (1.06, 1.51), 1.19 (1.01, 1.39), and 1.45 (1.20, 1.80). The groups of patients with low educational level showed relatively high excess mortality burdens from CRC.ConclusionsOur analytic approach provided rational support for targeted intervention when implementing CRC screening, aiming at optimizing participation in groups with low educational level.  相似文献   
53.
黑叶猴是栖息于喀斯特石山地区的珍稀濒危灵长类动物,由于非法捕杀和栖息地丧失,种群数量急剧减少。2017年10-12月,我们采用样方法调查恩城国家级自然保护区及其周边地区黑叶猴的种群分布、数量及保护现状,分析其致危因素,为制定有效管理行动计划提供理论依据。本次调查共记录到黑叶猴夜宿地89处,其中有43处夜宿地下方有新鲜排泄物。结合本次调查和访问,确定黑叶猴的种群数量为18群100~105只,包括1只全白化黑叶猴成年雌性个体;其中11群分布在保护区内,7群分布在保护区外。猴群由3~12只个体组成。栖息地破碎化和退化是威胁恩城保护区黑叶猴种群生存的主要因素。  相似文献   
54.
为了解海南岛油茶(Camellia oleifera)种质资源的遗传多样性,采用SRAP分子标记,对海南岛油茶主要分布区的31个居群进行了遗传多样性和亲缘关系分析。结果表明,海南岛油茶资源的遗传多样性低,物种水平的多态性百分率(PPB)为98.30%,Nei’s基因多样性(H)为0.222 8,Shannon信息指数(I)为0.353 8;居群水平的PPB=40.96%,观测等位基因数(Na)为1.409 6,有效等位基因数(Ne)为1.237 1, H=0.138 5, I=0.208 3,这与海南岛油茶丰富的表型多样性水平不一致。海南岛油茶资源遗传分化较大,居群间基因交流有限,不同居群间的遗传分化指数(Gst)为0.380,基因流(Nm)为0.813 91。遗传变异主要发生在居群内,有38.05%的变异存在居群间,61.95%存在于居群内。遗传距离为0.022 6~0.276 4,平均为0.107 7,居群间的亲缘关系较近。UPGMA聚类分析表明,在遗传距离为0.11处,可将31个油茶居群聚为6类,表现出明显的行政区域性,而与地理距离关系不大。因此,海南岛油茶资源遗传多样性低,亲缘关系近可能导致自交或近交不亲和,可能是海南油茶林分花量大而结实低的主要内在原因。  相似文献   
55.
mtDNA COII gene sequences were identified and analyzed using different types of software, namely, MEGA5.0, DNAMAN, and DnaSP5.0 in four Chinese provinces, namely, Sichuan, Zhejiang, Guizhou and Shanghai. Analysis of molecular genetic variation and its genetic structure and differentiation, combined with NJ tree, MP tree analysis and analysis of molecular variance (AMOVA), at Fst = 0.0582 conclude that the genetic differentiation is low, gene flow is Nm = 8.0911, and gene exchange is sufficient. However, for the geographic populations of Pseudoregma bambucicola in the four provinces, their gene exchange is relatively weak at Nm = 0.8284, whereas the genetic differentiation is high at Fst = 0.3764. Based on the data, total nucleotide diversity between the populations is 0.00158 ± 0.00021. The results showed that the total population of Tajima’s D and Fu’s Fs results are D = ?0.885 and Fs = 0.226, respectively. The experimental numerical results showed that this total population is not significant (P > 0.10), indicating that nine different geographic populations are short-term. No expansion occurred in the internal population. This study provided a theoretical and practical basis for the comprehensive prevention and control of P. bambucicola.  相似文献   
56.
采用扫描电镜对来自北美和东亚的变豆菜属植物(美国7个种,中国8个种)的果实表面微形态进行观察,结合前人对伞形科其他类群果实微形态特征的研究及近年来分子系统学证据,对变豆菜属的种间分类进行研究。结果显示,变豆菜属植物果实的形态大小、皮刺弯曲程度和果柄蜡质纹饰丰富多样。研究表明变豆菜属是一个自然类群,果实表面钩刺结构有利于该属植物的传播及扩散,果实微形态特征具有分类学价值,可作为种间分类的依据。根据变豆菜属果实表面微形态特征,编制了该属植物分种检索表。  相似文献   
57.
