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991.
Mikael Hedrén 《Plant Systematics and Evolution》1996,201(1-4):31-55
Taxa endemic to North-western Europe are rare, but the orchid genusDactylorhiza contains several species restricted to this area. Evidence from morphological and cytological studies have indicated that some species may have arisen recently and may be of hybrid origin. In the present report, I use allozymes to characterize the genomes in various species ofDactylorhiza and evaluate the possibilities for rapid evolutionary change in the genus. Allotetraploid species have evolved repeatedly from two principal diploid ancestral lineages. These lineages include extant diploid and autotetraploid species, from which allotetraploid derivatives may still arise. It is suggested that allotetraploidization dominates over introgression as speciation mechanism in the genus. The more common and widespread allotetraploid species could be characterized by their allozyme characters over considerable distances, indicating that each of them may have a unique origin and that they have spread from their ancestral populations to the present distribution areas. However, it is also possible that some allotetraploid species contain local populations that have been independently derived from the ancestral lineages. 相似文献
992.
Abstract The global regulators agr and sar control expression of cell wall and extracellular proteins. Inactivation of either sar and/or agr in a typical heterogeneously methicillin-resistant Staphylococcus aureus resulted in a small but reproducible decrease in the number of cells in the subpopulation expressing high methicillin resistance. The amount of low affinity penicillin-binding protein PBP2', the prerequisite for methicillin resistance, was apparently not affected, however, a reduction in PBP1 and PBP3 production was observed, suggesting that these resident PBPs of the cells might be involved somehow together with PBP2' in high level methicillin resistance. 相似文献
993.
J. Miranda J. Membrillo-Hernández M. L. Tabche M. Soberón 《Applied microbiology and biotechnology》1996,45(1-2):182-188
A method to isolate mutants with derepressed expression of cytochrome oxidases and better symbiotic performance is presented.
A mutant of Rhizobium etli, CFN030, isolated by its azide-resistant phenotype, was obtained by transposon Tn5-mob mutagenesis. This mutant has a derepressed expression of cytochrome aa3, higher respiratory activities when cultured microaerobically and an improved symbiotic nitrogen fixation capacity. This
phenotype was similar to the previously described mutant CFN037, which was isolated by its increased capacity to oxidize N,N,N′,N′-tetramethyl-p-phenylenediamine (TMPD) [Soberón M et al. (1990) J Bacteriol 172:1676–1680]. We show here that although both mutants have
a similar symbiotic phenotype, they are affected in different genes. Strain CFN030 has the Tn5 inserted in the chromosome while in strain CFN037 the transposon was located in plasmid b. Cytochrome spectral analysis of
both mutant strains in the post-exponential phase of growth, showed the expression of an additional terminal oxidase (cbb3) that is not expressed in the wild-type strain.
Received: 10 April 1995/Received revision: 21 August 1995/Accepted: 7 September1995 相似文献
994.
M. T. Fernández-Espinar S. Vallés F. Piñaga J. A. Pérez-González D. Ramón 《Applied microbiology and biotechnology》1996,45(3):338-341
Using recombinant DNA techniques, an Aspergillus nidulans multicopy transformant for the gene xlnB coding for the minor X24 xylanase has been constructed. When grown on glucose as sole carbon source this transformant secretes 114 U of xylanase (mg
protein)-1. In this culture condition, X24 is the only xylanase secreted and the predominant protein in the culture filtrate. This strategy has been used to purify
the X24 enzyme to homogeneity. The purified xylanase showed a single band on sodium dodecyl sulphate/ polyacrylamide gel electrophoresis
with a molecular mass of 24 kDa and had an isoelectric point of approximately 3.5. The enzyme was a non-debranching endo-1,4-β-xylan
xylanohydrolase highly specific for xylans and showed optimal activity at pH 5.5 and 52°C. The X24 xylanase had a Michaelis constant, K
m, of 12.43 mg oat spelt xylan ml-1 and a V
max of 1639 μmol min-1 (mg protein)-1.
Received: 17 May 1995/Received last revision: 25 September 1995/Accepted: 29 September 1995 相似文献
995.
M. J. Taherzadeh G. Lidén L. Gustafsson C. Niklasson 《Applied microbiology and biotechnology》1996,46(2):176-182
Physiological effects of deficiency of pantothenate, a necessary precursor in the synthesis of coenzyme A, were studied using
the yeast strain Saccharomyces cerevisiae CBS 8066. Cells were grown on defined media in anaerobic batch cultures with glucose (50 g/l) as the carbon and energy source.
Batch cultures containing more than 60 μg/l pantothenate showed no significant differences with respect to growth rates and
product yields. However, with an initial pantothenate concentration of 30 μg/l, the average glucose consumption rate was 50%
lower than in rich medium and, at even lower concentrations of pantothenate, the culture did not consume all the glucose in
the medium. Furthermore, pantothenate deficiency caused the acetate and pyruvate yields to increase and the biomass yield
to decrease, compared to the yields in pantothenate-rich medium. The increased acetate formation could be counteracted by
initial addition of acetate to the medium, and thereby the glycerol yield could be decreased. An initial addition of acetate
of 1.6 g/l to pantothenate-deficient medium (30 μg/l) caused a 35% decrease in glycerol yield and a 6% increase in ethanol
yield. Furthermore, the time required for complete conversion of the glucose decreased by 40%. Acetate addition affected the
acetate and glycerol yields in a similar way in pantothenate-rich medium (1000 μg/l) also.
