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   529581篇
  免费   56242篇
  国内免费   384篇
  2018年   5013篇
  2016年   6686篇
  2015年   8270篇
  2014年   9969篇
  2013年   14301篇
  2012年   15874篇
  2011年   16588篇
  2010年   11266篇
  2009年   10074篇
  2008年   14482篇
  2007年   15109篇
  2006年   14164篇
  2005年   13659篇
  2004年   13555篇
  2003年   13210篇
  2002年   12944篇
  2001年   24170篇
  2000年   24309篇
  1999年   19099篇
  1998年   6292篇
  1997年   6485篇
  1996年   6213篇
  1995年   5868篇
  1994年   5805篇
  1993年   5679篇
  1992年   15842篇
  1991年   15320篇
  1990年   15192篇
  1989年   14752篇
  1988年   13707篇
  1987年   13155篇
  1986年   11807篇
  1985年   12182篇
  1984年   9932篇
  1983年   8642篇
  1982年   6501篇
  1981年   5856篇
  1980年   5504篇
  1979年   9627篇
  1978年   7327篇
  1977年   6748篇
  1976年   6451篇
  1975年   7166篇
  1974年   7706篇
  1973年   7782篇
  1972年   7011篇
  1971年   6521篇
  1970年   5652篇
  1969年   5386篇
  1968年   4959篇
排序方式: 共有10000条查询结果,搜索用时 62 毫秒
951.
952.
953.
954.
955.
Species are generally described from morphological features, but there is growing recognition of sister forms that show substantial genetic differentiation without obvious morphological variation and may therefore be considered ‘cryptic species’. Here, we investigate the field vole (Microtus agrestis), a Eurasian mammal with little apparent morphological differentiation but which, on the basis of previous sex‐linked nuclear and mitochondrial DNA (mtDNA) analyses, is subdivided into a Northern and a Southern lineage, sufficiently divergent that they may represent two cryptic species. These earlier studies also provided limited evidence for two major mtDNA lineages within Iberia. In our present study, we extend these findings through a multilocus approach. We sampled 163 individuals from 46 localities, mainly in Iberia, and sequenced seven loci, maternally, paternally and biparentally inherited. Our results show that the mtDNA lineage identified in Portugal is indeed a distinct third lineage on the basis of other markers as well. In fact, multilocus coalescent‐based methods clearly support three separate evolutionary units that may represent cryptic species: Northern, Southern and Portuguese. Divergence among these units was inferred to have occurred during the last glacial period; the Portuguese lineage split occurred first (estimated at c. 70 000 bp ), and the Northern and Southern lineages separated at around the last glacial maximum (estimated at c. 18 500 bp ). Such recent formation of evolutionary units that might be considered species has repercussions in terms of understanding evolutionary processes and the diversity of small mammals in a European context.  相似文献   
956.
957.
958.
P Jemth  B Mannervik 《Biochemistry》1999,38(31):9982-9991
The reaction mechanism of rat glutathione transferase T2-2 has been studied using pre-steady-state and steady-state kinetics. Several parts of the catalytic cycle including binding of substrates, product formation, and product release were investigated. Under saturating conditions, a two-step product release was found to be rate limiting in the enzyme-catalyzed reactions between the nucleophilic substrate glutathione and either of the two electrophilic substrates 1-menaphthyl sulfate and 4-nitrobenzyl chloride. The rate constant for pre-steady-state product formation on rat glutathione transferase T2-2 has an observed pK(a) value of 5.7 apparently due to ionization of the sulfhydryl group of glutathione. This rate constant is approximately 2 orders of magnitude higher than k(cat) at pH values of >6. It can be predicted from the pH dependence that product formation would be the sole rate-limiting step at pH values of <3. A hysteretic mechanism of rGST T2-2 is proposed based on a slow conformational transition detected in pre-steady-state displacement experiments.  相似文献   
959.
Sign language studies of cross-fostered chimpanzees measure the effect of special rearing conditions on the development of very young chimpanzees. Cross-fostered chimpanzees, like human children, develop gradually in a process that takes many years. Here we discuss details of the procedure, the overlap between human and chimpanzee infants in the contents of the first 50-item vocabularies, and the ways in which the signs of the chimpanzees exhibit the fuzziness of natural language categories. We also compare the cross-fostering approach with more traditional modular approaches to the study of language-like behavior in nonhuman animals. Project Washoe was originally supported by grants MH-12154 from the National Institute of Mental Health and GB-7432 from the National Science Foundation. We gratefully acknowledge this support and the support that later sign language studies of chimpanzees have received since then from NIH, NSF, the National Geographic Society, the Grant Foundation, the Spencer Foundation, the University of Nevada, and the UNR Foundation.  相似文献   
960.
Methylisocyanate (MIC) induced mutagenic responses in the absence of exogenous activation in the mouse lymphoma cell forward mutation assay at concentrations as low as 8-24 microM. MIC produced predominantly small mutant colonies, suggesting the possibility of clastogenic activity. The intermediate hydrolysis product, methylamine, was also mutagenic without exogenous activation but required several hundred-fold higher concentrations (ca. 3 mM). N,N'-Dimethylurea, the final product in the reaction of methylisocyanate and water, was totally refractory in either the presence or absence of S9 for concentrations up to 57 mM (5 mg/ml). The ethyl ester of N-methylcarbamic acid was also tested since it was the only available analogue to the highly reactive N-methylcarbamic acid intermediate. This compound was mutagenic only in the presence of S9 at doses exceeding 5-40 microM, which suggested the possibility that the free acid, produced by enzymatic hydrolysis, is also mutagenic. The mutagenic activity of the ester resulted solely in the production of small mutant colonies.  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