全文获取类型
收费全文 | 1846篇 |
免费 | 212篇 |
国内免费 | 1篇 |
专业分类
2059篇 |
出版年
2021年 | 16篇 |
2019年 | 16篇 |
2018年 | 17篇 |
2017年 | 22篇 |
2016年 | 33篇 |
2015年 | 38篇 |
2014年 | 61篇 |
2013年 | 75篇 |
2012年 | 83篇 |
2011年 | 98篇 |
2010年 | 62篇 |
2009年 | 46篇 |
2008年 | 65篇 |
2007年 | 59篇 |
2006年 | 46篇 |
2005年 | 69篇 |
2004年 | 67篇 |
2003年 | 63篇 |
2002年 | 57篇 |
2001年 | 62篇 |
2000年 | 63篇 |
1999年 | 48篇 |
1998年 | 36篇 |
1997年 | 20篇 |
1996年 | 27篇 |
1995年 | 22篇 |
1994年 | 25篇 |
1993年 | 27篇 |
1992年 | 35篇 |
1991年 | 38篇 |
1990年 | 44篇 |
1989年 | 49篇 |
1988年 | 45篇 |
1987年 | 23篇 |
1986年 | 30篇 |
1985年 | 38篇 |
1984年 | 26篇 |
1983年 | 28篇 |
1982年 | 22篇 |
1981年 | 19篇 |
1980年 | 14篇 |
1979年 | 31篇 |
1977年 | 16篇 |
1976年 | 29篇 |
1975年 | 14篇 |
1974年 | 15篇 |
1973年 | 23篇 |
1971年 | 22篇 |
1970年 | 22篇 |
1968年 | 15篇 |
排序方式: 共有2059条查询结果,搜索用时 0 毫秒
101.
Kim EK Tucker DF Yun SJ Do KH Kim MS Kim JH Kim CD Birnbaum MJ Bae SS 《Cellular signalling》2008,20(11):2030-2037
The phosphatidylinositol 3-kinase (PI3K) signaling pathway(s) is activated by a variety of agonists to regulate cell migration. Here, we show that the stimulation of mouse embryonic fibroblasts with platelet-derived growth factor (PDGF) induces migration in a PI3K-dependent manner. Cells lacking Akt1/PKBalpha exhibit impaired migration and peripheral ruffling in response to PDGF stimulation, whereas cells lacking Akt2/PKBbeta are normal. In addition, over-expression of Akt1/PKBalpha but not Akt2/PKBbeta is sufficient to restore PDGF-induced cell migration in an Akt1/PKBalpha and Akt2/PKBbeta deficient background. In response to PDGF stimulation, Akt1/PKBalpha selectively translocates to membrane ruffles, however, this localization is abrogated by substituting the linker region of Akt2/PKBbeta. Similarly, expression of an Akt2/PKBalpha chimera containing the linker region of Akt1/PKBalpha restored PDGF-induced migration in cells lacking both Akt1/PKBalpha and Akt2/PKBbeta. Finally, over-expression of constitutively active Rac rescues PDGF-induced migration defects in cells lacking Akt1/PKBalpha. Given these results, we suggest that Akt1/PKBalpha controls cell migration by selectively translocating to the leading edge and activating Rac. 相似文献
102.
