全文获取类型
收费全文 | 42201篇 |
免费 | 3683篇 |
国内免费 | 3065篇 |
出版年
2024年 | 106篇 |
2023年 | 526篇 |
2022年 | 1065篇 |
2021年 | 1770篇 |
2020年 | 1231篇 |
2019年 | 1516篇 |
2018年 | 1501篇 |
2017年 | 1073篇 |
2016年 | 1537篇 |
2015年 | 2538篇 |
2014年 | 2883篇 |
2013年 | 3153篇 |
2012年 | 3802篇 |
2011年 | 3504篇 |
2010年 | 2099篇 |
2009年 | 1869篇 |
2008年 | 2252篇 |
2007年 | 1999篇 |
2006年 | 1781篇 |
2005年 | 1469篇 |
2004年 | 1339篇 |
2003年 | 1160篇 |
2002年 | 1045篇 |
2001年 | 861篇 |
2000年 | 785篇 |
1999年 | 691篇 |
1998年 | 395篇 |
1997年 | 394篇 |
1996年 | 378篇 |
1995年 | 302篇 |
1994年 | 310篇 |
1993年 | 224篇 |
1992年 | 383篇 |
1991年 | 343篇 |
1990年 | 294篇 |
1989年 | 261篇 |
1988年 | 231篇 |
1987年 | 182篇 |
1986年 | 168篇 |
1985年 | 185篇 |
1984年 | 175篇 |
1983年 | 117篇 |
1982年 | 105篇 |
1981年 | 64篇 |
1980年 | 64篇 |
1979年 | 82篇 |
1978年 | 80篇 |
1976年 | 73篇 |
1975年 | 67篇 |
1974年 | 86篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
261.
Subunit structure of the erythropoietin receptor 总被引:4,自引:0,他引:4
P J McCaffery J K Fraser F K Lin M V Berridge 《The Journal of biological chemistry》1989,264(18):10507-10512
Chemical cross-linking of the red blood cell hormone, erythropoietin (Epo), to its receptor on erythroid cells has revealed the presence of two proteins closely associated with Epo, but the relationship between these two proteins is controversial. Using the cross-linking reagents disuccinimidyl suberate and dithiobissuccinimidyl propionate, we show that 125I-Epo can be specifically conjugated in a complex of 224kDa using mouse fetal liver cells, bone marrow cells, and Friend virus-induced splenic erythroblasts as demonstrated by electrophoresis on sodium dodecyl sulfate-polyacrylamide gels under nonreducing conditions. Under reducing conditions, the 224-kDa complex appeared as two Epo conjugates of 136 kDa and 119 kDa, and these bands were also observed to a variable extent in some nonreducing gels. Disulfide linking of the 136-kDa and 119-kDa bands was confirmed by two-dimensional sodium dodecyl sulfate-polyacrylamide gel electrophoresis run under nonreducing followed by reducing conditions. With increasing time of 125I-Epo binding to Friend virus erythroblasts in the presence of sodium azide to inhibit receptor internalization, the 136-kDa and 119-kDa bands seen under reducing conditions increased markedly in intensity, whereas the 224-kDa band seen under nonreducing conditions declined. These results suggest that the 224-kDa Epo conjugate is inefficiently solubilized under nonreducing conditions following prolonged periods of Epo binding. A single class of saturable, high affinity receptors for Epo on each of the cell types tested is demonstrated. It is concluded that the two disulfide-linked Epo-binding proteins which can be independently cross-linked to Epo form a single ligand binding site. 相似文献
262.
The beta actin promoter. High levels of transcription depend upon a CCAAT binding factor 总被引:10,自引:0,他引:10
W W Quitschke Z Y Lin L DePonti-Zilli B M Paterson 《The Journal of biological chemistry》1989,264(16):9539-9546
263.
264.
265.
