全文获取类型
收费全文 | 88105篇 |
免费 | 7893篇 |
国内免费 | 46篇 |
专业分类
96044篇 |
出版年
2023年 | 287篇 |
2022年 | 716篇 |
2021年 | 1573篇 |
2020年 | 897篇 |
2019年 | 1121篇 |
2018年 | 1466篇 |
2017年 | 1256篇 |
2016年 | 2173篇 |
2015年 | 3692篇 |
2014年 | 4119篇 |
2013年 | 4892篇 |
2012年 | 6654篇 |
2011年 | 6586篇 |
2010年 | 4233篇 |
2009年 | 3852篇 |
2008年 | 5529篇 |
2007年 | 5510篇 |
2006年 | 5323篇 |
2005年 | 5116篇 |
2004年 | 4967篇 |
2003年 | 4743篇 |
2002年 | 4476篇 |
2001年 | 944篇 |
2000年 | 699篇 |
1999年 | 1024篇 |
1998年 | 1267篇 |
1997年 | 854篇 |
1996年 | 781篇 |
1995年 | 689篇 |
1994年 | 635篇 |
1993年 | 699篇 |
1992年 | 597篇 |
1991年 | 561篇 |
1990年 | 509篇 |
1989年 | 436篇 |
1988年 | 455篇 |
1987年 | 393篇 |
1986年 | 354篇 |
1985年 | 431篇 |
1984年 | 557篇 |
1983年 | 435篇 |
1982年 | 528篇 |
1981年 | 515篇 |
1980年 | 436篇 |
1979年 | 328篇 |
1978年 | 348篇 |
1977年 | 301篇 |
1976年 | 278篇 |
1975年 | 218篇 |
1974年 | 257篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
191.
Tenshuk A. Kadima Susan E. Jensen Michael A. Pickard 《Journal of industrial microbiology & biotechnology》1995,14(1):35-40
Summary -(l--Aminoadipyl)-l-cysteinyl-d-valine (ACV)-synthetase fromStreptomyces clavuligerus was studied under conditions that enabled the reuse of the enzyme. Coupling of ACV-synthetase to DEAE-Trisacryl and aminopropyl-glass resulted in an immobilized enzyme product of little or no catalytic activity. However, an enzyme reactor was designed by physical confinement of partially-purified ACV-synthetase in an ultrafiltration cell. This system was stimulated by phosphoenolpyruvate at lower concentrations of ATP, an effect not observed with purified enzyme. Up to 30% conversion of the limiting substrate, cysteine, to ACV occurred under semi-continuous conditions. Reaction products were investigated as potential inhibitors: AMP was the most inhibitory, but only when used at concentrations in excess of those produced in reaction mixtures. Under a nitrogen atmosphere, both product and enzyme stabilities were greatly improved and the enzyme retained 45–46% of its initial activity after five uses at room temperature during a 24-h period. Extrapolations based on these data suggest that 1.3 g partially purified enzyme (0.13 U g–1) would be capable of producing 411 mg of ACV in a 1-L reaction mixture in this period. 相似文献
192.
Michael W. Palmer 《植被学杂志》1995,6(3):447-447
Abstract. This note is to apologize for an error in the computer program used to evaluate the random data used in Fuzzy Set Ordination according to Zhang & Oxley. After correction of this error no artifacts could be detected any longer. However, the basic conclusion of the earlier critical note still stands: if one selects environmental variables after analyzing the results of a multivariate gradient analysis, and then uses these variables as input into a multiple univariate gradient analysis, the results are expected to be comparable. 相似文献
193.
HH2A,an immortalized bovine mammary epithelial cell line,expresses the gene encoding mammary derived growth inhibitor (MDGI) 总被引:3,自引:0,他引:3
Summary We have established and partially characterized a spontaneously immortalized bovine mammary epithelial cell line, designated
HH2a. The cells express the gene encoding for mammary derived growth inhibitor (MDGI) when grown on released collagen gels
in the presence of lactogenic hormones. This is the first report of a cell line that expresses MDGI. Immunohistochemical studies
showed that HH2a cells contain keratin intermediate filaments and desmosomes. When plated on confluent monolayer of live fibroblasts,
HH2a cells extensively contacted with fibroblasts. When embedded in the collagen gels, they rearranged themselves to produce
three-dimensional duct-like outgrowths extending into the matrix. The HH2a cell line should be useful in investigations of
the roles of cell-cell and cell-extracellular interactions in regulation of breast epithelial cell proliferation, and of the
hormonal regulation of MDGI gene expression. 相似文献
194.
195.