Wen-Qing Li Nan Hu Zhaoming Wang Kai Yu Hua Su Lemin Wang Chaoyu Wang Stephen J. Chanock Laurie Burdett Ti Ding You-Lin Qiao Jin-Hu Fan Yuan Wang Yi Xu Carol Giffen Xiaoqin Xiong Gwen Murphy Margaret A. Tucker Sanford M. Dawsey Neal D. Freedman Christian C. Abnet Alisa M. Goldstein Philip R. Taylor 《PloS one》2013,8(7)
The epidermal growth factor receptor (EGFR) signaling pathway regulates cell proliferation, differentiation, and survival, and is frequently dysregulated in esophageal and gastric cancers. Few studies have comprehensively examined the association between germline genetic variants in the EGFR pathway and risk of esophageal and gastric cancers. Based on a genome-wide association study in a Han Chinese population, we examined 3443 SNPs in 127 genes in the EGFR pathway for 1942 esophageal squamous cell carcinomas (ESCCs), 1758 gastric cancers (GCs), and 2111 controls. SNP-level analyses were conducted using logistic regression models. We applied the resampling-based adaptive rank truncated product approach to determine the gene- and pathway-level associations. The EGFR pathway was significantly associated with GC risk (P = 2.16×10−3). Gene-level analyses found 10 genes to be associated with GC, including FYN, MAPK8, MAP2K4, GNAI3, MAP2K1, TLN1, PRLR, PLCG2, RPS6KB2, and PIK3R3 (P<0.05). For ESCC, we did not observe a significant pathway-level association (P = 0.72), but gene-level analyses suggested associations between GNAI3, CHRNE, PAK4, WASL, and ITCH, and ESCC (P<0.05). Our data suggest an association between specific genes in the EGFR signaling pathway and risk of GC and ESCC. Further studies are warranted to validate these associations and to investigate underlying mechanisms. 相似文献
103.
François Hug Paul W. Hodges Kylie J. Tucker 《Journal of electromyography and kinesiology》2013,23(6):1413-1420
IntroductionWe aimed to determine whether the changes in muscle activity (in terms of both gross electromyography (EMG) and motor unit (MU) discharge characteristics) observed during pain are spatially organized with respect to pain location within a muscle which is the main contributor of the task.MethodsSurface and fine-wire EMG was recorded during matched low-force isometric plantarflexion from soleus (from four quadrants with fine-wire EMG and from the medial/lateral sides with surface EMG), both gastrocnemii heads, peroneus longus, and tibialis anterior. Four conditions were tested: two control conditions that each preceded contractions with pain induced in either the lateral (PainL) or medial (PainM) side of soleus.ResultsNeither the presence (p = 0.28) nor location (p = 0.19) of pain significantly altered gross muscle activity of any location (lateral/medial side of soleus, gastrocnemii, peroneus longus and tibialis anterior). Group data from 196 MUs show redistribution of MU activity throughout the four quadrants of soleus, irrespective of pain location. The significant decrease of MU discharge rate during pain (p < 0.0001; PainL: 7.3 ± 0.9–6.9 ± 1.1 Hz, PainM: 7.0 ± 1.1 to 6.6 ± 1.1 Hz) was similar for all quadrants of the soleus (p = 0.43), regardless of the pain location (p = 0.98). There was large inter-participant variation in respect to the characteristics of the altered MU discharge with pain.ConclusionResults from both surface and fine-wire EMG recordings do not support the hypothesis that muscle activity is reorganized in a simple systematic manner with respect to pain location. 相似文献
104.
Samad Abdul Zhang Ji Chen Da Chen Xiaowen Tucker Melvin Liang Yanna 《Journal of industrial microbiology & biotechnology》2017,44(3):353-362
Journal of Industrial Microbiology & Biotechnology - To make the process of producing sophorolipids by Candida bombicola truly sustainable, we investigated production of these biosurfactants on... 相似文献
105.
