全文获取类型
收费全文 | 42648篇 |
免费 | 4361篇 |
国内免费 | 12篇 |
出版年
2023年 | 196篇 |
2022年 | 391篇 |
2021年 | 769篇 |
2020年 | 531篇 |
2019年 | 638篇 |
2018年 | 772篇 |
2017年 | 699篇 |
2016年 | 1146篇 |
2015年 | 1899篇 |
2014年 | 2056篇 |
2013年 | 2537篇 |
2012年 | 3126篇 |
2011年 | 2976篇 |
2010年 | 1989篇 |
2009年 | 1725篇 |
2008年 | 2393篇 |
2007年 | 2379篇 |
2006年 | 2265篇 |
2005年 | 2020篇 |
2004年 | 1983篇 |
2003年 | 1772篇 |
2002年 | 1741篇 |
2001年 | 674篇 |
2000年 | 626篇 |
1999年 | 641篇 |
1998年 | 444篇 |
1997年 | 362篇 |
1996年 | 350篇 |
1995年 | 365篇 |
1994年 | 320篇 |
1993年 | 315篇 |
1992年 | 415篇 |
1991年 | 366篇 |
1990年 | 358篇 |
1989年 | 369篇 |
1988年 | 377篇 |
1987年 | 342篇 |
1986年 | 276篇 |
1985年 | 313篇 |
1984年 | 312篇 |
1983年 | 277篇 |
1982年 | 266篇 |
1981年 | 235篇 |
1980年 | 218篇 |
1979年 | 219篇 |
1978年 | 187篇 |
1977年 | 188篇 |
1976年 | 184篇 |
1975年 | 201篇 |
1973年 | 169篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
151.
Summary M1 is a virulent bacteriophage of Methanobacterium thermoautotrophicum strain Marburg. Restriction enzyme analysis of the linear, 30.4 kb phage DNA led to a circular map of the 27.1 kb M1 genome. M1 is thus circularly permuted and exhibits terminal redundancy of approximately 3 kb. Packaging of M1 DNA from a concatemeric precursor initiates at the pac site which was identified at coordinate 4.6 kb on the circular genome map. It proceeds clockwise for at least five packaging rounds. Headful packaging was also shown for M2, a phage variant with a 0.7 kb deletion at coordinate 23.25 on the map. 相似文献
152.
Pulsed field gel electrophoresis and physical mapping of large DNA fragments in the Tm-2a region of chromosome 9 in tomato 总被引:2,自引:0,他引:2
Martin W. Ganal Nevin D. Young Steven D. Tanksley 《Molecular & general genetics : MGG》1989,215(3):395-400
Summary A method has been developed which allows the isolation of very high molecular weight DNA (>2 million bp) from leaf protoplasts of tomato (Lycopersicon esculentum). The DNA isolated in this manner was digested in agarose with rare-cutting restriction enzymes and separated by pulsed field gel electrophoresis. The size range of the reslting fragments was determined by hybridization to a number of single copy clones and the suitability of these enzymes for the mapping of large DNA fragments was evaluated. Furthermore, five genetically tightly linked single copy clones have been used to begin the construction of a physical map in a region of the genome containing the Tm-2a gene which confers resistance to tobacco mosaic virus. Two of the five clones were found to be on the same 560 kb SalI fragment and therefore are no further apart than that distance. The remaining three markers are distributed over at least 3 million bp, so that the total minimum physical distance of that cluster is at least 4 million bp. The results are discussed with respect to correlations between recombination frequencies and physical distance as well as physical mapping large regions of a complex plant genome like tomato. 相似文献
153.
Summary The population dynamics of a slow- and a fast-growing strain of the rotifer Brachionus calyciflorus are compared. Rotifers were grown in steady-states, at various specific growth rates (), in both two-stage chemostat and turbidostat cultures on the green alga Chlorella pyrenoidosa. Population variables, including specific ingestion (I), loss (L) and filtration (F) rates, yield (Y), production (P) and half-saturation coefficient of growth (K
g), were calculated using a growth model based on saturation kinetics. I, L, F and K
g were shown to be higher and Y and P lower for the fast-growing strain. Differences between the two strains with regard to these variables may represent tradeoffs associated with the faster potential growth rate. Steady-state relationships between these values and for the fast-growing strain, however, deviated from model predictions which suggest a possible shift from carbon to non-carbon growth limitation. 相似文献
154.
