全文获取类型
收费全文 | 2439篇 |
免费 | 190篇 |
出版年
2023年 | 26篇 |
2021年 | 35篇 |
2020年 | 29篇 |
2019年 | 27篇 |
2018年 | 87篇 |
2017年 | 78篇 |
2016年 | 138篇 |
2015年 | 82篇 |
2014年 | 116篇 |
2013年 | 224篇 |
2012年 | 89篇 |
2011年 | 80篇 |
2010年 | 98篇 |
2009年 | 60篇 |
2008年 | 57篇 |
2007年 | 22篇 |
2006年 | 71篇 |
2005年 | 34篇 |
2004年 | 29篇 |
2003年 | 36篇 |
2002年 | 30篇 |
2001年 | 51篇 |
2000年 | 50篇 |
1999年 | 50篇 |
1998年 | 56篇 |
1997年 | 32篇 |
1996年 | 45篇 |
1995年 | 38篇 |
1994年 | 46篇 |
1993年 | 52篇 |
1992年 | 28篇 |
1991年 | 28篇 |
1989年 | 24篇 |
1988年 | 30篇 |
1987年 | 24篇 |
1985年 | 25篇 |
1984年 | 21篇 |
1983年 | 21篇 |
1981年 | 26篇 |
1980年 | 33篇 |
1978年 | 24篇 |
1975年 | 21篇 |
1974年 | 34篇 |
1973年 | 26篇 |
1972年 | 22篇 |
1971年 | 23篇 |
1969年 | 22篇 |
1968年 | 20篇 |
1967年 | 20篇 |
1966年 | 23篇 |
排序方式: 共有2629条查询结果,搜索用时 31 毫秒
91.
An auxotrophic strain ofEscherichia coli with therecB recC sbcB genotype was transformed by chromosomal DNA of the prototrophic strain and by plasmid DNA carrying genes for antibiotic resistance
(R1drd 19). The donor plasmid DNA obtained by cell lysis in the presence of Triton X-100 and subsequent centrifugation in a caesium
chloride-ethidium bromide gradient was shown to have a circulaf molecule and to retain its completeness after penetration
into the recipient. Experiments with mixtures or plasmid and chromosomal DNA indicate a competition between these two DNA
types during the transformation reaction in the given system. 相似文献
92.
Two new recombinant haplotypes of the rat major histocompatibility system,RT1, have been detected in [LEW.1A (RT1
a
) ×LEW.1W (RT1
u
)] × LEW 1N(RT1
n
) segregating hybrids. Recombinantr3 carries theRTL1. A region (determining classical transplantation antigens) and theRT1.B region (determining strong mixed lymphocyte reactivity and genetic control of antipolypeptide immune responsiveness) of the RT1a parent, bur rejects RT1a skin grafts. Recombinantr4 carries theA andB regions of the RT1u parent, but rejects RT1u skin grafts. The two histocompatibility genes detected are allelic to each other. The relevant locus, designated asH-C, maps to theB-region side of theRT1 system and appears to mark a thirdRT1 gene region,RT1.C. Availability of haplotypes r3 andr4 allowed the definition of a histocompatibility locus in theB region,H-B. The products ofH-C, H-B and of the previously describedH-A gene vary in antigenic strength. 相似文献
93.
Vytas K. Švedas Alexei L. Margolin Ilya V. Berezin 《Enzyme and microbial technology》1980,2(2):138-144
The equilibrium constants and the respective standard Gibbs energy changes for hydrolysis of some β-lactam antibiotics have been determined. Native and immobilized penicillin amidase (EC 3.5.1.11) from Escherichia coli has been used as a catalyst. The values of standard Gibbs energy changes corresponding to the pH-independent product of equilibrium concentrations (ΔG0c = ? RT ln Kc) have been calculated. The differences in the structure of the antibiotics nucleus hardly ever affect the value of the pH-independent component of the standard Gibbs energy change (ΔG0c) and value of apparent standard Gibbs energy change at a fixed pH (ΔG0′c). At the same time, the value of ΔG0c is more sensitive to the structure of the acyl moiety of the antibiotic; when ampicillin is used instead of benzylpenicillin, ΔG0c increases by ~6.3 kJ mol?1 (1.5 kcal mol?1). pH-dependences of the apparent standard Gibbs energy changes for hydrolysis of β-lactam antibiotics have been calculated. The pH-dependences of ΔG0′c for hydrolysis of all β-lactam antibiotics have a similar pattern. The thermodynamic pH optimum of the synthesis of these compounds is in the acid pH range (pH < 5.0). The breakage of the β-lactam ring leads to a sharp decrease in the ΔG0′c value and a change in the pattern of the pH-dependence. For example, at pH 5.0 ΔG0′c decreases from 14.4 kJ mol?1 for benzylpenicillin to ?1.45 kJ mol?1 for benzylpenicilloic acid. The reason for these changes is mainly a considerable increase in the pK of the amino group of the nucleus of the antibiotic and, as a consequence, a decrease in the component of standard Gibbs energy change, corresponding to the ionization of the system. The thermodynamic potentials of the enzymatic synthesis of semisynthetic penicillins and cephalosporins on the basis of both free acids and their derivatives (N-acylated amino acids, esters) are discussed. It is shown that with esters of the acids, a high yield of the antibiotic can, in principle, be achieved at higher pH values. 相似文献
94.