Nathanson KL Kanetsky PA Hawes R Vaughn DJ Letrero R Tucker K Friedlander M Phillips KA Hogg D Jewett MA Lohynska R Daugaard G Richard S Chompret A Bonaïti-Pellié C Heidenreich A Olah E Geczi L Bodrogi I Ormiston WJ Daly PA Oosterhuis JW Gillis AJ Looijenga LH Guilford P Fosså SD Heimdal K Tjulandin SA Liubchenko L Stoll H Weber W Rudd M Huddart R Crockford GP Forman D Oliver DT Einhorn L Weber BL Kramer J McMaster M Greene MH Pike M Cortessis V Chen C Schwartz SM Bishop DT Easton DF 《American journal of human genetics》2005,77(6):1034-1043
Testicular germ cell tumor (TGCT) is the most common cancer in young men. Despite a considerable familial component to TGCT risk, no genetic change that confers increased risk has been substantiated to date. The human Y chromosome carries a number of genes specifically involved in male germ cell development, and deletion of the AZFc region at Yq11 is the most common known genetic cause of infertility. Recently, a 1.6-Mb deletion of the Y chromosome that removes part of the AZFc region—known as the “gr/gr” deletion—has been associated with infertility. In epidemiological studies, male infertility has shown an association with TGCT that is out of proportion with what can be explained by tumor effects. Thus, we hypothesized that the gr/gr deletion may be associated with TGCT. Using logistic modeling, we analyzed this deletion in a large series of TGCT cases with and without a family history of TGCT. The gr/gr deletion was present in 3.0% (13/431) of TGCT cases with a family history, 2% (28/1,376) of TGCT cases without a family history, and 1.3% (33/2,599) of unaffected males. Presence of the gr/gr deletion was associated with a twofold increased risk of TGCT (adjusted odds ratio [aOR] 2.1; 95% confidence interval [CI] 1.3–3.6; P = .005) and a threefold increased risk of TGCT among patients with a positive family history (aOR 3.2; 95% CI 1.5–6.7; P = .0027). The gr/gr deletion was more strongly associated with seminoma (aOR 3.0; 95% CI 1.6–5.4; P = .0004) than with nonseminoma TGCT (aOR 1.5; 95% CI 0.72–3.0; P = .29). These data indicate that the Y microdeletion gr/gr is a rare, low-penetrance allele that confers susceptibility to TGCT. 相似文献
106.
A study of bacterial surface oligosaccharides were investigated among
different strains of Neisseria gonorrhoeae to correlate structural features
essential for binding to the MAb 2C7. This epitope is widely expressed and
conserved in gonococcal isolates, characteristics essential to an effective
candidate vaccine antigen. Sample lipooligosaccharides (LOS), was prepared
by a modification of the hot phenol-water method from which de-O-acetylated
LOS and oligosaccharide (OS) components were analyzed by ES-MS-CID-MS and
ES-MSnin a triple quadrupole and an ion trap mass spectrometer,
respectively. Previously documented natural heterogeneity was apparent from
both LOS and OS preparations which was admixed with fragments induced by
hydrazine and mild acid treatment. Natural heterogeneity was limited to
phosphorylation and antenni extensions to the alpha-chain. Mild acid
hydrolysis to release OS also hydrolyzed the beta(1-->6) glycosidic
linkage of lipid A. OS structures were determined by collisional and
resonance excitation combined with MS and multistep MSn which provided
sequence information from both neutral loss, and nonreducing terminal
fragments. A comparison of OS structures, with earlier knowledge of MAb
binding, enzyme treatment, and partial acid hydrolysis indicates a generic
overlapping domain for 2C7 binding. Reoccurring structural features include
a Hepalpha(1-->3)Hepbeta(1-->5)KDO trisaccharide core branched on the
nonreducing terminus (Hep-2) with an alpha(1-->2) linked GlcNAc
(gamma-chain), and an alpha-linked lactose (beta-chain) residue. From the
central heptose (Hep-1), a beta(1-->4) linked lactose (alpha-chain),
moiety is required although extensions to this residue appear unnecessary.
相似文献
107.
Murine embryonic stem (ES) cells are commonly cultured on feeder layers of primary murine embryonic fibroblasts (MEFs). Because gene targeting experiments often involve sequential selection for multiple-drug resistance in single ES cell lines, we have developed a new mouse strain which represents an economical donor for the production of multiple-drug resistant MEFs. MEFs prepared from the DR-4 mouse strain displayed resistance to concentrations of the drugs G418, 6-thioguanine, puromycin and hygromycin well above those used normally for the selection of drug-resistant ES cells. 相似文献
108.