Activities of the pentose phosphate pathway and enzymes of proline metabolism in legume root nodules 总被引:1,自引:1,他引:0
下载免费PDF全文
![点击此处可从《Plant physiology》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Based on localization and high activities of pyrroline-5-carboxylate reductase and proline dehydrogenase activities in soybean nodules, we previously suggested two major roles for pyrroline-5-carboxylate reductase in addition to the production of the considerable quantity of proline needed for biosynthesis; namely, transfer of energy to the location of biological N2 fixation, and production of NADP+ to drive the pentose phosphate pathway. The latter produces ribose-5-phosphate which can be used in de novo purine synthesis required for synthesis of ureides, the major form in which biologically fixed N2 is transported from soybean root nodules to the plant shoot. In this paper, we report rapid induction (in soybean nodules) and exceptionally high activities (in nodules of eight species of N2-fixing plants) of pentose phosphate pathway and pyrroline-5-carboxylate reductase. There was a marked increase in proline dehydrogenase activity during soybean (Glycine max) ontogeny. The magnitude of proline dehydrogenase activity in bacteroids of soybean nodules was sufficiently high during most of the time course to supply a significant fraction of the energy requirement for N2 fixation. Proline dehydrogenase activity in bacteroids from nodules of other species was also high. These observations support the above hypothesis. However, comparison of pentose phosphate pathway and pyrroline-5-carboxylate reductase activities of ureide versus amide-exporting nodules offers no support. The hypothesis predicts that pyrroline-5-carboxylate and pentose phosphate pathway activities should be higher in ureide-exporting nodules than in amide-exporting nodules. This predicted distinction was not observed in the results of in vitro assays of these activities. 相似文献
266.
Purification, characterization, and amino terminal sequence of xylose reductase from Candida shehatae. 总被引:1,自引:0,他引:1
D-Xylose is a major component of the carbohydrates derived from agricultural residues and forest products. Among more than two hundred known xylose-utilizing yeasts, only a few species are known to be able to ferment xylose anaerobically. Candida shehatae is one of such xylose-fermenting yeasts. Xylose reductase (E.C. 1.1.1.21) is a key enzyme responsible for xylose metabolism in xylose-utilizing as well as xylose-fermenting yeasts. In this paper, we report the development of a convenient and reliable procedure for the purification of xylose reductase from C. shehatae to near homogeneity. The amino acid composition and N-terminal sequence of the enzyme have also been analyzed. C. shehatae seems to contain only a single xylose reductase, but the enzyme has a dual coenzyme specificity for both NADPH and NADH. The enzyme is remarkably stable at room temperature and 4 degrees C. 相似文献
267.
Cellular gene expression in papillomas of the choroid plexus from transgenic mice that express the simian virus 40 large T antigen. 总被引:3,自引:1,他引:2
下载免费PDF全文
![点击此处可从《Journal of virology》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Transgenic mice that contain the simian virus 40 (SV40) enhancer-promoter and large tumor (T) antigen gene develop papillomas of the choroid plexus. The tumors remain well differentiated on histological examination and express normal levels of tissue-specific mRNAs for transthyretin (TTR) and the 5-HT1C serotonin receptor, two differentiated cell markers. Both Northern (RNA) blot analysis and in situ cytohybridization have been used to monitor the steady-state levels of the mRNAs from the viral oncogene (T antigen) and from several cellular oncogenes. In situ hybridization demonstrated, in serial sections, increased levels of both T antigen mRNA and p53 mRNA localized in the tumor tissue but not in the normal brain tissue. The ratios of the steady-state levels of mRNA for p53/TTR and p53/L32, a ribosomal protein gene, were 2- to 20-fold higher in the tumor tissue than in the normal choroid plexus tissue. Several other oncogenes did not show elevated levels of mRNA in these tumors. p53 protein levels were not detectable in normal brain tissue, but p53 levels were very high in tumor tissue in which all of the p53 was found in a complex with the SV40 large T antigen. These data continue to show a close relationship between SV40 T-antigen-mediated tumorigenesis and the role of p53 in these tumors. 相似文献
268.
