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   8805篇
  免费   666篇
  国内免费   660篇
  2024年   10篇
  2023年   103篇
  2022年   259篇
  2021年   491篇
  2020年   295篇
  2019年   371篇
  2018年   377篇
  2017年   301篇
  2016年   366篇
  2015年   568篇
  2014年   686篇
  2013年   702篇
  2012年   830篇
  2011年   720篇
  2010年   431篇
  2009年   402篇
  2008年   463篇
  2007年   378篇
  2006年   294篇
  2005年   249篇
  2004年   193篇
  2003年   184篇
  2002年   160篇
  2001年   143篇
  2000年   128篇
  1999年   106篇
  1998年   71篇
  1997年   83篇
  1996年   81篇
  1995年   58篇
  1994年   48篇
  1993年   45篇
  1992年   63篇
  1991年   49篇
  1990年   49篇
  1989年   33篇
  1988年   39篇
  1987年   26篇
  1986年   23篇
  1985年   36篇
  1984年   33篇
  1983年   14篇
  1982年   15篇
  1981年   8篇
  1978年   11篇
  1974年   11篇
  1973年   7篇
  1971年   9篇
  1967年   7篇
  1966年   9篇
排序方式: 共有10000条查询结果,搜索用时 375 毫秒
971.
972.
973.
974.
Tumour radioresistance is a major problem for cancer radiation therapy. To identify the underlying mechanisms of this resistance, we used human non-small cell lung cancer (NSCLC) cell lines and focused on the Inhibitor of Apoptosis Protein (IAP) family, which contributes to tumourigenesis and chemoresistance. We investigated the possible correlation between radioresistance in six NSCLC cell lines and IAP protein levels and tested the radiosensitizing effect of birinapant in vitro, a molecule that mimics the second mitochondria-derived activator of caspase. We found that birinapant-induced apoptosis and inhibited the proliferation of NSCLC cells after exposure to radiation. These effects were induced by birinapant downregulation of cIAP protein levels and changes of cIAP gene expression. Overall, birinapant can inhibit tumour growth of NSCLC cell lines to ironizing radiation and act as a promising strategy to overcome radioresistance in NSCLC.  相似文献   
975.
We tested the prediction that global warming has caused recent decreases in body weight (Bergmann's rule) and increases in wing length (Allen's rule) in 14 species of passerine birds at two localities in England: Wicken Fen (1968–2003) and Treswell Wood (1973–2003).
Predicted long-term linear decreases in residual body weight occurred in four species: dunnocks (Wicken Fen), and great tits, blue tits and bullfinches (Treswell Wood). Non-linear decreases also occurred in reed warblers and blackcaps at Wicken Fen, which also had a surprising linear increase in residual body weight in blackbirds.
Residual wing lengths increased linearly, as predicted, in six of seven species at Wicken Fen. Whereas there were non-linear long-term increases in wrens, dunnocks and blackbirds in Treswell Wood. Unexpected linear decreases also occurred in residual wing lengths in willow warblers (Wicken Fen), and blue tits, great tits and chaffinches (Treswell Wood).
The most parsimonious explanation for such long-term changes in body weight is global warming, as predicted by Bergmann's rule. Greater site and species-specific effects on wing length (e.g. non-linear changes plus shorter wings in the woodland habitat) suggest a less straightforward conclusion concerning Allen's rule, probably because wing length involved variation in both bone and feather growth.
Changes in residual body weights and wing lengths often differed between species and were sometimes non-linear, perhaps reflecting short-term modifications in selection pressures. Human-induced influences are discussed, such as avian predator population densities and land-use change. Short-term variation in temperature had little effect, but rainfall did explain the unusual increase in blackbird body weight, possibly as a result of improving food (earthworm) availability.  相似文献   
976.
977.