The effect of sucrose on the levels of abscisic acid, indoleacetic acid and zeatin/zeatin riboside in wheat ears growing in liquid culture 总被引:1,自引:0,他引:1
The influence of a varied sucrose supply on grain size and hormonal contents of detached wheat ears ( Triticwn aestivum L. cv. Schirokko) was investigated throughout grain development. In ears led limited amounts, or no sucrose, grain weights in both proximal and distal grain positions of the ear were reduced. Radioimmunoassay for abscisic acid, indoleacetic acid and zeatin/zeatin riboside showed that the changes in the levels of these hormones in grains and bracts were comparable to intact ears when detached ears were well supplied with sucrose. Under conditions of limited sucrose supply, higher abscisic acid levels in the distal and proximal grains of detached ears were found compared to ears supplied with adequate sucrose. Limiting sucrose supply to the ear did not alter the levels of indoleacetic acid or zeatin/zeatin riboside in either the grains or bracts of detached ears. 相似文献
155.
Zeatin Glycosylation Enzymes in Phaseolus: Isolation of O-Glucosyltransferase from P. lunatus and Comparison to O-Xylosyltransferase from P. vulgaris 总被引:4,自引:2,他引:2
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)
An enzyme catalyzing the formation of O-glucosylzeatin in immature embryos of Phaseolus lunatus was purified 2500-fold using ammonium sulfate precipitation followed by affinity and anion exchange chromatography. The enzyme uses trans-zeatin as substrate (Km 28 micromolar) but not cis-zeatin, ribosylzeatin, or dihydrozeatin. Both UDP-glucose and UDP-xylose can serve as glycosyl donors, with Kms of 0.2 and 2.7 millimolar, respectively, for the formation of O-glucosylzeatin and O-xylosylzeatin. In comparison, the UDPxylose-zeatin:O-xylosyltransferase (JE Turner, DWS Mok, MC Mok, G Shaw [1987] Proc Natl Acad Sci USA 84: 3714-3717) isolated by the same procedures from P. vulgaris embryos uses only UDP-xylose as donor substrate and the Kms for both zeatin and UDP-xylose are much lower (2 and 3 micromolar, respectively). The chromatographic behavior on affinity columns and molecular weights (approximate Mr 44,000 daltons) of the two enzymes are similar. Results from substrate competition experiments and enzyme separation by anion exchange HPLC indicate a single, distinct, zeatin O-glycosylation enzyme occurs in embryos of each of these Phaseolus species. 相似文献
156.
Photoaffinity labeling and partial purification of the putative plant receptor for the fungal wilt-inducing toxin,fusicoccin 总被引:1,自引:0,他引:1
The high-affinity fusicoccin-binding protein (FCBP) was solubilized from plasma-membrane vesicles prepared from leaves of Vicia faba L. by aqueous two-phase partitioning. Conditions for the solubilization of intact FCBP-radioligand complexes were worked out. About 60–70% of the complexes can be solubilized with 50–60 mM nonanoyl-N-methylglucamide in the presence of 1 mg· ml-1 soybean phosphatidylcholine, type IV S, and 20% (v/v) glycerol at pH 5.5. The slow dissociation of the radioligand, 9-nor-fusicoccin-8-alcohol-[3H] from the FCBP at low temperatures permits the purification of FCBP-radioligand complexes at 4–10° C by fast protein liquid chromatography on anion-exchange and gel permeation columns. The FCBP, extracted from plasma membranes with cholate and chromatographed in the presence of this detergent, gave an apparent molecular mass (Mr) of 80±20 kDa on gel permeation columns under the conditions used. By comparison of the elution profiles of the fraction most enriched in FCBP-radioligand complexes with polypeptide patterns obtained on sodium dodecyl sulfate-polyacrylamide gels, a polypeptide with an Mr of approx. 34kDa co-separated with the radioactivity profile. A second, faint band of approx. 31 kDa was sometimes also observed co-electrophoresing. Photoaffinity labeling of plasma-membrane vesicles with the new compound 9-nor-8[(3,5-[3H]-4-azidobenzoy)ethylenediamine]-fusicoccin ([3H]ABE-FC) and subsequent separation by sodium dodecyl sulfate-polyacrylamide gel electrophoresis labeled a single band with an Mr of 35±1 kDa. Labeling in this band was strongly reduced when the membranes were incubated with [3H]ABE-FC in the presence of 0.1–1 M fusicoccin. From our data, we conclude (i) that the 34-35-kDa polypeptide represents the FCBP and (ii) that in detergent extracts of plasma membranes this polypeptide is probably present as a di- or trimeric structure.Abbreviations ABE-FC
[(4-azidobenzoyl)-ethylenediamine]-fusicoccin
- ABE-NHS
(4-azidobenzoyl)-N-hydroxysuccinimide ester
- FC
fusicoccin
- FCBP
fusicoccin-binding protein
- FCol
9-norfusicoccin-8-alcohol
- MAB
monoclonal antibody
- Mega-9(10)
nonanoyl(decanoyl)-N-methylglucamide
- Mr
apparent molecular mass
- PMSF
phenylmethyl-sulfonyl fluoride
- SDS-PAGE
sodium dodecyl sulfate-polyacrylamide gel electrophoresis
- TCA
trichloroacetic acid
- Tris
2-amino-2-(hydroxymethyl)-1,3-propanediol 相似文献
157.