许多长距离迁徙的雀形目鸟类的种群数量正在持续下降,田鹀(Emberiza rustica)种群数量下降趋势更为突出。通过对田鹀种群数量长期监测和迁徙动态分析,可为此物种保护提供科学依据。从2001年开始,陆续在黑龙江省高峰、青峰、帽儿山、新青和大沾河,吉林省珲春和吉林市,辽宁省的辽宁鸟类研究中心(大连)和旅顺老铁山,以及内蒙古乌尔其汗鸟类栖息的临水林缘处布网环志。截至2018年,累计环志田鹀184181只,其中春季88571只,秋季95610只;各年度环志数量波动较大,总体呈现急速下降趋势。幼鸟的越冬损失率高达41.3%。田鹀106只次的回收信息表明,自然条件下田鹀寿命可达11年以上;日迁飞距离最快可达到300 km,飞行速度可达30 km/h。中国东北地区是田鹀等鸟类的重要迁徙途经地;田鹀的迁徙路线相对稳定,在瑞典北部繁殖的种群经中国东北地区迁徙到天津以南越冬。通过环志发现,近些年田鹀种群数量急速下降。通过比对,发现中国东北地区田鹀的环志数量变化趋势与瑞典的田鹀环志数量变化趋势相似;相对于环志数量最多的年份,环志数量下降95%以上,值得关注。栖息地破碎化、非法猎捕等是影响田鹀生存的主要受威胁因素。建议依据田鹀等鸟类生物学习性,加强鸟类栖息地的保护,坚持长期标准化的鸟类环志监测,进一步探索鸟类迁徙规律,以助于鸟类种群的恢复。  相似文献   
58.
Identification of environment specific marker-features is one of the key objectives of many metagenomic studies. It aims to identify such features in microbiome datasets that may serve as markers of the contrasting or comparable states. Hypothesis testing and black-box machine learnt models which are conventionally used for identification of these features are generally not exhaustive, especially because they generally do-not provide any quantifiable relevance (context) of/between the identified features. We present MarkerML web-server, that seeks to leverage the emergence of interpretable machine learning for facilitating the contextual discovery of metagenomic features of interest. It does so through a comprehensive and automated application of the concept of Shapley Additive Explanations in companionship to the compositionality accounted hypothesis testing for the multi-variate microbiome datasets. MarkerML not only helps in identification of marker-features, but also enables insights into the role and inter-dependence of the identified features in driving the decision making of the supervised machine learnt model. Generation of high quality and intuitive visualizations spanning prediction effect plots, model performance reports, feature dependency plots, Shapley and abundance informed cladograms (Sungrams), hypothesis tested violin plots along-with necessary provisions for excluding the participant bias and ensuring reproducibility of results, further seek to make the platform a useful asset for the scientists in the field of microbiome (and even beyond). The MarkerML web-server is freely available for the academic community at https://microbiome.igib.res.in/markerml/.  相似文献   
59.
周德庆 《微生物学通报》2022,49(3):1195-1199
《微生物学教程》获“首届全国教材奖”(二等奖)的消息后,为响应学院领导提出的总结、交流等要求,特撰写了本文。作者以“愿终生能炼成一个称职的微生物学教师”为指导思想,历经半个多世纪的努力,在教书育人的同时,编写了较多的文章和书籍,其中的代表作当属《微生物学教程》。该书至今已出了4版,被300余所高等院校选作教材,总印数超百万册。主要特色为:一人独撰,具中国特色,体系稳定,紧跟学科前沿,重视训练学生的长效记忆和战略思维,关心教学方法的改进和交流,以及注重理论联系实际等。作者对教材编写的体会主要是:干一行,爱一行;自认笨,大有益;轻功利,求实绩;勤厚积,方薄发;花可发,柳自荫;青自蓝,更胜蓝。  相似文献   
60.
《遗传学报》2022,49(1):54-62
The global “myopia boom” has raised significant international concerns. Despite a higher myopia prevalence in Asia, previous large-scale genome-wide association studies (GWASs) were mostly based on European descendants. Here, we report a GWAS of spherical equivalent (SE) in 1852 Chinese Han individuals with extreme SE from Guangzhou (631 < ?6.00D and 574 > 0.00D) and Wenzhou (593 < ?6.00D and 54 > ?1.75D), followed by a replication study in two independent cohorts with totaling 3538 East Asian individuals. The discovery GWAS and meta-analysis identify three novel loci, which show genome-wide significant associations with SE, including 1q25.2 FAM163A, 10p11.22 NRP1/PRAD3, and 10p11.21 ANKRD30A/MTRNR2L7, together explaining 3.34% of SE variance. 10p11.21 is successfully replicated. The allele frequencies of all three loci show significant differences between major continental groups (P < 0.001). The SE reducing (more myopic) allele of rs10913877 (1q25.2 FAM163A) demonstrates the highest frequency in East Asians and much lower frequencies in Europeans and Africans (EAS = 0.60, EUR = 0.20, and AFR = 0.18). The gene-based analysis additionally identifies three novel genes associated with SE, including EI24, LHX5, and ARPP19. These results provide new insights into myopia pathogenesis and indicate the role of genetic heterogeneity in myopia epidemiology among different ethnicities.  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