Michael A. Rogers Jürgen Schweizer Thomas Krieg Hermelita Winter 《Molecular biology reports》1994,20(3):155-161
We present the nucleotide and amino acid sequence for a novel human type I hair keratin, which could be identified through its high sequence homology and strict carboxyterminal length identity as a human ortholog of the murine hair keratin mHa3. Our hHa3 sequence differs, however, from that of a previously described hHa3 hair keratin (published only as an amino acid sequence; [13]) in 24 amino acid positions, 8 of which occur in the middle of the carboxyterminal domain. PCR of genomic DNA from 25 normal human subjects using a primer pair derived from sequence segments located in the 3-region of our hHa3 clone that encode conserved amino acid sequences in both keratins, resulted in the amplification of two distinct products of 0.38 kbp and 1.0 kbp. DNA sequence analysis of the cloned PCR products allowed identification of the 0.38 kb sequence as that originating from Yuet al. [13] and the 1.0 kb sequence as that being derived from our data. The difference in fragment length was due to unique intron 6 sequences, indicating that these two keratin species are encoded by genes of their own. Moreover, extensive Southern blot analyses with DNA from 25 unrelated individuals of different races using a 3-noncoding sequence from our keratin and the intron 6 sequence of the keratin of Yuet al. [13], as hybridization probes showed that both keratin genes are present as single copy sequences occurring ubiquitously and without gross alterations in the human genome. Collectively, these data demonstrate that the human type I hair keratin described in this paper represents an isoform of the previously described hHa3 keratin. We propose that these hHa3 isoforms be named in chronological order of discovery hHa3-I and hHa3-II. 相似文献
196.
Michael V. Angrosino 《American anthropologist》1994,96(4):824-832
This essay examines how the Roman Catholic Church uses a version of the concept of culture in its evangelizing mission. The field of Christian evangelization is not commonly studied by anthropologists, but it represents a situation parallel to that of international and political development, which has similarly been informed by the anthropological concept of culture but translates that concept into action in ways that are not always consonant with anthropological theory. The great size and international reach of the Roman Catholic Church commend it to our attention insofar as it draws on a version of the anthropological perspective in its dealings with people in a multicultural society. 相似文献
197.
Michael Snyder 《Chromosoma》1994,103(6):369-380
Microtubule organizing centers play an essential cellular role in nucleating microtubule assembly and establishing the microtubule array. The microtubule organizing center of yeast, the spindle pole body (SPB), shares many functions and properties with those other organisms. In recent years considerable new information has been generated concerning components associated with the SPB, and the mechanism by which it duplicates. This article reviews our current view of the cytology and molecular composition of the SPB of the budding yeast, Saccharomyces cerevisiae, and the fission yeast, Schizosaccharomyces pombe. Genetic studies in these organisms has revealed information about how the SPB duplicates and separates, and its roles during vegetative growth, mating and meiosis. 相似文献
198.
Michael Nee 《Brittonia》1994,46(4):265-269
Talauma boliviana is described as new and illustrated. This species, first collected in 1989, is the only Magnoliaceae known from Bolivia.
It seems to be most closely related toT. sambuensis of northwestern Colombia and eastern Panama. 相似文献
199.
Arthur O. Tucker Michael J. Maciarello Pearl W. Burbage George Sturtz 《Economic botany》1994,48(3):333-336
The essential oils of fresh leaves, twigs, and/or fruits of spicebush cultivated in Oregon (field distilled) and Delaware (laboratory distilled) are examined by GC/MS. The oil of leaves of spicebush is notably high in 6-methyl-5-hepten-2-one (1.94% in Oregon to 34.83 ± 9.69% in Delaware). β-caryophyllene (15.26% in Oregon to 48.44 ± 1.35% in Delaware), and/or (E)-nerolidol (10.20% in Oregon to 12.05 ± 2.02% in Delaware). The oil of the twigs of spicebush is notably high in 1,8-cineole (45.41 ± 0.35% in Delaware), while the oil of the fruits is notably high in a-phellandrene (64.62 ± 0.66% in Delaware) and β-phellandrene (11.23+0.17% in Delaware). 相似文献
200.
The homologous operons for P1 and P7 plasmid partition are autoregulated from dissimilar operator sites 总被引:12,自引:3,他引:9
Finbarr Hayes Lyndsay Radnedge Michael A. Davis Stuart J. Austin 《Molecular microbiology》1994,11(2):249-260
The plasmid-partition regions of the P1 and P7 plasmid prophages in Escherichia coli are homologues which each encode two partition proteins, ParA and ParB. The equivalent PI and P7 proteins are closely related. In each case, the proteins are encoded by an operon that is autoregulated by the ParA and ParB proteins in concert. This regulation is species-specific, as the P1 proteins are unable to repress the P7 par operon and vice versa. The homologous ParA proteins are primarily responsible for repression and bind to regions that overlap the operon promoter in both cases. The DNA-binding domain of the P7 auto-repressor lies in the amino-terminal end of the P7 ParA protein. This region includes a helix-turn-helix motif that has a clear counterpart in the P1 ParA sequence. However, despite the common regulatory mechanism and the similarity of the proteins involved in repression, the promoter-operator sequences of these two operons are very different in sequence and organization. The operator is located downstream of the promoter in P1 and upstream of it in P7, and the two regions show little, if any, homology. How these differences may have arisen from a common ancestral form is discussed. 相似文献