全文获取类型
收费全文 | 281675篇 |
免费 | 30656篇 |
国内免费 | 255篇 |
专业分类
312586篇 |
出版年
2018年 | 2464篇 |
2016年 | 3340篇 |
2015年 | 4610篇 |
2014年 | 5234篇 |
2013年 | 7796篇 |
2012年 | 8399篇 |
2011年 | 8586篇 |
2010年 | 5672篇 |
2009年 | 5223篇 |
2008年 | 7684篇 |
2007年 | 7745篇 |
2006年 | 7360篇 |
2005年 | 7097篇 |
2004年 | 6970篇 |
2003年 | 6732篇 |
2002年 | 6526篇 |
2001年 | 11846篇 |
2000年 | 11871篇 |
1999年 | 9395篇 |
1998年 | 3421篇 |
1997年 | 3654篇 |
1996年 | 3550篇 |
1995年 | 3152篇 |
1994年 | 3140篇 |
1993年 | 3127篇 |
1992年 | 7914篇 |
1991年 | 7898篇 |
1990年 | 7561篇 |
1989年 | 7512篇 |
1988年 | 6888篇 |
1987年 | 6635篇 |
1986年 | 6070篇 |
1985年 | 6241篇 |
1984年 | 5177篇 |
1983年 | 4365篇 |
1982年 | 3398篇 |
1981年 | 3166篇 |
1980年 | 2962篇 |
1979年 | 4965篇 |
1978年 | 3837篇 |
1977年 | 3746篇 |
1976年 | 3448篇 |
1975年 | 3814篇 |
1974年 | 4191篇 |
1973年 | 4107篇 |
1972年 | 3679篇 |
1971年 | 3448篇 |
1970年 | 3091篇 |
1969年 | 3003篇 |
1968年 | 2751篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
71.
Escherichia coli RecBC pseudorevertants lacking chi recombinational hotspot activity 总被引:24,自引:7,他引:17
下载免费PDF全文
![点击此处可从《Journal of bacteriology》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Pseudorevertants of an Escherichia coli exonuclease V (RecBC enzyme)-negative mutant have been isolated after ethyl methane sulfonate mutagenesis of a recC73 (presumed missense) mutant. The remedial mutations in each of the four pseudorevertants studied in detail map and complement as recC mutations. By several criteria, such as recombination proficiency, support of phage growth, RecBC nuclease activity, and cell viability, the pseudorevertants appear to have regained partially or completely various aspects of RecBC activity. However, chi recombinational hotspots, which stimulate exclusively the RecBC pathway of recombination, have no detectable activity in lambda vegetative crosses in the pseudorevertants. The properties of these mutants, in which the RecBC pathway of recombination is active yet in which chi is not active, are consistent with the hypothesis that wild-type RecBC enzyme directly interacts with chi sites; alternatively, the mutants may block or bypass the productive interaction of another recombinational enzyme with chi. 相似文献
72.
D Laky S Constantinescu G Filipescu N M Constantinescu E Ratea F Halalau 《Morphologie et embryologie》1980,26(2):173-177
The first ten days' evolution of post-ischaemic lesions of the premonitory or angina pectoris syndrome type was experimentally studied by the challenge of a short-term (10 and 15 min) ischaemia, of an adaptation to ischaemia and an adaptation followed by prolonged ischaemia (20 and 35 min). Worthy of note was the persistence of reversible lesions after short-term ischaemia and adaptation, and the progressive evolution towards cytolysis and cicatrization of some pancicellular foci after adaptation followed by prolonged ischaemia. The role of mitochondrial lesions, of lysosomal hydrolases, the inefficiency of renewed circulation, as well as problems of diagnosis are discussed. 相似文献
73.
74.
75.
Selective diapedesis of Th1 cells induced by endothelial cell RANTES. 总被引:16,自引:0,他引:16
T Kawai M Seki K Hiromatsu J W Eastcott G F Watts M Sugai D J Smith S A Porcelli M A Taubman 《Journal of immunology (Baltimore, Md. : 1950)》1999,163(6):3269-3278
Differentiated CD4 T cells can be divided into Th1 and Th2 types based on the cytokines they produce. Differential expression of chemokine receptors on either the Th1-type or the Th2-type cell suggests that Th1-type and Th2-type cells differ not only in cytokine production but also in their migratory capacity. Stimulation of endothelial cells with IFN-gamma selectively enhanced transmigration of Th1-type cells, but not Th2-type cells, in a transendothelial migration assay. Enhanced transmigration of Th1-type cells was dependent on the chemokine RANTES produced by endothelial cells, as indicated by the findings that Ab neutralizing RANTES, or Ab to its receptor CCR5, inhibited transmigration. Neutralizing Ab to chemokines macrophage-inflammatory protein-1alpha or monocyte chemotactic protein-1 did not inhibit Th1 selective migration. Whereas anti-CD18 and anti-CD54 blocked basal levels of Th1-type cell adherence to endothelial cells and also inhibited transmigration, anti-RANTES blocked only transmigration, indicating that RANTES appeared to induce transmigration of adherent T cells. RANTES seemed to promote diapedesis of adherent Th1-type cells by augmenting pseudopod formation in conjunction with actin rearrangement by a pathway that was sensitive to the phosphoinositol 3-kinase inhibitor wortmannin and to the Rho GTP-binding protein inhibitor, epidermal cell differentiation inhibitor. Thus, enhancement of Th1-type selective migration appeared to be responsible for the diapedesis induced by interaction between CCR5 on Th1-type cells and RANTES produced by endothelial cells. Further evidence that CCR5 and RANTES play a modulatory role in Th1-type selective migration derives from the abrogation of this migration by anti-RANTES and anti-CCR5 Abs. 相似文献
76.
77.
78.
79.
80.
Stewart A. Hill Stephen E. Scheckler James F. Basinger 《American journal of botany》1997,84(1):85-103
A new fern-like fossil plant is described from the lower Upper Devonian of southern Ellesmere Island, Canadian Arctic Archipelago. The plant occurs in an Archaeopteris-dominated flora preserved in the Nordstrand Point Formation (Mid-Late Frasnian) near Bird Fiord. The plant has a pinnate vegetative system with three branch orders and laminate sphenopteroid pinnules. Primary pinnae usually diverge from the main axis in distichous pairs (quadriseriate), but can depart singly (biseriate). Each primary pinna bears a basal catadromic aphlebia. Anatomically, the plant exhibits a mesarch, bipolar protostele that is ribbon- to clepsydropsoid-shaped in the main axis. Primary pinna traces are also initially bipolar and crescent-shaped, but may become four-ribbed before dividing into a pair of bipolar traces. The morphology and anatomy of this plant are nongymnospermous and are most similar to Zygopteridales (particularly Rhacophytaceae and Zygopteridaceae). The Frasnian age of Ellesmeris shows that laminated foliage had evolved in some zygopterid ferns much earlier than previously recognized. The Sphenopteris-like pinnules of Ellesmeris indicate the need for caution when attributing such a convergent foliar design to other plant groups, such as the Devonian gymnosperms. 相似文献