The Epstein-Barr virus (EBV) BZLF1 immediate-early gene product differentially affects latent versus productive EBV promoters. 总被引:28,自引:25,他引:3
下载免费PDF全文
![点击此处可从《Journal of virology》网站下载免费的PDF全文](/ch/ext_images/free.gif)
S Kenney J Kamine E Holley-Guthrie J C Lin E C Mar J Pagano 《Journal of virology》1989,63(4):1729-1736
The Epstein-Barr virus (EBV) BZLF1 gene product is thought to mediate the disruption of latent EBV infection. We have examined the regulatory effects of BZLF1 by studying its transactivating effects on seven different EBV promoters. We find that whereas the BZLF1 gene product increases the activity of the two early promoters, BMLF1 and BMRF1, it decreases the activity of three latent promoters (the BamHI-C and BamHI-W Epstein-Barr nuclear antigen promoters and the latent membrane protein promoter). The BZLF1-induced changes in promoter-directed chloramphenicol acetyltransferase activity occur in EBV-negative as well as EBV-positive cell lines and are accompanied by a similar change in chloramphenicol acetyltransferase mRNA. Deletion analysis of the BamHI Z fragment indicates that in a portion of the amino-terminal half of the BZLF1 gene product (amino acids 24 to 86) is not essential for positive transactivating effects but is required for down-regulating effects. Thus, different domains of the same EBV immediate-early gene product can either increase the function of EBV promoters active in productive infection or decrease the function of key promoters active in latent infection. 相似文献
269.
A comparison was made of ethanol's effects on the order of plasma membranes in intact cells and some isolated membrane preparations. Order was assessed by steady-state fluorescence polarization techniques using the non-permeant probe, TMA-DPH. The data show that two cultured cells, rat neonatal astroglial and N2A neuroblastoma, were sensitive to significant ethanol-induced disordering within the anesthetically relevant range (100 - 200 mM). Human erythrocytes, cultured fibroblasts and homogenized astroglial cells required higher ethanol concentrations (greater than 250 mM) to produce a similar effect. Intact erythrocytes were approximately twice as sensitive as erythrocyte ghost membranes to ethanol-induced perturbation. The neonatal glial and N2A cells were approximately five times more sensitive than synaptic membranes to ethanol effects. DMPC and DMPC + cholesterol liposomes and myelin membranes were insensitive to ethanol's effects. The incorporation of 10 mole % ganglioside GM1 sensitized the liposomes to ethanol-induced perturbation. 相似文献
270.
Hypophysectomy decreases and growth hormone increases the turnover and mass of rat liver glutamine synthetase 总被引:1,自引:0,他引:1
Hypophysectomy diminishes rat liver glutamine synthetase (GS) activity and growth hormone (GH) administration restores this activity to normal levels; brain GS is unaffected. We have now investigated the effects of long-term hypophysectomy (45-day) and GH treatment on the GS mass (amount of enzyme) and turnover in rat liver and brain. Labeled GS was isolated by immunoprecipitation at intervals between one and six days after pulse administration of [U-14C] leucine and the GS half-life (t1/2) was determined. The GS mass was obtained by immunoassay and by calculation using the specific activity of purified GS. GS turnover was calculated by multiplying the GS mass by the first-order rate constant of degradation (kd). During the time course of each experiment, the GS mass did not change, indicating that in each of the three hormonal states studied, a steady state existed. Hypophysectomy increased the t1/2 of hepatic GS from 3.8 to 8.8 days and decreased GS turnover from 0.38 to 0.1 microgram/100 g body wt/day; the GH regimen used restored the turnover to above normal levels, 0.6 microgram/100 g body wt/day. The GS mass decreased from 2.0 to 1.2 micrograms/100 g body wt and GH restored the GS mass to normal levels. The brain enzyme was not affected by hypophysectomy or GH. 相似文献