全文获取类型
收费全文 | 2895篇 |
免费 | 363篇 |
出版年
2022年 | 22篇 |
2021年 | 39篇 |
2020年 | 24篇 |
2019年 | 25篇 |
2018年 | 38篇 |
2017年 | 28篇 |
2016年 | 59篇 |
2015年 | 112篇 |
2014年 | 101篇 |
2013年 | 105篇 |
2012年 | 150篇 |
2011年 | 164篇 |
2010年 | 91篇 |
2009年 | 87篇 |
2008年 | 135篇 |
2007年 | 136篇 |
2006年 | 130篇 |
2005年 | 137篇 |
2004年 | 120篇 |
2003年 | 121篇 |
2002年 | 118篇 |
2001年 | 129篇 |
2000年 | 99篇 |
1999年 | 97篇 |
1998年 | 33篇 |
1997年 | 35篇 |
1996年 | 41篇 |
1995年 | 30篇 |
1994年 | 25篇 |
1993年 | 24篇 |
1992年 | 61篇 |
1991年 | 54篇 |
1990年 | 55篇 |
1989年 | 52篇 |
1988年 | 36篇 |
1987年 | 44篇 |
1986年 | 34篇 |
1985年 | 31篇 |
1984年 | 35篇 |
1983年 | 17篇 |
1982年 | 23篇 |
1980年 | 20篇 |
1979年 | 40篇 |
1978年 | 25篇 |
1975年 | 18篇 |
1974年 | 16篇 |
1973年 | 18篇 |
1972年 | 18篇 |
1970年 | 17篇 |
1969年 | 16篇 |
排序方式: 共有3258条查询结果,搜索用时 15 毫秒
21.
Flow cytometric determination of atypical antigen expression in acute leukemia for the study of minimal residual disease. 总被引:6,自引:0,他引:6
The aim of this study was to investigate to which extent acute leukemias could be monitored for residual disease by using atypical antigen combinations as leukemia-related markers. Atypical antigenic features were determined by double color flow cytometry and included coexpression of lymphoid and myeloid related antigens, unphysiological coexpression of immature and mature antigens, and lack of an antigen that is normally expressed during maturation. Atypical immunophenotypes were detected in 35 of 68 patients with AML (51.5%) and 15 of 24 patients with ALL (62.5%). When 12 patients with leukemia-associated markers were again analyzed at relapse, the relevant antigen combinations were retained in 11 of them. The sensitivity of this two color flow cytometric assay as determined in dilution experiments was 1 in 10(3) to 10(4) cells. Follow-up studies of bone marrow samples revealed that, after induction chemotherapy cells with leukemia-associated markers were detectable in several patients at a frequency of 0.5 to 4%, but only patients in whom the cells with atypical antigens never disappeared suffered from relapse. In contrast, patients who became negative for the atypical cells remained in complete remission (median remission duration after the first negative bone marrow assessment by flow cytometry 52 weeks, range 20-102). We conclude that atypical antigen combinations, which are present in a meaningful number of acute leukemias, are a valuable means of monitoring acute leukemia patients during follow-up. This flow cytometric approach can complement other strategies to get a more accurate definition of remission in acute leukemia. 相似文献
22.
Flooding the roots of greenhouse-grown muskmelon (Cucumis meloL. cv. Noy Yizreel) plants for 4 days reduced sucrose accumulation36% in the inner mesocarp and 88% in the outer mesocarp of developingfruit. Concentration of the translocated sugars raffinose andstachyose were also lower in fruit on flooded plants than inthose from nonflooded plants. In contrast, fruit hexose concentrationwas similar in both flooded and nonflooded plants. There wasno alteration in activities of enzymes associated with sucrosemetabolism in the fruit which could explain the decreased sucroseconcentration. Four days of root flooding caused no reductionin leaf carbon exchange rate or assimilate export rate, indicatingthat the reduction in fruit sucrose accumulation was not dueto source limitation. Root respiration, measured as CO2 evolution,was approximately 30% lower in anaerobic roots than in aerobicroots. When viewed as carbohydrate consumed, a doubling of glycolyticactivity occurred in the anaerobic root mass. Increased demandfor carbohydrates by anaerobic roots may lead to a reductionin translocated carbohydrates available for sucrose biosynthesisin the developing fruit. (Received August 29, 1990; Accepted February 21, 1991) 相似文献
23.
