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   1864篇
  免费   156篇
  国内免费   181篇
  2024年   5篇
  2023年   24篇
  2022年   47篇
  2021年   93篇
  2020年   80篇
  2019年   84篇
  2018年   60篇
  2017年   62篇
  2016年   74篇
  2015年   115篇
  2014年   137篇
  2013年   148篇
  2012年   173篇
  2011年   146篇
  2010年   112篇
  2009年   88篇
  2008年   118篇
  2007年   69篇
  2006年   62篇
  2005年   56篇
  2004年   46篇
  2003年   39篇
  2002年   43篇
  2001年   36篇
  2000年   34篇
  1999年   37篇
  1998年   18篇
  1997年   13篇
  1996年   14篇
  1995年   25篇
  1994年   18篇
  1993年   10篇
  1992年   14篇
  1991年   15篇
  1990年   10篇
  1989年   14篇
  1988年   16篇
  1987年   6篇
  1986年   5篇
  1985年   8篇
  1984年   6篇
  1983年   4篇
  1982年   5篇
  1981年   3篇
  1980年   2篇
  1979年   2篇
  1974年   1篇
  1973年   1篇
  1967年   1篇
  1956年   1篇
排序方式: 共有2201条查询结果,搜索用时 15 毫秒
131.
Objective: We needed to establish an embryonic fibroblast cell line from the Langshan chicken (LSCEF61) to preserve their important genetic resources at the cellular level. Material and methods: The cell line was established from 9‐day‐old embryos by direct explant culture and cryopreservation techniques. Cell morphology, dynamic proliferation and any contamination present were tested, and the karyotype and levels of isoenzymes of lactic dehydrogenase and malic dehydrogenase were analysed. Four types of fluorescent protein exogenous genes for pEGFP‐C1, pEGFP‐N3, pEYFP‐N1 and pDsRed1‐N1 were transfected into the cells. Results: Showed that the cells were healthy and were of spindle shaped structure, without change in morphology. Cell growth curves were of typical S‐shape. Assays for microbial contamination were negative. The LSCEF61 line showed no cross‐contamination when assessed by isoenzyme analysis. Chromosome number (2n) = 78 on more than 90% of occasions. The four types of fluorescent protein extro‐genes appeared to be expressed effectively with high transfection efficiency between 15.6% and 38.6%. Conclusion: The cell line met each of the quality control standards required for the American Type Culture Collection. It had not only preserved the genetic resources of the important Langshan chicken at the cellular level, but also provided valuable material for genomic, post‐genomic and somatic cell cloning research and other applications.  相似文献   
132.
Several analogs of aristolochic acids were isolated and derivatized into their lactam derivatives to study their inhibition in CDK2 assay. The study helped to derive some conclusions about the structure–activity relation around the phenanthrin moiety. Semi-synthetic aristolactam 21 showed good activity with inhibition IC50 of 35 nM in CDK2 assay. The activity of this compound was comparable to some of the most potent synthetic compounds reported in the literature.  相似文献   
133.
The diterpenoids (+)-ferruginol (1), ent-kaur-16-en-15-one (2), ent-8(14),15-sandaracopimaradiene-2α,18-diol (3), 8(14),15-sandaracopimaradiene-2α,18,19-triol (4), and (+)-sugiol (5) and the triterpenoids 3β-methoxycycloartan-24(241)-ene (6), 3β,23β-dimethoxycycloartan-24(241)-ene (7), 3β,23β-dimethoxy-5α-lanosta-24(241)-ene (8), and 23(S)-23-methoxy-24-methylenelanosta-8-en-3-one (9), isolated from Amentotaxus formosana, showed inhibitory effects on xanthine oxidase (XO). Of the compounds tested, compound 5 was a potent inhibitor of XO activity, with an IC50 value of 6.8 ± 0.4 μM, while displaying weak ABTS radical cation scavenging activity. Treatment of the bladder cancer cell line, NTUB1, with 3–10 μM of compound 5 and 10 μM cisplatin, and immortalized normal human urothelial cell line, SV-HUC1, with 0.3–1 μM and 10–50 μM of compound 5 and 10 μM cisplatin, respectively, resulted in increased viability of cells compared with cytotoxicity induced by cisplatin. Treatment of NTUB1 with 20 μM cisplatin and 10 or 30 μM of compound 5 resulted in decreased ROS production compared with ROS production induced by cisplatin. These results indicate that 10 or 30 μM of compound 5 in NTUB1 cells may mediate through the suppression of XO activity and reduction of reactive oxygen species (ROS) induced by compound 5 cotreated with 20 μM cisplatin and protection of subsequent cell death.  相似文献   
134.
135.