When flax seedlings are decapitated above cotyledons and three days later one of the two cotyledons is removed then the remaining
cotyledon stimulates in four to five days growth of its axillary bud. It has been found that content of endogenous cytokinins
was higher in the stimulated bud as compared with the other one already 12 h after the cotyledon removal.
Flax seedlings decapitated under cotyledons regenerate adventitious buds on thy hypocotyl stump during 5–6 days. The endogenous
fytohormonal preparation of this regeneration was investigated in the 20 mm apical part of the hypocotyl stump. Decrease in
auxin and increase in gibberellins was already found during the first day after decapitation while the level of cytokinins
increased as late as three days after the apex removal. 相似文献
95.
96.
97.
On integrating experimental data published previously, the following picture of the mitochondrial adenine nucleotide (AdN) translocation system is being presented: 1. The AdN translocation system serves not only to transport ATP synthesized within mitochondria into the cytosol but also to transport cytosolic ATP into the mitochondria when oxidative phosphorylation is not functioning. 2. The AdN translocator is coded for by nuclear genes and the mitochondrial protein synthesis is not involved in its formation. 3. The AdN translocation system must be preserved and functioning even in cells which could dispense with oxidative phosphorylation. It assures appropriate concentrations of intramitochondrial ATP. 4. The intramitochondrial ATP is required for normal replication of mitochondrial DNA. Tis supports the view that the mitochondrion is a self-replicating semi-autonomous organelle. 5. The appropriate concentration of ATP must be present in mitochondria to make possible cell growth or multiplication. This points to a direct or indirect role of mitochondria in the control of cell proliferation. 相似文献
98.
Miloš Sedlák Jaromír Kaválek Vladimír Machácek Vojeslav Šterba 《Molecules and cells》1992,1(10):170-174
The cyclization reactions of N-methyl-N’-(2-methoxycarbonylphenyl)sulfamide (1a), N-methyl-N-(2-methoxycarbonylphenyl)-sulfamide (2a), and 2-methoxycarbonylphenylsulfamide (3a) were studied in aqueous amine buffers (butylamine, ethanolamine, morpholine, glycinamide). The dependences observed between the rate constants and buffer concentrations show that the reactions are subject to base catalysis in all the three cases, the decomposition of the tetrahedral intermediate being rate limiting. The ratio of the relative rate constants of the base catalyzed cyclizations reactions of the three derivatives is 1a: 2a: 3a = 1: 20000: 100. The logarithm of rate constants of the base catalyzed cyclization reactions was plotted against the pKa values of conjugated acids of the individual amines used as the buffers in the cyclization of compound 1a, and the value of the Brönsted coefficient obtained was about 0.1, which means that the proton transfer from the intermediate to the basic buffer component is thermodynamically favorable. The intermediate is a much weaker base, and the reaction is controlled by diffusion. The slope of an analogous dependence for compound 2a gradually decreases from values near to 0.5 to values near to zero, which means that the intermediate formed from compound 2a (pKa ≈ 9.3) has a pKa value comparable with that of the acid buffer component. 相似文献
99.
No significant differences in the acetylene-reducing activity and evolution of H2 and CO2 nodulated roots ofPisum sativum inoculated with soilRhizobium populations from two soils with different acidities (Ruzyně soil 7.6; Lukavec soil 4.9) were observed.Rhizobium population from Lukavec soil formed nodules, exhibiting a higher H2 evolution. Co-inoculation with the Hup+ strain 128C30 (7×107 cells per seedling) eliminated, to some extent, the effect of soil populations on physiological activity.
Translated by Č. Novotny 相似文献
100.
Darja Barlič Maganja Borut Štrukelj Jože Pungerčar Franc Gubenšek Vito Turk Igor Kregar 《Plant molecular biology》1992,20(2):311-313
A genomic DNA clone encoding an aspartic proteinase inhibitor of potato was isolated from a lambda EMBL3 phage library using the aspartic proteinase inhibitor cDNA as a hybridization probe. The gene has all characteristic sequences normally found in eucaryotic genes. Typical CAAT and TATA box sequences were found in the 5-upstream region. In this part are also two putative regulatory AGGA box sequences located. In the genomic sequence there are no intron sequences interrupting the coding region. An open reading frame of the gene encodes a precursor protein of 217 amino acids which shows high percent identity with the aspartic proteinase inhibitor cDNA. 相似文献