全文获取类型
收费全文 | 6427篇 |
免费 | 533篇 |
国内免费 | 5篇 |
专业分类
6965篇 |
出版年
2023年 | 27篇 |
2022年 | 64篇 |
2021年 | 101篇 |
2020年 | 73篇 |
2019年 | 82篇 |
2018年 | 103篇 |
2017年 | 128篇 |
2016年 | 164篇 |
2015年 | 285篇 |
2014年 | 390篇 |
2013年 | 433篇 |
2012年 | 552篇 |
2011年 | 488篇 |
2010年 | 324篇 |
2009年 | 283篇 |
2008年 | 336篇 |
2007年 | 372篇 |
2006年 | 347篇 |
2005年 | 327篇 |
2004年 | 301篇 |
2003年 | 294篇 |
2002年 | 273篇 |
2001年 | 67篇 |
2000年 | 60篇 |
1999年 | 61篇 |
1998年 | 75篇 |
1997年 | 60篇 |
1996年 | 59篇 |
1995年 | 47篇 |
1994年 | 59篇 |
1993年 | 49篇 |
1992年 | 70篇 |
1991年 | 41篇 |
1990年 | 47篇 |
1989年 | 45篇 |
1988年 | 28篇 |
1987年 | 35篇 |
1986年 | 22篇 |
1985年 | 29篇 |
1984年 | 39篇 |
1983年 | 32篇 |
1982年 | 28篇 |
1981年 | 37篇 |
1980年 | 18篇 |
1979年 | 14篇 |
1978年 | 15篇 |
1976年 | 18篇 |
1974年 | 26篇 |
1973年 | 13篇 |
1972年 | 20篇 |
排序方式: 共有6965条查询结果,搜索用时 15 毫秒
51.
Denitrification and oxygen respiration in biofilms studied with a microsensor for nitrous oxide and oxygen 总被引:4,自引:0,他引:4
Lars Peter Nielsen Peter Bondo Christensen Niels Peter Revsbech Jan Sørensen 《Microbial ecology》1990,19(1):63-72
Depth distributions of O2 respiration and denitrification activity were studied in 1- to 2-mm thick biofilms from nutrient-rich Danish streams. Acetylene was added to block the reduction of N2O, and micro-profiles of O2 and N2O in the biofilm were measured simultaneously with a polarographic microsensor. The specific activities of the two respiratory processes were calculated from the microprofiles using a one-dimensional diffusion-reaction model. Denitrification only occurred in layers where O2 was absent or present at low concentrations (of a fewM). Introduction of O2 into deeper layers inhibited denitrification, but the process started immediately after anoxic conditions were reestablished. Denitrification activity was present at greater depth in the biofilm when the NO3
– concentration in the overlying water was elevated, and the deepest occurrence of denitrification was apparently determined by the depth penetration of NO3
–. The denitrification rate within each specific layer was not affected by an increase in NO3
– concentration, and the half-saturation concentration (Km) for NO3
– therefore considered to be low (<25M). Addition of 0.2% yeast extract stimulated denitrification only in the uppermost 0.2 mm of the denitrification zone indicating a very efficient utilization of the dissolved organic matter within the upper layers of the biofilm. 相似文献
52.
Lars Nitschke Karin Heeger Hans Bender Georg E. Schulz 《Applied microbiology and biotechnology》1990,33(5):542-546
Summary The -cyclodextrin glycosyltransferase (-CGTase) gene was isolated from a -library prepared from Bacillus circulans strain no. 8. It was subcloned into plasmid pTZ and expressed by its endogenous regulatory sequences in Escherichia coli JM 103. The structural gene was sequenced and showed an open reading frame for a polypeptide of 718 amino acid residues. The recombinant -CGTase had the same enzymatic properties as the extracellular CGTase (684 amino acid residues, corresponding to a mol. wt. of 74416) produced by B. circulans strain no. 8. The amino acid sequence showed the highest homology (74.6% identical amino acids) with the CGTase of B. circulans strain F-2, which had been erroneously described as an amylase. The homology with the enzyme from the alkalophilic Bacillus sp. strain no. 1011 was 71.4%. The amino acid sequence derived will be used for elucidating the three-dimensional structure of the enzyme.
Offprint requests to: H. Bender 相似文献
53.