Background  

A hip replacement with a cemented or cementless femoral stem produces an effect on the bone called adaptive remodelling, attributable to mechanical and biological factors. All of the cementless prostheses designs try to achieve an optimal load transfer in order to avoid stress-shielding, which produces an osteopenia.  相似文献   
136.

Background  

The proximal interphalange joint (PIP) is fundamental for the functional nature of the hand. The contracture in flexion of the PIP, secondary to traumatisms or illnesses leads to an important functional loss. The use of correcting splints is the common procedure for treating this problem. Its functioning is based on the application of a small load and a prolonged stress which can be dynamic, static progressive or static serial.  相似文献   
137.

Introduction

To investigate whether accelerated hand bone mineral density (BMD) loss is associated with progressive joint damage in hands and feet in the first year of rheumatoid arthritis (RA) and whether it is an independent predictor of subsequent progressive total joint damage after 4 years.

Methods

In 256 recent-onset RA patients, baseline and 1-year hand BMD was measured in metacarpals 2-4 by digital X-ray radiogrammetry. Joint damage in hands and feet were scored in random order according to the Sharp-van der Heijde method at baseline and yearly up to 4 years.

Results

68% of the patients had accelerated hand BMD loss (>-0.003 g/cm2) in the first year of RA. Hand BMD loss was associated with progressive joint damage after 1 year both in hands and feet with odds ratios (OR) (95% confidence intervals [CI]) of 5.3 (1.3-20.9) and 3.1 (1.0-9.7). In univariate analysis, hand BMD loss in the first year was a predictor of subsequent progressive total joint damage after 4 years with an OR (95% CI) of 3.1 (1.3-7.6). Multivariate analysis showed that only progressive joint damage in the first year and anti-citrullinated protein antibody positivity were independent predictors of long-term progressive joint damage.

Conclusions

In the first year of RA, accelerated hand BMD loss is associated with progressive joint damage in both hands and feet. Hand BMD loss in the first year of recent-onset RA predicts subsequent progressive total joint damage, however not independent of progressive joint damage in the first year.  相似文献   
138.
Huang SX  Pu JX  Xiao WL  Li LM  Weng ZY  Zhou Y  Han QB  Peng SL  Ding LS  Lou LG  Sun HD 《Phytochemistry》2007,68(5):616-622
ent-Abietane diterpenoids, hebeiabinins A-F (1-5), together with seven known diterpenoids were isolated from leaves of Isodon rubescens var. rubescens. The structures of 1-5 were established on the basis of spectroscopic analyses, including application of 2D NMR spectroscopic techniques. The diterpenoids isolated were evaluated for the cytotoxicity against A549, HT-29, and K562 tumor cells. Compound 5 was the most active with IC(50) value of 0.91 microM against A549 cells.  相似文献   
139.
PII is an important signal protein for regulation of nitrogen metabolism in bacteria and plants. We constructed a mutant of glnB, encoding PII, in a heterocystous cyanobacterium, Anabaena sp. PCC 7120, with a cre-loxP system. The mutant (MP2alpha) grew more slowly than the wild type under all nitrogen regimens. It excreted a large amount of ammonium when grown on nitrate due to altered activities of glutamine synthetase and nitrate reductase. MP2alpha had a low nitrogenase activity but was able to form heterocysts under diazotrophic conditions, suggesting that PII is not required for heterocyst differentiation. Analysis of the PII with mass spectroscopy found tyrosine nitration at Tyr-51 under diazotrophic conditions while no phosphorylation at Ser-49 was detected. The strains 51F and 49A, which have PII with mutations of Y51F and S49A, respectively, were constructed to analyze the functions of the two key residues on the T-loop. Like MP2alpha, they had low nitrogenase activity and grew slowly under diazotrophic conditions. 49A was also impaired in nitrate uptake and formed heterocysts in the presence of nitrate. The up-regulation of ntcA after nitrogen step-down, which was present in the wild type, was not observed in 51F and 49A. While our results showed that the Ser-49 residue is important to the function of PII in Anabaena sp. PCC 7120, evidence from the PII pattern of the wild type and 49A in non-denaturing gel electrophoresis suggested that Ser-49 is not modified. The possible physiological roles of tyrosine nitration of PII are discussed.  相似文献   
140.
采用紫外线对现有生产菌株进行诱变处理,再运用筛选剂丙酸、丁酸等对其进行选育,得到高产菌株M-3-01。投入中试车间发酵罐中,发酵效价达到50560×103u.L-1,其发酵能力比出发菌株提高了26%。  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