Terminal meristems of Pisum sativum (garden pea) transit from vegetative to inflorescence development, and begin producing floral axillary meristems. Determination for inflorescence development was assessed by culturing excised buds and meristems. The first node of floral initiation (NFI) for bud expiants developing in culture and for adventitious shoots forming on cultured meristems was compared with the NFI of intact control buds. When terminal buds having eight leaf primordia were excised from plants of different ages (i.e., number of unfolded leaves) and cultured on 6-benzylaminopurine and kinetin-supplemented medium, the NFI was a function of the age of the source plant. By age 3, all terminal buds were determined for inflorescence development. Determination occurred at least eight nodes before the first axillary flower was initiated. Thus, the axillary meristems contributing to the inflorescence had not formed at the time the bud was explanted. Similar results were obtained for cultured axillary buds. In addition, meristems excised without leaf primordia from axillary buds three nodes above the cotyledons of age-3 plants gave rise to adventitious buds with an NFI of 8.3 ±0.3 nodes. In contrast seed-derived plants had an NFI of 16.5 ±0.2. Thus cells within the meristem were determined for inflorescence development. These findings indicate that determination for inflorescence development in P. sativum is a stable developmental state, separable from determination for flower development, and occurring prior to initiation of the inflorescence at the level of meristems. 相似文献
24.
Gabi Wegmann Regula Huber Else Zanolla Hans M. Eppenberger Theo Wallimann 《Differentiation; research in biological diversity》1991,46(2):77-87
The expression and the cellular- as well as subcellular-distribution of brain-type B-CK and mitochondrial Mi-CK during development of the chicken retina was studied by immunoblotting, immunofluorescence and immunogold methods. B-CK expression and accumulation in retina was high from early stages of embryonic development on, decreased slightly around hatching and remained high again during adulthood. At early stages of development (days 2-5), B-CK was more or less evenly distributed over the entire retina with the exception of ganglion cells, which were stained more strongly for B-CK than other retinal precursor cells. Then, at around day 10, the beginning of stratified immunostaining by anti-B-CK antibody was noted concomitant with progressing differentiation. Finally, a dramatic increase in staining of the differentiating photoreceptor cells was seen before hatching (day 18) with weaker staining of other cell types. At hatching, as in the adult state, most of the B-CK was localized within rods and cones. Thus, during retinal development marked changes in the immunostaining pattern for B-CK were evident. By contrast, Mi-CK expression was low during development in ovo and rose just before hatching with a predominant accumulation of this isoenzyme within the ellipsoid portion of the inner photoreceptor cell segments. Mi-CK accumulation in the retina coincided with functional maturation of photoreceptors and therefore represents a good marker for terminal differentiation of these cells. B-CK, present from early stages of retina development, seems to be relevant for the energetics of retinal cell proliferation, migration and differentiation, whereas the simultaneous expression of both B- and Mi-CK around the time of hatching indicates a coordinated function of the two CK isoforms as constituents of a PCr-circuit involved in the energetics of vision, which, in autophagous birds, has to be operational at this point in time. 相似文献
25.
Site-directed mutagenesis of beta-galactosidase (E. coli) reveals that tyr-503 is essential for activity 总被引:1,自引:0,他引:1
M Ring D E Bader R E Huber 《Biochemical and biophysical research communications》1988,152(3):1050-1055
By using the technique of site-directed mutagenesis we have succeeded in replacing tyr-503 of beta-galactosidase (E. coli) with a phe. A study of the kinetic and stability properties of this mutant enzyme (F-503 beta-galactosidase) showed that the loss in activity upon this change is due to the loss of a catalytic group (rather than a detrimental change in the enzyme's overall structure or a change in the enzyme's binding capacity). This confirms previous suggestions that this tyr residue is involved in catalysis. 相似文献
26.
Identification of two histidines as copper ligands in Streptomyces glaucescens tyrosinase 总被引:2,自引:0,他引:2
The physiochemical properties of wild type and two mutants of Streptomyces glaucescens tyrosinase are reported. The native enzyme contains two coppers at the active site which are EPR nondetectable. The two coppers react stoichiometrically with one hydrogen peroxide molecule giving rise to oxytyrosinase. Its optical features are similar to those reported earlier for a molluscan hemocyanin. The two mutants in which histidine-62 and -189 were changed to asparagine by site-directed mutagenesis have lost their enzymatic activity and their ability to bind oxygen and contain only one copper ion which is fully EPR detectable. The EPR parameters indicate that the remaining copper is in a tetragonally distorted ligand environment. These data are in agreement with His-62 and His-189 serving as copper ligands in S. glaucescens tyrosinase. 相似文献
27.
