全文获取类型
收费全文 | 3744篇 |
免费 | 591篇 |
国内免费 | 1篇 |
出版年
2021年 | 51篇 |
2019年 | 39篇 |
2018年 | 45篇 |
2016年 | 61篇 |
2015年 | 103篇 |
2014年 | 113篇 |
2013年 | 134篇 |
2012年 | 172篇 |
2011年 | 179篇 |
2010年 | 141篇 |
2009年 | 100篇 |
2008年 | 155篇 |
2007年 | 146篇 |
2006年 | 127篇 |
2005年 | 129篇 |
2004年 | 129篇 |
2003年 | 113篇 |
2002年 | 138篇 |
2001年 | 120篇 |
2000年 | 111篇 |
1999年 | 97篇 |
1998年 | 46篇 |
1997年 | 41篇 |
1996年 | 46篇 |
1995年 | 43篇 |
1994年 | 48篇 |
1993年 | 41篇 |
1992年 | 88篇 |
1991年 | 92篇 |
1990年 | 76篇 |
1989年 | 81篇 |
1988年 | 96篇 |
1987年 | 75篇 |
1986年 | 70篇 |
1985年 | 55篇 |
1984年 | 52篇 |
1983年 | 51篇 |
1982年 | 38篇 |
1981年 | 54篇 |
1980年 | 43篇 |
1979年 | 62篇 |
1978年 | 47篇 |
1977年 | 44篇 |
1976年 | 41篇 |
1975年 | 37篇 |
1974年 | 62篇 |
1973年 | 60篇 |
1972年 | 39篇 |
1970年 | 37篇 |
1967年 | 39篇 |
排序方式: 共有4336条查询结果,搜索用时 265 毫秒
91.
The kinetics of reduced nicotinamide adenine dinucleotide phosphate (NADPH) binding to fatty acid synthase from chicken liver and of the reduction of enzyme-bound acetoacetyl by NADPH (beta-ketoacyl reductase) and the steps leading to formation of the acetoacetyl-enzyme have been studied in 0.1 M potassium phosphate-1 mM ethylenediaminetetraacetic acid (EDTA), pH 7.0, at 25 degrees C by monitoring changes in NADPH fluorescence with a stopped-flow apparatus. Improved fluorescence detection has permitted the use of NADPH concentrations as low as 20 nM. The kinetics of the binding of NADPH to the enzyme is consistent with a simple bimolecular binding mechanism and four equivalent sites on the enzyme (presumably two beta-ketoacyl reductase sites and two enoyl reductase sites). The bimolecular rate constant is 12.7 X 10(6) M-1 s-1, and the dissociation rate constant is 76.7 s-1, which gives an equilibrium dissociation constant of 6.0 microM. The formation of the acetoacetyl-enzyme and its subsequent reduction by NADPH could be analyzed as two consecutive pseudo-first-order reactions by mixing enzyme-NADPH with acetyl-CoA and malonyl-CoA under conditions where [acetyl-CoA], [malonyl-CoA] much greater than [enzyme] much greater than [NADPH]. From the dependence of the rate of reduction of aceto-acetyl-enzyme by NADPH on enzyme concentration, an independent estimate of the equilibrium dissociation constant for NADPH binding to the enzyme of 5.9 microM is obtained, and the rate constant for the reduction is 17.5 s-1.(ABSTRACT TRUNCATED AT 250 WORDS) 相似文献
92.
Mutations in the uncE gene affecting assembly of the c-subunit of the adenosine triphosphatase of Escherichia coli. 总被引:11,自引:2,他引:9 下载免费PDF全文
D A Jans A L Fimmel L Langman L B James J A Downie A E Senior G R Ash F Gibson G B Cox 《The Biochemical journal》1983,211(3):717-726
The amino acid substitutions in the mutant c-subunits of Escherichia coli F1F0-ATPase coded for by the uncE429, uncE408 and uncE463 alleles affect the incorporation of these proteins into the cell membrane. The DNA sequence of the uncE429 allele differed from normal in that a G leads to A base change occurred at nucleotide 68 of the uncE gene, resulting in glycine being replaced by aspartic acid at position 23 in the c-subunit. The uncE408 and uncE463 mutant DNA sequences were identical and differed from normal in that a C leads to T base change occurred at nucleotide 91 of the uncE gene, resulting in leucine being replaced by phenylalanine at position 31 in the c-subunit. An increased gene dosage of the uncE408 or uncE463 alleles resulted in the incorporation into the membranes of the mutant c-subunits. The results are discussed in terms of the 'Helical Hairpin Hypothesis' of Engelman & Steitz [(1981) Cell 23,411-422]. 相似文献
93.
