全文获取类型
收费全文 | 4619篇 |
免费 | 29篇 |
国内免费 | 49篇 |
专业分类
系统科学 | 121篇 |
丛书文集 | 232篇 |
教育与普及 | 162篇 |
理论与方法论 | 5篇 |
现状及发展 | 400篇 |
研究方法 | 791篇 |
综合类 | 2978篇 |
自然研究 | 8篇 |
出版年
2018年 | 6篇 |
2017年 | 7篇 |
2016年 | 8篇 |
2014年 | 16篇 |
2013年 | 15篇 |
2012年 | 366篇 |
2011年 | 448篇 |
2010年 | 81篇 |
2009年 | 32篇 |
2008年 | 294篇 |
2007年 | 327篇 |
2006年 | 350篇 |
2005年 | 380篇 |
2004年 | 325篇 |
2003年 | 304篇 |
2002年 | 257篇 |
2001年 | 216篇 |
2000年 | 312篇 |
1999年 | 101篇 |
1998年 | 12篇 |
1997年 | 16篇 |
1996年 | 12篇 |
1995年 | 7篇 |
1994年 | 15篇 |
1993年 | 18篇 |
1992年 | 26篇 |
1991年 | 26篇 |
1990年 | 31篇 |
1989年 | 16篇 |
1988年 | 16篇 |
1987年 | 21篇 |
1986年 | 14篇 |
1985年 | 24篇 |
1984年 | 21篇 |
1983年 | 16篇 |
1982年 | 16篇 |
1981年 | 18篇 |
1980年 | 11篇 |
1979年 | 9篇 |
1977年 | 15篇 |
1971年 | 10篇 |
1970年 | 28篇 |
1966年 | 5篇 |
1959年 | 49篇 |
1958年 | 99篇 |
1957年 | 79篇 |
1956年 | 58篇 |
1955年 | 50篇 |
1954年 | 59篇 |
1948年 | 18篇 |
排序方式: 共有4697条查询结果,搜索用时 427 毫秒
221.
222.
Tan L Li X Liu F Sun X Li C Zhu Z Fu Y Cai H Wang X Xie D Sun C 《Nature genetics》2008,40(11):1360-1364
The transition from the prostrate growth of ancestral wild rice (O. rufipogon Griff.) to the erect growth of Oryza sativa cultivars was one of the most critical events in rice domestication. This evolutionary step importantly improved plant architecture and increased grain yield. Here we find that prostrate growth of wild rice from Yuanjiang County in China is controlled by a semi-dominant gene, PROG1 (PROSTRATE GROWTH 1), on chromosome 7 that encodes a single Cys(2)-His(2) zinc-finger protein. prog1 variants identified in O. sativa disrupt the prog1 function and inactivate prog1 expression, leading to erect growth, greater grain number and higher grain yield in cultivated rice. Sequence comparison shows that 182 varieties of cultivated rice, including 87 indica and 95 japonica cultivars from 17 countries, carry identical mutations in the prog1 coding region that may have become fixed during rice domestication. 相似文献
223.
The Pristionchus pacificus genome provides a unique perspective on nematode lifestyle and parasitism
Dieterich C Clifton SW Schuster LN Chinwalla A Delehaunty K Dinkelacker I Fulton L Fulton R Godfrey J Minx P Mitreva M Roeseler W Tian H Witte H Yang SP Wilson RK Sommer RJ 《Nature genetics》2008,40(10):1193-1198
Here we present a draft genome sequence of the nematode Pristionchus pacificus, a species that is associated with beetles and is used as a model system in evolutionary biology. With 169 Mb and 23,500 predicted protein-coding genes, the P. pacificus genome is larger than those of Caenorhabditis elegans and the human parasite Brugia malayi. Compared to C. elegans, the P. pacificus genome has more genes encoding cytochrome P450 enzymes, glucosyltransferases, sulfotransferases and ABC transporters, many of which were experimentally validated. The P. pacificus genome contains genes encoding cellulase and diapausin, and cellulase activity is found in P. pacificus secretions, indicating that cellulases can be found in nematodes beyond plant parasites. The relatively higher number of detoxification and degradation enzymes in P. pacificus is consistent with its necromenic lifestyle and might represent a preadaptation for parasitism. Thus, comparative genomics analysis of three ecologically distinct nematodes offers a unique opportunity to investigate the association between genome structure and lifestyle. 相似文献
224.
A cis-acting regulatory mutation causes premature hair graying and susceptibility to melanoma in the horse 总被引:1,自引:0,他引:1
Rosengren Pielberg G Golovko A Sundström E Curik I Lennartsson J Seltenhammer MH Druml T Binns M Fitzsimmons C Lindgren G Sandberg K Baumung R Vetterlein M Strömberg S Grabherr M Wade C Lindblad-Toh K Pontén F Heldin CH Sölkner J Andersson L 《Nature genetics》2008,40(8):1004-1009
In horses, graying with age is an autosomal dominant trait associated with a high incidence of melanoma and vitiligo-like depigmentation. Here we show that the Gray phenotype is caused by a 4.6-kb duplication in intron 6 of STX17 (syntaxin-17) that constitutes a cis-acting regulatory mutation. Both STX17 and the neighboring NR4A3 gene are overexpressed in melanomas from Gray horses. Gray horses carrying a loss-of-function mutation in ASIP (agouti signaling protein) had a higher incidence of melanoma, implying that increased melanocortin-1 receptor signaling promotes melanoma development in Gray horses. The Gray horse provides a notable example of how humans have cherry-picked mutations with favorable phenotypic effects in domestic animals. 相似文献
225.