Linker insertion analysis of the FimH adhesin of type 1 fimbriae in an Escherichia coli fimH-null background 总被引:2,自引:0,他引:2
Mark A. Schembri Lars Pallesen Hugh Connell David L. Hasty Per Klemm 《FEMS microbiology letters》1996,137(2-3):257-263
Abstract The gene encoding the Escherichia coli FimH adhesin of type 1 fimbriae has been subjected to linker insertion mutagenesis. Amino acid changes were introduced at a number of positions spanning the entire sequence in order to probe the structure-function relationship of the FimH protein. The effect of these mutations on the ability of bacteria to express a D-mannose binding phenotype was assessed in a fimH null mutant (MS4) constructed by allelic exchange in the E. coli K-12 strain PC31. Mutations mapping at amino acid residues 36, 58 and 279 of the mature FimH protein were shown to completely abolish binding to D-mannose receptors. Differences in the level of fimbriation were also observed as a result of some of the mutations in the fimH gene. These mutants may prove useful in dissecting receptor-ligand interactions by defining regions of the FimH protein that are important in erythrocyte binding. 相似文献
54.
A second immunoglobulin light chain isotype in the rainbow trout 总被引:4,自引:0,他引:4
55.
The region encompassing theMa, Mb1, Mb2, andLmp2 genes of the mouse class II major histocompatibility complex (MHC) was sequenced. Since this region contains clusters of genes required for efficient class I and class II antigen presentation, it was interesting to search for putative additional genes in the 21 kilobase gap between theMb1 andLmp2 genes. Computer predictions of coding regions and CpG islands, exon trapping experiments, and cross-species comparison with the corresponding human sequence indicate that no additional functional gene is present in that stretch. However, computer analysis revealed the possible existence of an alternative 3 exon forMb1. Except for the fact that the mouse MHC contains twoMb genes, the genomic organization of theH2-M loci was found to be almost identical to the organization of the humanHLA-DM genes. The promoter regions of theMa andMb genes also resemble classical class II promoters, containing typical S, X, and Y boxes. Like the human genes, the threeH2-M genes displayed very limited polymorphism when we compared the cDNA sequences from six haplotypes. Finally, comparison ofDMB withMb1 andMb2, both at the genomic level and in their coding regions, suggests that theMb gene was recently duplicated, probably only in certain rodents. 相似文献
56.
Lars Öhman Maria Alarcon Jan Ljunggren Anna-Karin Ramqvist Lena Häggström 《Biotechnology letters》1996,18(7):765-770
Summary
Spodoptera frugiperda (Sf-9) insect cells are fully capable of growth and proliferation in a glutamine, glutamate and aspartate-free medium, provided that ammonium ions are supplied. S. frugiperda (Sf-21) and Mamestra brassicae cells (IZD-MB-0503) also grow in glutamine-free media but not Trichoplusia ni cells (BTI-TN 5B1-4). The yield of -galactosidase in Sf-9 cells infected with a recombinant baculovirus under glutamine-free conditions was even higher than the yield obtained in glutamine containing cultures. 相似文献
57.
ZmHox: a novel class of maize homeobox genes 总被引:2,自引:0,他引:2
Bettina Klinge Bärbel Überlacker Christian Korfhage Wolfgang Werr 《Plant molecular biology》1996,30(3):439-453
58.
59.
The influence of various culture parameters on the attachment of a recombinant baculovirus to suspended insect cells was examined under normal culture conditions. These parameters included cell density, multiplicity of infection, and composition of the cell growth medium. It was found that the fractional rate of virus attachment was independent of the multiplicity of infection but dependent on the cell density. A first order mathematical model was used to simulate the adsorption kinetics and predict the efficiency of virus attachment under the various culture conditions. This calculated efficiency of virus attachment was observed to decrease at high cell densities, which was attributed to cell clumping. It was also observed that virus attachment was more efficient in Sf900II serum free medium than it was in IPL-41 serum-supplemented medium. This effect was attributed to the protein in serum which may coat the cells and so inhibit adsorption. A general discussion relating the observations made in-these experiments to the kinetics of recombinant baculovirus adsorption to suspended insect cells is presented. 相似文献
60.
Sture Forsgren Lars Franzén Yan Liang Hans Gustafsson Roger Henriksson 《The Histochemical journal》1994,26(8):630-640
Summary It is well-known that a large number of factors can influence the expression of neuropeptides in the nervous system. In the present study, the effects of unilateral and bilateral irradiation to the rat head and neck on the expression of neuropeptides in the innervation of the submandibular gland and in the ganglionic cells of the submandibular ganglion was examined ten days and six months after treatment. Antisera directed against enkephalin and bombesin and immunohistochemical methods were used. The effects of bilateral irradiation on the staining pattern of various neuropeptides in the cervical spinal cord were also studied. In the submandibular gland and in the submandibular ganglionic cells, there was a markedly increased neuropeptide expression ten days after bilateral treatment, as seen after staining with both antisera used, while no changes occurred after unilateral treatment. Six months after treatment, the pattern of neuropeptide expression in the submandibular gland/ganglion corresponded to that seen in controls. Irradiation did not lead to any changes in the staining pattern of neuropeptides in the spinal cord. The observations show that there is a great complexity in the susceptibility of nervous tissues to radiotherapy with respect to influences on the expression of neuropeptides. 相似文献