全文获取类型
收费全文 | 2811篇 |
免费 | 368篇 |
出版年
2021年 | 26篇 |
2019年 | 31篇 |
2018年 | 32篇 |
2017年 | 32篇 |
2016年 | 51篇 |
2015年 | 85篇 |
2014年 | 71篇 |
2013年 | 133篇 |
2012年 | 124篇 |
2011年 | 131篇 |
2010年 | 72篇 |
2009年 | 66篇 |
2008年 | 99篇 |
2007年 | 122篇 |
2006年 | 109篇 |
2005年 | 126篇 |
2004年 | 111篇 |
2003年 | 80篇 |
2002年 | 78篇 |
2001年 | 71篇 |
2000年 | 97篇 |
1999年 | 84篇 |
1998年 | 46篇 |
1997年 | 54篇 |
1996年 | 35篇 |
1995年 | 34篇 |
1994年 | 35篇 |
1993年 | 30篇 |
1992年 | 76篇 |
1991年 | 49篇 |
1990年 | 65篇 |
1989年 | 61篇 |
1988年 | 55篇 |
1987年 | 52篇 |
1986年 | 43篇 |
1985年 | 49篇 |
1984年 | 29篇 |
1983年 | 36篇 |
1982年 | 36篇 |
1981年 | 30篇 |
1980年 | 36篇 |
1979年 | 39篇 |
1978年 | 30篇 |
1977年 | 32篇 |
1976年 | 31篇 |
1975年 | 26篇 |
1974年 | 31篇 |
1973年 | 27篇 |
1972年 | 28篇 |
1970年 | 25篇 |
排序方式: 共有3179条查询结果,搜索用时 31 毫秒
41.
Glycosylation at specific sites of erythropoietin is essential for biosynthesis, secretion, and biological function 总被引:10,自引:0,他引:10
The glycoprotein hormone erythropoietin (Ep), the primary regulator of erythropoiesis, is synthesized by the kidney and secreted as the mature protein with three N-linked and one O-linked oligosaccharide chains. To investigate the role(s) of each carbohydrate moiety in the biosynthesis and function of Ep, we have used oligonucleotide-directed mutagenesis of a cDNA for human Ep to alter the amino acids at each of the carbohydrate attachment sites. Each mutated cDNA construct was expressed in stably transfected sublines of a kidney cell line, baby hamster kidney. We show, by preventing attachment of N-linked carbohydrate at asparagines 38 or 83, or preventing O-linked glycosylation at serine 126, that glycosylation of each of these specific sites is critical for proper biosynthesis and secretion of Ep. Fractionation of cellular extracts demonstrated that the mutant proteins lacking glycosylation at each of these three sites, (38, 83, and 126) were associated mainly with membrane components or were degraded rapidly. Less than 10% of these three mutant proteins were processed properly and secreted from the cells. The Ep protein lacking N-linked glycosylation at asparagine 24 is synthesized and secreted as efficiently as native Ep. The carbohydrates at positions 24 and 38 may be involved in the biological activity of Ep, since the absence of either of the oligosaccharide side chains at these positions reduced the hormone's biological activity. 相似文献
42.
F J Castellino V S de Serrano J R Powell W R Johnson J M Beals 《Archives of biochemistry and biophysics》1986,247(2):312-320
The structure of a small region of human plasminogen (F4), consisting of amino acid residues Val354-Ala439 and containing its kringle 4 (K4) domain (residues Cys357-Cys434), has been predicted from Chou-Fasman calculations and hydropathy profiles, and compared to circular dichroism (CD) measurements on the isolated fragment. Calculations, by the Chou-Fasman method, of the probabilities of various types of secondary structures that exist in this region reveal that no helical structures are present. Of the total of 86 amino acid residues present in this K4-containing peptide region, 37% can adopt conformations of beta-pleated sheets, 48% of the amino acids can exist in beta-turns, and 15% of the residues can be present as coils. The structure of F4 in dilute aqueous solution has been experimentally evaluated by CD measurements. At pH = 7.4, in dilute salt solutions, a total of 64% beta-structures, 30% beta-turns, and 6% coiled structures is estimated to be present in this peptide region. Consideration of the marginal stability of many of the conformational regions of F4, as predicted by Chou-Fasman calculations, suggests that secondary structural flexibility is present in this fragment, which could result in ready adoption of new conformations. The hydropathy profile of F4 has been determined and suggests that this polypeptide is highly hydrophilic, especially in the regions of residues His387-Tyr396 and Cys406-Lys413. Thus, it appears as though a large portion of the surface of F4 can be exposed to solvent in its native conformation. 相似文献
43.