Characterization of leaf senescence and pod development in soybean explants 总被引:2,自引:0,他引:2 下载免费PDF全文
Excised soybean (Glycine max [L.] Merrill) cv Anoka leaf discs tend to remain green even after the corresponding intact leaves have turned yello on fruiting plants. We have found that explants which include a leaf along with a stem segment (below the node) and one or more pods (maintained on distilled H2O) show similar but accelerated leaf yellowing and abscission compared with intact plants. In podded explants excised at pre-podfill, the leaves begin to yellow after 16 days, whereas those excised at late podfill begin to yellow after only 6 days. Although stomatal resistances remain low during the first light period after excision, they subsequently increase to levels above those in leaves of intact plants. Explants taken at mid to late podfill with one or more pods per node behave like intact plants in that pod load does not affect the time lag to leaf yellowing. Explant leaf yellowing and abscission are delayed by removal of the pods or seeds or by incubation in complete mineral nutrient solution or in 4.6 micromolar zeatin. Like chorophyll breakdown, protein loss is accelerated in the explants, but minerals or especially zeatin can retard the loss. Pods on explants show rates and patterns of color change (green to yellow to brown) similar to those of pods on intact plants. These changes start earlier in explants on water than in intact plants, but they can be delayed by adding zeatin. Seed dry weight increased in explants, almost as much as in intact plants. Explants appear to be good analogs of the corresponding parts of the intact plant, and they should prove useful for analyzing pod development and mechanisms of foliar senescence. Moreover, our data suggest that the flux of minerals and cytokinin from the roots could influence foliar senescence in soybeans, but increased stomatal resistance does not seem to cause foliar senescence. 相似文献
109.
Productivity and nutrient uptake of water hyacinth,Eichhornia crassipes I. Effect of nitrogen source
Water hyacinth,Eichhornia crassipes, growth and nutrient uptake rates, as influenced by different N sources and N transformations, were measured using microcosm aquaculture systems. Net productivity was highest in the system receiving equal amounts of NH4 + and NO3 - (at 10 mg N 1-1 each) and decreased in the order of NO3 -, NH4 +, urea (added at 20 mg N 1-1 each), and methane digestor effluent (at 6 mg N 1-1). During the first 7-wk study (average ambient air temperature was 26–28°C), biomass yields were in the range of 19–53 g dry wt m-2 day-1, while between the 8th and 12th wk (average ambient air temperature was 16–22°C), biomass yields were in the range of 10–33 g dry wt m-2 day-1. In the systems with either NH4 + or NO3 -, or both added in equal proportions, about 14–20% of the total yield was contributed by roots, whereas in the system with urea and digestor effluent, roots contributed about 23 and 44% of the total yield, respectively. Nitrogen and P uptake per unit area followed trends similar to biomass yields. Nitrogen uptake rates were in the range of 533–2, 161 mg N m-2 day-1 for the systems receiving NH4 +, NO3 -, and urea, while uptake rates were in the range of 124–602 mg N m-2 day-1 for the system receiving methane digestor effluent. Phosphorus uptake rates were found to be in the range of 59–542 mg P m-2 day-1. Under the most favorable conditions, maximum recorded biomass yield was 53 g dry wt m-2 day-1, with N and P removal rate of 2,161 mg N m-2 day-1 and 542 mg P m-2 day-1, indicating the potential of water hyacinth to produce large amounts of biomass which can be potentially used as a feedstock to produce methane. 相似文献
110.
Genetic relatedness among six bacterial strains currently classified as vibrios was determined by biochemical tests, deoxyribonucleic acid base composition analysis, and the detection of nucleic acid homology by the membrane filter method. Varying degrees of genetic homology existed among vibrios having similar base compositions. The bovine isolate which had a substantially different per cent guanine plus cytosine content was shown to be genetically different from the remaining vibrio strains. The use of nucleic acid homology studies in classifying vibrio strains is discussed. 相似文献