This study was conducted to compare the efficacy of four in vitro fertilization (IVF) media: Bracket and Oliphant's medium (BO), modified medium 199 (IVF-M199), modified Tyrode's medium (MTM), and modified KSOM (m-KSOM) on fertilization efficiency and blastocyst formation rate. In addition, we wanted to investigate the benefit of prolonging the IVF period (from 6 to 18 h) using the two most effective IVF media determined in our initial experiment; subsequently, blastocyst viability was assessed following vitrification. A higher incidence of polyspermic fertilization was observed in the MTM (6%) and in BO, in both the 6 and 18 h (7% and 11%, respectively) groups, than in the m-KSOM (1%) or in the IVF-M199 6 or 18 h (1 and 3%, respectively) groups. Cleavage rates were similar in BO, IVF-M199, and MTM 48 h post-fertilization; however, the lowest cleavage rate was observed for m-KSOM. A greater proportion of zygotes developed into 8-cell embryos in IVF-M199 than in other IVF media. Subsequently, a greater proportion of blastocyst formation and hatching was achieved in IVF-M199 (40% and 79%, respectively) or BO (35% and 74%, respectively) than in m-KSOM (18% and 58%, respectively) or MTM (22% and 66%, respectively). Prolonging IVF to 18 h did not alter cleavage rates; however, the highest rate of overall blastocyst formation was achieved in the IVF-M199 18 h (49%), rather than in the BO 18 h (20%) group. Vitrified/thawed blastocysts from IVF-M199 groups re-expanded and developed better, as compared to the BO 18 h group, and hatching rate and total cell number in IVF-M199 18 h group was comparable to the control groups (non-vitrified). Vitrification reduced survival compared to controls. In conclusion, IVF-M199 was successfully used for IVF, compared favorably to BO medium, and offered the advantage of an extended IVF period for up to 18 h that requires only one-half a dose of semen, and resulted in better quality blastocysts that endured vitrification with a hatching rate comparable to that of control groups.  相似文献   
978.
Continuous gradient elution chromatography (CGEC) was employed to purify and separate enzymes and polysaccharides from the sap of Rhus vernicifera Chinese lacquer tree. There are three different molecules with laccase enzyme activity. Two are enzymes of each other (L1, and L2), whereas the third (RL) is an entirely separate entity. Two polysaccharides (GP1 and GP2) were also found. The Rhus laccase (RL), and isoenzymes L1 and L2, have peak molecular masses of 109,100, 120,000, 103,000 respectively; each has four copper atoms per molecule, and the pI values were 8.2, 8.6, and 9.1, respectively. The structure of the laccases was studied by Fourier-transform infrared (FT-IR) and Matrix-assisted laser desorption/ionization time-of flight (MALDI-TOF) mass spectrometry. The typical amide I (1646 cm−1) and amide II (1545 cm−1) bands were observed. The results from MALDI-TOF were similar to those from CGEC, but the molecular mass from the MALDI-TOF was significantly different from that obtained from sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE).  相似文献   
979.
The ability to quantify infections provides a tool with which to perform comparative pathological research. The need exists for a simplistic standard method to compare infection levels of Batrachochytrium dendrobatidis, a major cause of global amphibian declines. Through examination of skin sloughs of the Cape river frog Afrana fuscigula, we present an accessible method that not only provides quantitative measurements of B. dendrobatidis, but also provides information that increases the confidence of detection through histological surveys. The method relies on the availability of live animals that are actively shedding skin. By employing a direct microscopic count of sporangia, it is possible to express infection in terms of density. Micro-spatial infection in the skin of A. fuscigula is characterised by significant differences in sporangium density among the different components of the foot, and by similar differences in site infection frequency. Notably, toe tips and tubercles contain higher infection densities and are more often infected than webbing or the base of the foot. This pattern of infection might facilitate disease transmission due to the increased exposure of these components to abrasion. Density data can be used with the Poisson frequency function to approximate binomial probabilities of detecting B. dendrobatidis through histology. The probability matrix produced for A. fuscigula indicated that foot-site selection for histology markedly influenced the number of sections required to detect B. dendrobatidis at a specific level of probability. Thus, examination of a test sample of skin tissue with direct-count quantification can help in planning the sampling of tissues for histological surveys.  相似文献   
980.
Du C  Niu R  Chu E  Zhang P  Lin X 《Journal of biochemistry》2006,139(5):913-920
The thymidylate synthase (TS), an important target for many anticancer drugs, has been cloned from different species. But the cDNA property and function of TS in zebrafish are not well documented. In order to use zebrafish as an animal model for screening novel anticancer agents, we isolated TS cDNA from zebrafish and compared its sequence with those from other species. The open reading frame (ORF) of zebrafish TS cDNA sequence was 954 nucleotides, encoding a 318-amino acid protein with a calculated molecular mass of 36.15 kDa. The deduced amino acid sequence of zebrafish TS was similar to those from other organisms, including rat, mouse and humans. The zebrafish TS protein was expressed in Escherichia coli and purified to homogeneity. The purified zebrafish TS showed maximal activity at 28 degrees C with similar K(m) value to human TS. Western immunoblot assay confirmed that TS was expressed in all the developmental stages of zebrafish with a high level of expression at the 1-4 cell stages. To study the function of TS in zebrafish embryo development, a short hairpin RNA (shRNA) expression vector, pSilencer 4.1-CMV/TS, was constructed which targeted the protein-coding region of zebrafish TS mRNA. Significant change in the development of tail and epiboly was found in zebrafish embryos microinjected pSilencer4.1-CMV/TS siRNA expression vector.  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