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   16316篇
  免费   1545篇
  国内免费   10篇
  2021年   149篇
  2019年   137篇
  2018年   174篇
  2017年   209篇
  2016年   263篇
  2015年   496篇
  2014年   488篇
  2013年   674篇
  2012年   887篇
  2011年   829篇
  2010年   579篇
  2009年   478篇
  2008年   739篇
  2007年   773篇
  2006年   683篇
  2005年   809篇
  2004年   692篇
  2003年   669篇
  2002年   638篇
  2001年   328篇
  2000年   313篇
  1999年   287篇
  1998年   218篇
  1997年   177篇
  1996年   192篇
  1995年   173篇
  1994年   147篇
  1993年   178篇
  1992年   221篇
  1991年   224篇
  1990年   214篇
  1989年   213篇
  1988年   204篇
  1987年   162篇
  1986年   175篇
  1985年   149篇
  1984年   141篇
  1983年   137篇
  1982年   131篇
  1981年   175篇
  1980年   133篇
  1979年   161篇
  1978年   145篇
  1977年   144篇
  1976年   126篇
  1975年   122篇
  1974年   153篇
  1973年   149篇
  1972年   128篇
  1970年   138篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
2.
Ffh is a component of a bacterial ribonucleoprotein complex homologous to the signal recognition particle (SRP) of eukaryotes. It comprises three domains that mediate both binding to the hydrophobic signal sequence of the nascent polypeptide and the GTP-dependent interaction of Ffh with a structurally homologous GTPase of the SRP receptor. The X-ray structures of the two-domain 'NG' GTPase of Ffh in complex with Mg2+GDP and GDP have been determined at 2.0 A resolution. The structures explain the low nucleotide affinity of Ffh and locate two regions of structural mobility at opposite sides of the nucleotide-binding site. One of these regions includes highly conserved sequence motifs that presumably contribute to the structural trigger signaling the GTP-bound state. The other includes the highly conserved interface between the N and G domains, and supports the hypothesis that the N domain regulates or signals the nucleotide occupancy of the G domain.  相似文献   
3.
4.
The coliform bacterial population in the Grand Forks, N.Dak. sewage system was examined for multiple-antibiotic-resistant organisms over a 1-year period. Multiple-antibiotic-resistant coliforms were found to be common in the sewage, and their numbers remained fairly constant relative to the total coliform population throughout the year. Resistance to kanamycin, tetracycline, and ampicillin was found to be transferable at variable rates. Transfer rates were found to be temperature sensitive and were optimal at 35 degrees C. Although 75% of the multiple-antibiotic-resistant coliforms were capable of transferring resistance at some level, only 25% were capable of transferring resistance at rates greater than 10(-3) transconjugants per initial donor.  相似文献   
5.
6.
The effect of the pentapeptide H-Cys-Tyr-Ile-Gln-Asn-OH to act like melanocyte-stimulating hormone-releasing factor (MSH-RF) was studied. This peptide decreases at ng amounts the MSH content of the rat pituitary and increases plasma MSH concentration. This agent also stimulates the release of MSH in animals with eminence lesion, indicating a direct action of the pentapeptide on the gland.  相似文献   
7.
8.
9.
Abstract. Degeneration of the archenteron in middle gastrulae occurred in the presence of α,α'-dipyridyl or Zn2+, inhibitors of prolyl hydroxylase. In the presence of these substances the archenteron degenerated and was eventually destroyed. Adding Fe2+ to the embryo culture containing α,α'-dipyridyl protected the archenteron from further degeneration, but the collapsed archenteron was not restored to the upright position. At the late gastrula stage, α,α'-dipyridyl did not cause the degeneration of the archenteron. Treatment of the embryos by α,α'-dipyridyl, starting at the swimming blastula stage, resulted in the production of many mesenchyme-like cells but archenteron was not produced in the embryos. Addition of Fe2+ to α,α'-dipyridyl culture, just before the beginning of gastrulation of normal embryos, resulted in the formation of normal archenteron. α,α'-Dipyridyl inhibited hydroxylation of proline residues of collagen in sea urchin embryos and Fe2+ prevented the inhibition by α,α'-dipyridyl. Respiration was not inhibited by α,α'-dipyridyl.  相似文献   
10.
The interaction between some polyhexamethylene biguanides and the cell envelope of Escherichia coli has been investigated. An amine-ended dimer, (AED, n = 2), a polydisperse mixture (ICI plc) available as the active ingredient of Vantocil IB, (PHMB, n = 5.5), and a high molecular weight fraction, (HMW, n = greater than or equal to 10) of PHMB were used. The sensitivity of batch cultures depleted of magnesium (M-dep), phosphorus (P-dep) or glycerol (C-dep) towards the biocides was assessed by monitoring the rate and extent of potassium ion leakage. P-dep suspensions were particularly resistant to all these agents and possessed less than half the quantity of phospholipid of other cell types. This was compensated for by a proportionate increase in fatty acid and neutral lipid content of the cells. The reduction in phospholipid content was accounted for by decreases in phosphatidylglycerol (PG) and phosphatidylethanolamine (PE). Diphosphatidylglycerol (DPG) and phosphatidylserine (PS) content of the cultures remained unaffected by the depleting nutrient. Fourier-transform n.m.r. spectroscopy was used to study proton nuclei during the interaction of HMW, AED and PHMB with various phospholipid-vesicle preparations. The results strongly suggest that the biocides acted preferentially on the acidic phospholipids PG and DPG, rather than towards PE or PS. Resistance of P-dep cultures therefore reflected reductions in PG content. A molecular basis for the interaction of these compounds and membranes is proposed.  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