The population of a strain ofEscherichia coli that was resistant to nalidixic acid and streptomycin declined rapidly in samples of sterile and nonsterile Cayuga Lake water and reached an undetectable level in nonsterile water at 24 and 72 hours when counted on eosin-methylene blue (EMB) agar and half-strength trypticase soy agar (TSA), respectively. In sterile lake water amended with 10g amino acids per ml or 0.1 M phosphate,E. coli multiplied exponentially for more than 24 hours. The addition ofRhizobium leguminosarum biovarphaseoli to unamended sterile lake water prevented the decline ofE. coli, and its addition to amended sterile lake water preventedE. coli multiplication. The cell density of this strain ofE. coli declined in the first 8 hours after its introduction into an inorganic salts solution, but the bacterium then grew extensively. This increase in abundance was not observed in the presence ofR. phaseoli, andE. coli counts on half-strength TSA remained unchanged between 8 hours and 6 days. When counted on EMB agar, the abundance of the antibiotic-resistant strain ofE. coli and a strain not selected for resistance increased in solutions containing phosphate and amino acids but declined in the presence of high densities ofR. phaseoli. Many of the cells of the antibiotic-resistantE. coli strain failed to grow on antibiotic-amended EMB agar after introduction of the organism into nonsterile or sterile lake water or into an inorganic salts solution containingR. phaseoli, although colonies appeared on TSA. The data suggest thatE. coli cells grown on rich media suffer a shock when introduced into lake water because of low hypotonicity, the indigenous competing flora, or both. This shock is prevented by either phosphate buffer or by amino acids at low concentration. The shocked bacteria formed colonies on half-strength TSA. Depending on environmental conditions, the presence of a second organism either has no effect or results in an increase or decrease inE. coli numbers. 相似文献
158.
Lokesh Bhattacharyya Martin Haraldsson Nathan Sharon Halina Lis Fred Brewer 《Glycoconjugate journal》1989,6(1):141-150
Erythrina lectins possess similar structural and carbohydrate binding properties. Recently, tri- and tetra-antennary complex type carbohydrates with non-reducing terminal galactose residues have been shown to be precipitated as tri- and tetravalent ligands, respectively, with certainErythrina lectins [Bhattacharyya L, Haraldsson M, Brewer CF (1988) Biochemistry 271034-41]. The present work describes a comparative study of the binding and precipitating activities of fourErythrina lectins,viz. E. corallodendron, E. cristagalli, E. flabelliformis, andE. indica, with multi-antennary complex type carbohydrates and synthetic cluster glycosides. The results show that though their binding affinities are very similar, theErythrina lectins show large differences in their precipitating activities with the carbohydrates. The results also indicate significant dependence of the precipitating activities of the lectins on the core structure of the carbohydrates. These findings provide a new dimension to the structure-activity relationship of the lectins and their interactions with asparagine-linked carbohydrates.Abbreviations EAL, ECorL, ECL, EFL, and EIL
represent the lectins from the seeds ofErythrina arborescens,
-
E. corallodendron, E. cristagalli, E. flabelliformis, andE. indica
respectively
- AFOS
thetri-antennary complex type oligosaccharide from asialofetuin
- AFGP
the tri-antennary glycopeptide from asialofetuin
- MeGal
methyl -d-galactopyranoside
Unless stated otherwise all sugars are in thed-configuration. 相似文献
159.
160.
Youngdo Won Richard A. Friesner Martin R. Johnson Jonathan L. Sessler 《Photosynthesis research》1989,22(3):201-210
The Soret absorption spectra of six synthetic rigid porphyrin dimers whose crystal structures have been determined are simulated using simple exciton theory. The objective is to test the validity of the point dipole and associated approximations; the electronic interaction parameters are thus calculated using data obtained from the monomer spectra, with no adjustable parameters. Satisfactory agreement between theory and experiment is obtained for one class of dimers but not for a second. This poses a challenge for semiempirical electronic structure methods as to whether improvements over the point dipole calculations can be obtained. 相似文献