全文获取类型
收费全文 | 393562篇 |
免费 | 30655篇 |
国内免费 | 1056篇 |
出版年
2018年 | 16033篇 |
2017年 | 14980篇 |
2016年 | 11861篇 |
2015年 | 5114篇 |
2014年 | 5315篇 |
2013年 | 7872篇 |
2012年 | 13357篇 |
2011年 | 23740篇 |
2010年 | 19938篇 |
2009年 | 15725篇 |
2008年 | 19494篇 |
2007年 | 21881篇 |
2006年 | 7566篇 |
2005年 | 8544篇 |
2004年 | 8208篇 |
2003年 | 8238篇 |
2002年 | 7703篇 |
2001年 | 12065篇 |
2000年 | 12068篇 |
1999年 | 9366篇 |
1998年 | 3405篇 |
1997年 | 3647篇 |
1996年 | 3544篇 |
1995年 | 3128篇 |
1994年 | 3117篇 |
1993年 | 3142篇 |
1992年 | 7891篇 |
1991年 | 7909篇 |
1990年 | 7525篇 |
1989年 | 7491篇 |
1988年 | 6869篇 |
1987年 | 6620篇 |
1986年 | 6020篇 |
1985年 | 6207篇 |
1984年 | 5159篇 |
1983年 | 4364篇 |
1982年 | 3389篇 |
1981年 | 3144篇 |
1980年 | 2948篇 |
1979年 | 4951篇 |
1978年 | 3811篇 |
1977年 | 3725篇 |
1976年 | 3432篇 |
1975年 | 3796篇 |
1974年 | 4176篇 |
1973年 | 4092篇 |
1972年 | 4311篇 |
1971年 | 4038篇 |
1970年 | 3093篇 |
1969年 | 2996篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
91.
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
92.
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
93.
94.
95.
The review summarizes the authors’ and literature data on accumulation of DNA breaks in differentiating cells. Large 50-kb free DNA fragments were observed by several research teams in non-apoptotic insect, mammal, and plant cells. More intense DNA breakage was observed during maturation of spermatides, embryo development, and differentiation of myotubes, epidermal cells, lymphocytes, and neutrophils. In general, accumulation of DNA breaks in differentiating cells cannot be attributed to a decrease in the DNA repair efficiency. Poly(ADP)ribose synthesis often follows the DNA breakage in differentiating cells. We hypothesize that DNA fragmentation is an epigenetic tool for regulating the differentiation process. Scarce data on localization of the differentiation-associated DNA breaks indicate their preferable accumulation in specific DNA sequences including the nuclear matrix attachment sites. The same sites are degraded at early stages of apoptosis. Recent data on non-apoptotic function of caspases provide more evidence for possible existence of a DNA breakage mechanism in differentiating cells, resembling the initial stage of apoptosis. Excision of methylated cytosine and recombination are other possible explanations of the phenomenon. Elucidation of mechanisms of differentiation-induced DNA breaks appears to be a prospective research direction. 相似文献
96.
97.
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
98.
99.
100.