The use of limited proteolysis to probe interdomain and active site regions of beta-galactosidase (Escherichia coli) 总被引:4,自引:0,他引:4
L A Edwards M R Tian R E Huber A V Fowler 《The Journal of biological chemistry》1988,263(4):1848-1854
Limited proteolysis by pancreatic elastase (EC 3.4.21.36) and chymotrypsin (EC 3.4.21.1) was used to study the domain structure and active site of beta-galactosidase (EC 3.2.1.23) (Escherichia coli). Treatment with elastase resulted in a rapid cleavage between residues Ala-732 and Ala-733. No inactivation accompanied this cleavage suggesting that this bond is in a hinge region of the protein. Some slow cleavages beyond the initial one were observed to occur and were accompanied by inactivation. Treatment of beta-galactosidase with chymotrypsin resulted in cleavages first between Trp-585 and Ser-586 and then between Phe-601 and Cys-602. The first of these cleavages resulted in total inactivation of beta-galactosidase. The presence of monovalent ions or isopropyl-beta-D-thiogalactopyranoside protected against the cleavages but when Mg2+ or Mn2+ was present in the reaction mixture, the bond between Trp-585 and Ser-586 was more susceptible to the action of chymotrypsin. These data demonstrate that the conformation of beta-galactosidase around Trp-585 and Ser-586 is dramatically affected by the binding of ions and isopropyl-beta-D-thiogalactopyranoside. The mutant M15 beta-galactosidase, which is missing residues 11 through 41 and is an inactive dimer rather than an active tetramer, was found to be much more labile to proteases than native beta-galactosidase, but the same initial cleavages were found to occur. In addition, trypsin cleaved the M15 protein between Arg-431 and Trp-432 while native beta-galactosidase was stable to trypsin. 相似文献
28.
The reversion reactions of beta-galactosidase (Escherichia coli) produced beta-galactosyl-galactoses and beta-galactosyl-glucoses. About 10 beta-galactosyl-galactose and 10 beta-galactosyl-glucose gas-liquid chromatographic peaks were detected and it is thus very likely that every possible isomer of beta-galactosyl-galactose and beta-galactosyl-glucose was formed by the reversion reactions (taking into account both anomers for each isomer). The presence of lactose and allolactose among the beta-galactosyl-glucoses was confirmed with standards. An important finding relating to the role of allolactose as an inducer of the lac operon was that allolactose (beta-D-galactosyl-(1----6)-D-glucose) was the only disaccharide formed initially, and at equilibrium it was present in the largest amount (50%). Obviously the enzyme is specific in its ability to form allolactose, and allolactose is the most stable beta-galactosyl-glucose, both important inducer properties. The equilibrium constant (concentration of disaccharides divided by the concentration of reactants at equilibrium) of the reaction was about 9.5 mM-1. This is the first report of an equilibrium constant for the beta-galactosidase reaction. Of mechanistic significance is the fact that only three compounds were able to replace D-galactose as a reversion reactant. Two of these (L-arabinose and D-fucose) had alterations at carbon 6. The 6 position, therefore, is not essential for reactivity. The third compound was D-galactal. Any other sugars tested (even with very minor changes relative to D-galactose) did not react. Of special consequence is the 2 position. The results strongly suggest that there has to be either an equatorial hydroxyl at the 2 position of a sugar or a special reactivity (as with D-galactal) in order for the enzyme to catalyze the beta-galactosidase reaction. 相似文献
29.
Crystal structure determination, refinement and the molecular model of the alpha-amylase inhibitor Hoe-467A 总被引:5,自引:0,他引:5
The crystal and molecular structure of the alpha-amylase inhibitor Hoe-467A has been determined and refined at high resolution. The polypeptide chain is folded in two triple-stranded sheets, which form a barrel. The topology of folding is as found in the immunoglobulin domains. The amino acid triplet Trp18-Arg19-Tyr20 has an exceptional conformation and position in the molecule and is possibly involved in inhibitory activity. 相似文献
30.
Diurnal fluctuations in cotton leaf carbon export, carbohydrate content, and sucrose synthesizing enzymes 总被引:11,自引:3,他引:8
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)
In fully expanded leaves of greenhouse-grown cotton (Gossypium hirsutum L., cv Coker 100) plants, carbon export, starch accumulation rate, and carbon exchange rate exhibited different behavior during the light period. Starch accumulation rates were relatively constant during the light period, whereas carbon export rate was greater in the afternoon than in the morning even though the carbon exchange rate peaked about noon. Sucrose levels increased throughout the light period and dropped sharply with the onset of darkness; hexose levels were relatively constant except for a slight peak in the early morning. Sucrose synthase, usually thought to be a degradative enzyme, was found in unusually high activities in cotton leaf. Both sucrose synthase and sucrose phosphate synthetase activities were found to fluctuate diurnally in cotton leaves but with different rhythms. Diurnal fluctuations in the rate of sucrose export were generally aligned with sucrose phosphate synthase activity during the light period but not with sucrose synthase activity; neither enzyme activity correlated with carbon export during the dark. Cotton leaf sucrose phosphate synthase activity was sufficient to account for the observed carbon export rates; there is no need to invoke sucrose synthase as a synthetic enzyme in mature cotton leaves. During the dark a significant correlation was found between starch degradation rate and leaf carbon export. These results indicate that carbon partitioning in cotton leaf is somewhat independent of the carbon exchange rate and that leaf carbon export rate may be linked to sucrose formation and content during the light period and to starch breakdown in the dark. 相似文献