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   11009篇
  免费   104篇
  国内免费   186篇
系统科学   191篇
丛书文集   662篇
教育与普及   644篇
理论与方法论   139篇
现状及发展   681篇
研究方法   1046篇
综合类   7931篇
自然研究   5篇
  2023年   57篇
  2022年   58篇
  2021年   73篇
  2020年   62篇
  2019年   92篇
  2018年   86篇
  2017年   55篇
  2016年   58篇
  2015年   93篇
  2014年   201篇
  2013年   175篇
  2012年   662篇
  2011年   793篇
  2010年   318篇
  2009年   217篇
  2008年   696篇
  2007年   694篇
  2006年   696篇
  2005年   756篇
  2004年   613篇
  2003年   572篇
  2002年   492篇
  2001年   420篇
  2000年   567篇
  1999年   248篇
  1998年   69篇
  1997年   76篇
  1996年   106篇
  1995年   71篇
  1994年   62篇
  1993年   61篇
  1992年   55篇
  1991年   55篇
  1990年   55篇
  1989年   63篇
  1988年   75篇
  1987年   65篇
  1986年   79篇
  1985年   63篇
  1984年   77篇
  1983年   64篇
  1982年   74篇
  1981年   52篇
  1979年   51篇
  1959年   141篇
  1958年   233篇
  1957年   181篇
  1956年   134篇
  1955年   148篇
  1954年   170篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
81.
基于治理理论和委托—代理模型,本研究以35 个国际单项体育联合会善治 评价为对象,设计了以透明度、程序民主、制衡、团结等维度作为关键维度的指标体系, 并使用文献资料法、问卷调查法和访谈法等方法,对数据进行搜集整理。研究发现,国 际单项体育联合会平均得分为45.5(满分100),善治状况评价为“一般”,决策和工作 机制不透明、选举和工作程序失范、利益相关者参与不足等,导致组织内部投机主义盛 行,国际单项体育联合会合法性面临危机。研究建议:1)严格执行选举程序,健全主席 和高管资格审查机制;2)完善信息公开制度,提升组织透明度;3)规范工作流程,充 实组织行政程序正当性;4)多元主体参与,健全组织内外部审查制度;5)明确最长任 期,保证组织官员积极性。  相似文献   
82.
本研究通过新疆哈萨克羊NYD-SP27基因的克隆及序列分析,为研究绵羊NYD-SP27表达调控机制奠定基础。根据Genbank中公布的人类的NYD-SP27基因序列设计引物,本研究以新疆哈萨克羊的睾丸组织的总DNA为模板,采用RT-PCR扩增方法对绵羊NYD-SP27基因序列进行研究。结果显示:哈萨克羊NYD-SP27基因全长为1901 bp,其中只含有1617 bp组成的开放阅读框(ORF),共编码538个氨基酸;相对分子质量61995.66,理论PI值为6.16,正电荷残基数(Arg+Lys)为61,负电荷残基数(Asp+Glu)为67;分子式C2798H4319N737O819S19;体外半衰期为30 h,不稳定系数46.96(不稳定);总平均吸水性-0.403。结论:哈萨克羊NYD-SP27基因属于PLCζ家族;同时将所得序列已提交到Gene Bank(登录号:KX905090)。  相似文献   
83.
The pedigrees of three sequenced rice cultivars were analyzed to show that a majority of the genetic composition of 'Nipponbare' originates from japonica cultivars while the minority originates from indica cultivars. In contrast, '93-11' is derived mainly from indica cultivars with a smaller contribution from japonica cultivars. All ancestors of 'Guang lu ai 4' appeared to be indica lines. A set of molecular markers (46 InDels and 53 SSRs) polymorphic between 'Nipponbare' and '93-11' were examined in 46 typical indica and 47 typical japonica cultivars selected from 443 accessions according to Cheng's index. All cultivars were divided into indica and japonica groups without overlapping when clustered by Cheng's index, InDels and SSRs. Much higher InDel and SSR diversity between groups than within groups implies that the marker polymorphisms between 'Nipponbare' and '93-11' represent a large proportion of inter-subspecific diversity. About 85% of indica cultivars and more than 90% of japonica cultivars were confirmed to have the same PCR banding patterns as '93-11' and 'Nipponbare', respectively. Some polymorphic loci between 'Nipponbare' and '93-11' cannot be validated in other indica and japonica cultivars, either as subspecies-specific but not predominant alleles, or alleles not specific between the two groups. It was concluded that molecular markers developed from sequence polymorphism between 'Nipponbare' and '93-11' often represent inter-subspecific diversity, although some exceptions were sensitive to either particular marker loci or particular cultivars.  相似文献   
84.
Collagen VI is an extracellular matrix protein that forms a microfilamentous network in skeletal muscles and other organs. Inherited mutations in genes encoding collagen VI in humans cause two muscle diseases, Bethlem myopathy and Ullrich congenital muscular dystrophy. We previously generated collagen VI-deficient (Col6a1-/-) mice and showed that they have a muscle phenotype that strongly resembles Bethlem myopathy. The pathophysiological defects and mechanisms leading to the myopathic disorder were not known. Here we show that Col6a1-/- muscles have a loss of contractile strength associated with ultrastructural alterations of sarcoplasmic reticulum (SR) and mitochondria and spontaneous apoptosis. We found a latent mitochondrial dysfunction in myofibers of Col6a1-/- mice on incubation with the selective F1F(O)-ATPase inhibitor oligomycin, which caused mitochondrial depolarization, Ca2+ deregulation and increased apoptosis. These defects were reversible, as they could be normalized by plating Col6a1-/- myofibers on collagen VI or by addition of cyclosporin A (CsA), the inhibitor of mitochondrial permeability transition pore (PTP). Treatment of Col6a1-/- mice with CsA rescued the muscle ultrastructural defects and markedly decreased the number of apoptotic nuclei in vivo. These findings indicate that collagen VI myopathies have an unexpected mitochondrial pathogenesis that could be exploited for therapeutic intervention.  相似文献   
85.