A sulfurous brine seep at the East Flower Garden Bank, northwest Gulf of Mexico, produces conditions conducive to the growth of a luxuriant prokaryotic biota. Hydrodynamic cropping continually harvests this biota and distributes it to sandy-bottom and hard-bank benthic communities downstream of the seep. Consequently, both macro- and meiofaunal abundances are dramatically increased above the regional norm in parts of the seep system. When sulfide is present, the lower Bilaterian groups belonging to the meiofauna dominate the community; without sulfide, macrofaunal groups, particularly crustaceans, dominate the community. Outside the influence of the seep, meiofaunal copepods predominate. Changes in taxonomic composition and abundance indicate that the sandy-bottom benthos at 70–80 m depth at the East Flower Garden bank is foodlimited and that, under these conditions, meiofauna, particularly the higher Bilaterian groups, dominate the community numerically. Perhaps, under food-limiting conditions, meiofauna compete favorably with macrofauna for food. 相似文献
44.
Polymorphic phase behavior of cardiolipin derivatives studied by 31P NMR and X-ray diffraction 总被引:1,自引:0,他引:1
The polymorphic phase behavior of cardiolipin (diphosphatidylglycerol) analogues with two to five chains per phospholipid head group, namely, dilysocardiolipin, monolysocardiolipin, cardiolipin, and acylcardiolipin, respectively, has been studied by 31P NMR and X-ray diffraction. Dilysocardiolipin dispersions at low salt concentration are micellar, and a transition to a lamellar phase takes place between 1 and 2 M NaCl. From light-scattering measurements, it is also found that a transition takes place from the micellar state with a midpoint at 5.2 mM CaCl2, 0.95 M HClO4, and 1.5 M NaCl. Monolysocardiolipin dispersions are lamellar throughout the concentration range from zero to saturated NaCl. Cardiolipin dispersions undergo a transition from a lamellar to an inverted hexagonal phase between 1 and 2 M NaCl. Acylcardiolipin dispersions are in an inverted hexagonal phase throughout the concentration range from zero to saturated NaCl. The chemical shift anisotropies of both phosphate groups in dilysocardiolipin and of one of the phosphate groups in monolysocardiolipin are drastically reduced in the lamellar phase, indicating a different conformation of the phosphatidyl head group from that normally found in diacyl phospholipid bilayers. The results provide strong support for the "shape" concept of lipid polymorphism when viewed in its most general form including configurational entropy, hydrophobic effects, etc. and indicate the importance of head-group interactions in determining the lipid phase behavior. 相似文献
45.
J Caraux B Chichehian C Gestin B Longhi A C Lee J E Powell V C Stevens A Pourquier 《Journal of immunology (Baltimore, Md. : 1950)》1985,134(2):835-840
Noncross-reactive monoclonal antibodies specific for human chorionic gonadotropin (hCG) were obtained after pre-selection for submolecular specificity with a synthetic peptide immunogen. Mice were immunized with a synthetic peptide representing a segment unique to the beta-subunit of hCG (amino acid residues 109-145), conjugated to diphtheria toxoid. We then derived nine different hybridomas that secreted monoclonal antibodies reactive with both native hCG and isolated C-terminal peptide, after somatic cell hybridization of immune spleen cells with a nonsecretory myeloma cell line. None of the nine monoclonal antibodies, termed beta-hCG-CTPa1----a9, reacted with hLH, hFSH, or hTSH, although these pituitary hormones display extensive amino acid sequence homology with hCG. The noncross-reactive anti-beta-hCG monoclonal antibodies show apparent association constants on the order of 10(9) to 10(10) M-1. A sandwich-type enzyme-linked immunosorbent assay was set up with cut-off values of around 5 mIU/ml. These antibodies might have important implications for: a) improving the diagnosis and clinical management of pregnancy; b) monitoring the course of development of carcinomas which secrete the hormone, through in vitro assays or in vivo radioimmunodetection; c) evaluating the antibodies' therapeutic potential against such carcinomas; d) studying the biologic functions of the C-terminal segment of beta-hCG; and e) addressing the anti-fertility effect of antibodies raised against that segment. 相似文献
46.
Cytomegalovirus (CMV) strain AD-169 replicated in smooth muscle cell (SMC) cultures derived from human umbilical arteries, producing enveloped infectious virions. However, unlike the effects of CMV on fully permissive human lung fibroblasts, the effects of strain AD-169 on SMC cultures were delayed and prolonged, resulting in extended survival of a fraction of the starting population. This period of survival did not exceed the life-span of the control SMC cultures. Infectious CMV continued to be isolated from the surviving SMC cultures after extinction of the original inoculum by dilution and after treatment of the cultures with CMV neutralizing antibody. The implications of these findings for the pathogenesis of atherosclerosis are discussed. 相似文献
47.