High-throughput sequencing provides insights into genome variation and evolution in Salmonella Typhi 总被引:1,自引:0,他引:1
Holt KE Parkhill J Mazzoni CJ Roumagnac P Weill FX Goodhead I Rance R Baker S Maskell DJ Wain J Dolecek C Achtman M Dougan G 《Nature genetics》2008,40(8):987-993
Isolates of Salmonella enterica serovar Typhi (Typhi), a human-restricted bacterial pathogen that causes typhoid, show limited genetic variation. We generated whole-genome sequences for 19 Typhi isolates using 454 (Roche) and Solexa (Illumina) technologies. Isolates, including the previously sequenced CT18 and Ty2 isolates, were selected to represent major nodes in the phylogenetic tree. Comparative analysis showed little evidence of purifying selection, antigenic variation or recombination between isolates. Rather, evolution in the Typhi population seems to be characterized by ongoing loss of gene function, consistent with a small effective population size. The lack of evidence for antigenic variation driven by immune selection is in contrast to strong adaptive selection for mutations conferring antibiotic resistance in Typhi. The observed patterns of genetic isolation and drift are consistent with the proposed key role of asymptomatic carriers of Typhi as the main reservoir of this pathogen, highlighting the need for identification and treatment of carriers. 相似文献
226.
喜马拉雅山中段达索普粒雪芯中夏季风和尘埃信号记录研究 总被引:8,自引:2,他引:6
通过粒雪芯δ^18O和主要离子(Ca^2+,Mg^2+,NH^+4,SO^2-4和NO^-3)的分析表明,该粒雪芯年平均积累量为0.75m水当量。粒雪芯中δ^18O和主要离子浓度的季节变化表明,达索普冰川中高精度的记录了现代夏季风和尘埃信号。夏季风期间降水中δ^18O值由“降水量效应”所控制,雪层中较低δ^18O值则代表了夏季风信号。亚洲中部干旱半干旱区的春季尘暴输入是造成粒雪芯中Ca^2+,Mg 相似文献
227.
In human patients, blood coagulation disorders often associate with cancer, even in its early stages. Recently, in vitro and in vivo experimental models have shown that oncogene expression, or inactivation of tumour suppressor genes, upregulate genes that
control blood coagulation. These studies suggest that activation of blood clotting, leading to peritumoral fibrin deposition,
is instrumental in cancer development. Fibrin can indeed build up a provisional matrix, supporting the invasive growth of
neoplastic tissues and blood vessels. Interference with blood coagulation can thus be considered as part of a multifaceted
therapeutic approach to cancer.
Received 30 November 2005; received after revision 7 February 2005; accepted 8 February 2006 相似文献
228.
On the basis of evidence collected from the literature, we propose a general model by which protein kinase (PK) A and the
different PKC isoforms can inversely affect cell growth. Molecular switches, which are able to direct the signal towards antiproliferative
or mitogenic pathways, are the different isoforms of Raf and PKC. Conflicting data are also reported and discussed in an attempt
to reconcile them.
Received 10 November 2005; received after revision 28 December 2005; accepted 3 January 2006 相似文献
229.
Induction, suppression and requirement of RNA silencing pathways in virulent Agrobacterium tumefaciens infections 总被引:4,自引:0,他引:4
Regulation of gene expression through microRNAs (miRNAs) and antiviral defense through small interfering RNAs (siRNAs) are aspects of RNA silencing, a process originally discovered as an unintended consequence of plant transformation by disarmed Agrobacterium tumefaciens strains. Although RNA silencing protects cells against foreign genetic elements, its defensive role against virulent, tumor-inducing bacteria has remained unexplored. Here, we show that siRNAs corresponding to transferred-DNA oncogenes initially accumulate in virulent A. tumefaciens-infected tissues and that RNA interference-deficient plants are hypersusceptible to the pathogen. Successful infection relies on a potent antisilencing state established in tumors whereby siRNA synthesis is specifically inhibited. This inhibition has only modest side effects on the miRNA pathway, shown here to be essential for disease development. The similarities and specificities of the A. tumefaciens RNA silencing interaction are discussed and contrasted with the situation encountered with plant viruses. 相似文献
230.
Nras loss induces metastatic conversion of Rb1-deficient neuroendocrine thyroid tumor 总被引:2,自引:0,他引:2
Takahashi C Contreras B Iwanaga T Takegami Y Bakker A Bronson RT Noda M Loda M Hunt JL Ewen ME 《Nature genetics》2006,38(1):118-123
Mutations in the gene encoding the retinoblastoma tumor suppressor predispose humans and mice to tumor development. Here we have assessed the effect of Nras loss on tumor development in Rb1 heterozygous mice. Loss of one or two Nras alleles is shown to significantly reduce the severity of pituitary tumors arising in Rb1(+/-) animals by enhancing their differentiation. By contrast, C-cell thyroid adenomas occurring in Rb1(+/-) mice progress to metastatic medullary carcinomas after loss of Nras. In Rb1(+/-)Nras(+/-) animals, distant medullary thyroid carcinoma metastases are associated with loss of the remaining wild-type Nras allele. Loss of Nras in Rb1-deficient C cells results in elevated Ras homolog family A (RhoA) activity, and this is causally linked to the invasiveness and metastatic behavior of these cells. These findings suggest that the loss of the proto-oncogene Nras in certain cellular contexts can promote malignant tumor progression. 相似文献