We defined the genetic landscape of balanced chromosomal rearrangements at nucleotide resolution by sequencing 141 breakpoints from cytogenetically interpreted translocations and inversions. We confirm that the recently described phenomenon of 'chromothripsis' (massive chromosomal shattering and reorganization) is not unique to cancer cells but also occurs in the germline, where it can resolve to a relatively balanced state with frequent inversions. We detected a high incidence of complex rearrangements (19.2%) and substantially less reliance on microhomology (31%) than previously observed in benign copy-number variants (CNVs). We compared these results to experimentally generated DNA breakage-repair by sequencing seven transgenic animals, revealing extensive rearrangement of the transgene and host genome with similar complexity to human germline alterations. Inversion was the most common rearrangement, suggesting that a combined mechanism involving template switching and non-homologous repair mediates the formation of balanced complex rearrangements that are viable, stably replicated and transmitted unaltered to subsequent generations.  相似文献   
86.
Double-stranded RNA interference (RNAi) is an effective method for disrupting expression of specific genes in Caenorhabditis elegans and other organisms. Applications of this reverse-genetics tool, however, are somewhat restricted in nematodes because introduced dsRNA is not stably inherited. Another difficulty is that RNAi disruption of late-acting genes has been generally less consistent than that of embryonically expressed genes, perhaps because the concentration of dsRNA becomes lower as cellular division proceeds or as developmental time advances. In particular, some neuronally expressed genes appear refractory to dsRNA-mediated interference. We sought to extend the applicability of RNAi by in vivo expression of heritable inverted-repeat (IR) genes. We assayed the efficacy of in vivo-driven RNAi in three situations for which heritable, inducible RNAi would be advantageous: (i) production of large numbers of animals deficient for gene activities required for viability or reproduction; (ii) generation of large populations of phenocopy mutants for biochemical analysis; and (iii) effective gene inactivation in the nervous system. We report that heritable IR genes confer potent and specific gene inactivation for each of these applications. We suggest that a similar strategy might be used to test for dsRNA interference effects in higher organisms in which it is feasible to construct transgenic animals, but impossible to directly or transiently introduce high concentrations of dsRNA.  相似文献   
87.
Auditory neuropathy is a particular type of hearing impairment in which neural transmission of the auditory signal is impaired, while cochlear outer hair cells remain functional. Here we report on DFNB59, a newly identified gene on chromosome 2q31.1-q31.3 mutated in four families segregating autosomal recessive auditory neuropathy. DFNB59 encodes pejvakin, a 352-residue protein. Pejvakin is a paralog of DFNA5, a protein of unknown function also involved in deafness. By immunohistofluorescence, pejvakin is detected in the cell bodies of neurons of the afferent auditory pathway. Furthermore, Dfnb59 knock-in mice, homozygous for the R183W variant identified in one DFNB59 family, show abnormal auditory brainstem responses indicative of neuronal dysfunction along the auditory pathway. Unlike previously described sensorineural deafness genes, all of which underlie cochlear cell pathologies, DFNB59 is the first human gene implicated in nonsyndromic deafness due to a neuronal defect.  相似文献   
88.
<正> 在3、4月间,对产后20—31天正在泌乳的母羊诱导发情。发情时配种,配种后11、12和13天,用一种盐水或绒毛膜促性腺激素处理母羊(HCG;100国际单位)。对经过妊娠诊断的母羊,不论妊娠或未妊娠,发现配种后12、14和16天,用绒毛膜促性腺激素处理过的比用盐水处理过的血清中孕酮浓度增加(P<0.01)。用绒毛膜促性腺激素处理的母羊比用盐水(29%58标准液;P<0.05)处理的母羊妊娠率较高。从试验提供的资料指出,用绒毛膜  相似文献   
89.
我们每天都要跟铝打交道,家里的餐具、炊具、门窗、电器,办公室里的电脑、电话、文具、桌椅,工厂里的机器设备,出行乘坐的交通工具,以及潜艇、战机乃至火箭、飞船,铝如影随行、无处不在。铝是一种银白色金属,在地壳中含量仅次于氧和硅,排在第三位。铝的密度小,仅为铁的34.61%、铜的30.33%,因此又被称作轻金属,在世界上产量和用量都仅次于钢铁。它既是常用的生产生活物资,又是重要的战略物资。具有轻便性、导电性、导热性、可塑性(易拉伸、易延展)、耐腐蚀性(不生锈)等优良特性,所以,成为各种设施轻易化的首选金属材料。作为轻型结构材料,铝…  相似文献   
90.
关于转基因食品的争论   总被引:2,自引:0,他引:2  
植物的遗传操作目前在英国产生了极大的轰动效应,可谓是议论纷纷,褒贬不一。超级市场上的新型食品丰富多彩,满足了消费者的需求,感谢现代集约型农业所提供的这一切。但富余也同时带有自身的问题,这不仅是技术变化的加速。物质的丰富,寿命的延长,都对无风险环境提出了要求。和世纪初相比,当今的西方人更讲究健康食品;所以对用新法生产的同种食品,态度冷淡;尤其是对那些被怀疑有风险的食品,即使对健康稍有涉嫌,公众也不愿轻易接受。关于超级杂草fo]题英国医学会和环境保护人员,否定了对草甘脚含有抗性的GM油菜;认为一种特异…  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