Received: 27 December 1995/Received revision: 3 May 1996/Accepted: 9 May 1996 相似文献
996.
F. Fierro E. Montenegro S. Gutiérrez J. F. Martín 《Applied microbiology and biotechnology》1996,44(5):597-604
The organization of the genes of the penicillin cluster has been studied in three different mutants of P. chrysogenum impaired in penicillin biosynthesis. The three blocked mutants (derived from the parental strain P. chrysogenum Bb-1) lacked the genes pcbAB, pcbC and penDE of the penicillin biosynthetic pathway and were unable to form isopenicillin N synthase and isopenicillin N acyltransferase. All strains were identified as P. chrysogenum derivatives by fingerprinting analysis with (GTG)n as a probe. The borders of the deleted region were cloned and sequenced, showing the same junction point in the three mutants. The deleted DNA region was found to be identical to that described in P. chrysogenum npe10. The frequent deletion of the pen gene cluster at this point may indicate that this cluster is located in an unstable genetic region, flanked by hot spots of recombination, that is easily lost by mutagen-induced recombination. 相似文献
997.
B. Szajáni Z. Buzás K. Dallmann I. Gimesi J. Krisch M. Tóth 《Applied microbiology and biotechnology》1996,46(2):122-125
Saccharomyces cerevisiae cells were immobilized on preformed cellulose beads by adsorption. The fermentation capacity of the immobilized yeast cells
was found to be practically independent of the hydrogen ion concentration between pH 3.1 and 6.25. The fermentation capacity
was maximal at 30 °C. The immobilized yeast cells were used for continuous production of ethanol in a fluidized-bead reactor.
The average values characteristic for the process were an ethanol concentration of 41.9±0.1 g l-1, a fermentation efficiency of 82.9±2.1% and a volumetric productivity of 3.94±0.52 g l-1 h-1.
Received: 9 October 1995/Accepted: 22 April 1996 相似文献
998.
999.
Settler agriculture and the dynamics of resource allocation in frontier environments 总被引:1,自引:0,他引:1
Francisco J. Pichón 《Human ecology: an interdisciplinary journal》1996,24(3):341-371
This article attempts to conceptualize the dynamics of resource allocation by colonist farmers under the unique conditions of land abundance and labor scarcity which characterize frontier environments, such as the smallholder agricultural settlement areas in the Amazon basin. In contrast, most previous theoretical literature on household agricultural decision making and land-use change in rural areas considers conditions of high population density and land scarcity, and is not, therefore, adequate for understanding critical land-use changes which may be occurring in frontier regions. This article first discusses the appropriateness and inadequacies of the analytical frameworks commonly used to explain the expansion of settler agriculture into remote forest regions and the unsustainable land-use practices observed in these areas. This review serves as the basis for characterizing resource allocation under the particular conditions of frontier environments. A conceptual advance in the analysis is its consideration of the way institutional/policy factors and farm-level characteristics can interact to produce land-use outcomes. This knowledge is essential to understand not only the social and economic factors affecting present land use and choice of technology, but also those factors influencing farmers' demand for more optimal systems of land use which are consistent with varying agro-ecological potentials, demographic situations, and their own management capacity. 相似文献
1000.
V. Alvarez E. Coto S. González-Roces C. López-Larrea F. Setién C. López-Larrea 《Immunogenetics》1996,44(6):446-452
N-formyl peptides (FMLP) and complement fragment C5a are neutrophil chemoattractants. In humans, a single-copy gene was identified
for the C5a receptor, and the receptor for FMLP (FPR1) is encoded by a single gene that shows 53% amino acid similarity to
the C5aR. Two other humanFPR1 homologues,FPR-like 1 (FPR2/FPRL1) andFPR-like 2 (FPRL2) have been cloned. The human C5aR, FPR1, FPRL1, and FPRL2 are physically linked. By direct sequencing or by sequencing plasmid
clones we studied theC5aR andFPR genes from four non-human primates (chimpanzee, gorilla, orangutan, and macaque). The sequences showed 95%–99% similarity
to the human homologues, with the major divergences observed in macaque. In these genes, the transmembrane and the cytoplasmic
domains are highly conserved, while the highest divergence corresponded to the extracellular loops involved in ligand binding.
Additionally, we constructed a physical map of these genes in non-human primates. In all species the four genes were physically
linked and we defined the relative orientation of the four genes in primates:C5aR>FPR1>FPR2 (FPRL1)>FPRL2.
The nucleotide sequence data reported in this paper have been submitted to the EMBL/GenBank nucleotide sequence databases
and have assigned the accession numbers X97730 (PTC5aR), X97731 (MMC5aR), X97732 (PPC5aR), X97730 (GGC5aR), X97734 (MMFPR1), X97735 (PPFPR1), X97736 (GGFPR1), X97737 (MMFPRL1), X97738 (GGFPRL1), X97739 (PTFPRL1), X97740 (MMFPRL2), X97741 (PPFPRL2), X97742 (GGFPRL2), X97743 (PTFPRL2), X97744 (PPFPRL1), and X97745 (PTFPR1) 相似文献