W. Powell P. D. S. Caligari J. S. Swanston J. L. Jinks 《TAG. Theoretical and applied genetics. Theoretische und angewandte Genetik》1985,71(3):461-466
Summary Random inbred lines produced by doubled haploidy (DH) and single seed descent (SSD) have been used to investigate the genetics of -glucan (gum) content in barley (Hordeum vulgare). Genetical analyses indicated that gum content is controlled by a simple additive genetic system. Significant negative genetic correlations were observed between -glucan content, thousand grain weight and height in the DH samples. These correlations were much reduced in the SSD samples and would suggest linkage of the genes controlling these characters. The presence of repulsion linkages could be exploited in a barley breeding programme by producing F1 derived DH to generate recombinants with high thousand grain weight and low -glucan content. Genetical parameters estimated from DH and F3 samples have successfully been used to predict the number of inbred lines transgressing the parental range for -glucan content and bivariate combinations involving -glucan. 相似文献
48.
J W Neal D A Winfield T P Powell 《Proceedings of the Royal Society of London. Series B, Containing papers of a Biological character. Royal Society (Great Britain)》1985,225(1241):411-423
The basal dendrites of Meynert cells in the striate cortex have been studied with the Golgi method in the brains of monkeys that had been reared for varying periods with the eyelids closed over one eye. The lengths and arrangement of the dendrites were compared with those in normal brains. In the visually deprived brain almost half of the cells had basal dendrites that were apparently normal with the dendritic fields in the form of an ellipse and the long axes parallel to the direction of the ocular dominance bands. The other cells had dendritic fields that have rarely been seen in normal material and two distinct types could be recognized. The 'lop-sided' cell had an ellipsoidal dendritic field with the major axis parallel to the ocular dominance bands, but the extents of the dendrites along the minor axis were very asymmetric; the ratio of the means of the long and short arms of the minor axis of the 'lop-sided' cell is 2.3:1 compared with 1.1:1 in normal brains. The 'perpendicular' type of cell also had an ellipsoidal dendritic field but the relation of the major and minor axes to the direction of the ocular dominance bands was the reverse of the normal cell, with the long axis of the ellipse being aligned perpendicular to the bands. 'Lop-sided' cells formed approximately 18% of the total of Meynert cells studied and the 'perpendicular' 32%. The proportion of the cells with abnormal basal dendritic fields, and particularly the 'perpendicular', increased with longer durations of eyelid closure. It is suggested that the alterations in the dendritic fields of the 'lop-sided' and 'perpendicular' cells may be correlated with the changes in width of the ocular dominance bands that are known to occur after monocular eyelid suture. 相似文献
49.
Chemical modification of arginine residues of lung galaptin and fibronectin. Effects on fibroblast binding.
下载免费PDF全文
![点击此处可从《The Biochemical journal》网站下载免费的PDF全文](/ch/ext_images/free.gif)
J T Powell 《The Biochemical journal》1985,232(3):919-922
Lung galaptin bound to lung fibroblasts with a Kd of 190 nM, and this binding could be inhibited by 20 mM-lactose. Selective modifications of the arginine residues of galaptin with cyclohexane-1,2-dione did not change its lectin activity or its binding to fibroblasts. By contrast, modification of the arginine residues of plasma fibronectin resulted in a marked diminution of protein-fibroblast binding. Selective modification of arginine residues may provide a useful probe for -Arg-Gly-Asp-Xaa cell-binding sequences of proteins. 相似文献
50.
During 1981 and 1982, bollworm,Heliothis zea (Boddie), and tobacco budworm,H. virescens (F.), larvae (n=3,666) were collected from 41 cotton fields near Portland, Arkansas (USA) to assess the occurrence of parasitism.
Three strategies were employed to controlHeliothis spp. in these fields: (1) release ofTrichogramma pretiosum Riley; (2) insecticidal control; or (3) inaction (check). Insecticide use in nonchemical control fields was reduced, but
not eliminated.Heliothis spp. larvae collected in cotton had higher parasitism rates in 1981 (30.9%) and 1982 (50.1%) than had been reported for cotton
since the advent of organochlorine insecticide usage. Four species of larval parasites and 1 species of larval-pupal parasite
were recorded. The larval parasiteMicroplitis croceipes (Cresson) comprised 90.6% and 94.5% of all parasitic insects reared from field collectedHeliothis spp. in 1981 and 1982, respectively. No difference (P>0.05) in level of parasitism existed betweenH. zea andH. virescens. Differences between treatments occurred only in 1982 whenH. zea larvae were parasitized at a greater (P<0.05) rate in check fields (68.3%) than in insecticidal control fields (44.3%). Higher
levels of larval parasitism in cotton fields may be a consequence of reduced insecticide usage and changes in materials applied,
particularly the pyrethroids.
Mention of a trademark or proprietary product does not constitute a guarantee or warranty of the product by the U.S. Dept.
of Agriculture and does not imply its approval to the exclusion of other products that may also be suitable. 相似文献