Summary The thecal ultrastructure ofScrippsiella faeroense (Paulsen) Balech and Oliveira Soares as seen in the electron microscope is described. Additional structural detail of the thecal plate surface, plate connections, and the apical pore, is revealed. 相似文献
94.
95.
Conditional Mutator Gene in Escherichia coli: Isolation, Mapping, and Effector Studies 总被引:54,自引:22,他引:32
A mutator gene, mutD5, whose phenotype is conditional, has been identified in Escherichia coli. By P1 transduction it has been shown to lie at about 5.7 min on the chromosome, being co-transduced with proA and argF. In rich medium, streptomycin- and nalidixic acid-resistant mutation frequencies are 50 to 100 times higher than those in minimal medium. In minimal medium, the mutD5-induced mutation frequencies are still 50 to 100 times above co-isogenic wild-type (mut(+)) levels. Similar results were obtained with all markers tested. Mutant frequencies can be raised by thymidine in the medium at concentrations as low as 0.04 muM, or by the endogenous generation of thymidine from thymine plus a deoxyribosyl donor. Deoxyadenosine, various ribonucleosides, thymine, and 2-deoxyribose do not stimulate mutation. None of these effects are related to growth rate, since growth rate and mutation rate can be decoupled completely. 相似文献
96.
97.
Complementation and polarity suppression data are interpreted in terms of the genetic structure of the maltose B region. It is proposed that this region comprises two divergent operons. One operon includes malK, a cistron involved in maltose permeation, and lamB the only known cistron specifically involved in lambda receptor synthesis. The other operon includes malJ(1) and malJ(2) which are most probably two different cistrons, both involved in maltose permeation*. It is further assumed that expression of the two operons is controlled by malT, the positive regulatory gene of the maltose system, located in the malA region. The target(s) for the action of the malT product is (are) most likely to be located between malJ(1) and malK. There is an indication that the two operons might overlap in the region of their promoters. The structure of such an overlap as well as the possible function of the products of the different cistrons in malB are briefly discussed. 相似文献
98.
Liver and testis slices convert 6-N-trimethyl-lysine into 4-N-trimethylaminobutyrate and carnitine. Adipose, skeletal muscle, heart, or kidney tissues metabolize trimethyl-lysine into trimethylaminobutyrate but not into carnitine. Trimethylaminobutyrate hydroxylation, forming carnitine, occurs in liver and to a minor degree in testis. Liver is the primary site of carnitine biosynthesis in the rat. 相似文献
99.
100.
Purification, Ultrastructure, and Composition of Axial Filaments from Leptospira 总被引:22,自引:16,他引:6 下载免费PDF全文
The ultrastructure of three strains of water Leptospira was studied by negative staining, thin sectioning, and freeze-etching. The cells possessed a triple-layered sheath which covered two independent axial filaments, one inserted subterminally in each end of the cell. The protoplasmic cylinder was surrounded by a triple-layered cell wall and possessed ribosomes, lamellar structures, and a typical procaryotic nuclear region. The axial filament was comprised of several component structures. An axial fibril, with a diameter of 20 to 25 nm, consisted of a solid inner core (13 to 16 nm in diameter) surrounded by a coat. A terminal knob (40 to 70 nm in length) was connected to a series of disc insertion structures at the terminal end of the axial fibril. The axial fibril was surrounded by a helical outer coat (35 to 60 nm in diameter) which was composed of a continuously coiled fiber, 3 to 4 nm in diameter, embedded in an electron-dense material. A procedure for the purification of the axial fibrils was presented and their ultrastructural, physical, and chemical properties were determined. Similarities in ultrastructural, physical, and chemical properties were noted between the axial fibrils and bacterial flagella. A schematic model of the leptospiral axial filament is presented, and a mechanism is proposed for its function as a locomotor organelle. 相似文献