全文获取类型
收费全文 | 13749篇 |
免费 | 902篇 |
国内免费 | 3篇 |
专业分类
14654篇 |
出版年
2023年 | 80篇 |
2022年 | 162篇 |
2021年 | 273篇 |
2020年 | 204篇 |
2019年 | 273篇 |
2018年 | 334篇 |
2017年 | 304篇 |
2016年 | 476篇 |
2015年 | 701篇 |
2014年 | 754篇 |
2013年 | 962篇 |
2012年 | 1157篇 |
2011年 | 1073篇 |
2010年 | 682篇 |
2009年 | 639篇 |
2008年 | 831篇 |
2007年 | 796篇 |
2006年 | 789篇 |
2005年 | 675篇 |
2004年 | 613篇 |
2003年 | 608篇 |
2002年 | 539篇 |
2001年 | 106篇 |
2000年 | 87篇 |
1999年 | 105篇 |
1998年 | 123篇 |
1997年 | 124篇 |
1996年 | 113篇 |
1995年 | 91篇 |
1994年 | 90篇 |
1993年 | 85篇 |
1992年 | 69篇 |
1991年 | 65篇 |
1990年 | 50篇 |
1989年 | 50篇 |
1988年 | 38篇 |
1987年 | 30篇 |
1986年 | 29篇 |
1985年 | 47篇 |
1984年 | 39篇 |
1983年 | 41篇 |
1982年 | 43篇 |
1981年 | 45篇 |
1980年 | 32篇 |
1979年 | 30篇 |
1978年 | 28篇 |
1977年 | 21篇 |
1975年 | 22篇 |
1974年 | 19篇 |
1973年 | 24篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
81.
In the former part of the review the principal available data aboutHox genes, their molecular organisation and their expression in vertebrate embryos, with particular emphasis for mammals, are briefly summarized.In the latter part we analysed the expression of four mouse homeobox genes related to twoDrosophila genes expressed in the developing head of the fly: Emx1 and Emx2, related toems, and Otx1 and Otx2, related tootd. 相似文献
82.
Antonio Cuadrado Wolfgang Issing Timothy P. Fleming Christopher J. Molloy 《Journal of cellular physiology》1994,159(3):434-440
Protein kinase C (PKC) represents a family of structurally related Ser/Tre kinases which are involved in mitogenic signalling and may contribute to human neoplasia. To address this issue, the messenger RNA and protein levels of PKC isoenzymes α and β were analyzed in several human sarcoma- and carcinoma-derived cell lines. Carcinomas contained low or undetectable levels of either PKC-α or PKC-β. Sarcomas exhibited similar or increased PKC expression compared to human diploid fibroblasts. Moreover, sarcoma cell lines expressing one PKC isoform did not contain detectable levels of the other. When PKC was depleted from the tumor cells, we observed that the PKC overexpressing sarcomas had reduced their malignant properties as determined by their ability to grow in semisolid medium. In addition, epidermal growth factor-stimulated and erbB2-transformed fibroblasts exhibited enhanced cell growth in the absence of PKC. We propose a model for the effect of PKC as a negative regulator of proliferation in epithelial cells and a growth promoter in fibroblasts. © 1994 wiley-Liss, Inc. 相似文献
83.
We have studied by electron microscopy the size and morphology of the complexes obtained with different DNAs (between 500 and 5243 base pairs long) and four different proteins: sea urchin histone H1; sea cucumber histone ?0, chicken erythrocyte histone H5, and clupeine. Surprisingly, the type of protein used has only a marginal influence on the complexes formed. The molecular weight and topology of DNA do not show any influence. The size of the complexes depends strongly on the ratio of positive to negative charges and also on the ionic conditions. Our studies have been mainly carried out at a ratio of 0.4. Under these conditions the average thickness of rods and toroids observed varies between 165 Å at 1.5 mM salt to 290 Å at 100 mM salt, with minor variations around these values depending on the type of DNA and protein used. We conclude that the formation of DNA condensates is mainly determined by a balance of electrostatic and intermolecular forces, the influence of specific interactions is only marginal. This conclusion seems to apply not only to the complexes described here, but also to chromatin fibers and to DNA condensed by low molecular weight counterions and other compounds (polyamines, inorganic ions, ethanol, etc.). © 1994 John Wiley & Sons, Inc. 相似文献
84.
Gabriel Gutiérrez Josep Casadesús Jose L. Oliver Antonio Marine 《Journal of molecular evolution》1994,39(4):340-346
E. coli genes that contain a high frequency of the tetranucleotide CTAG are also rich in the tetramers CTTG, CCTA, CCAA, TTGG, TAGG, and CAAG (group-I tetramers). Conversely, E. coli genes lacking CTAG are rich in the tetranucleotides CCTG, CCAG, CTGG, and CAGG (group-II tetramers). These two gene samples differ also in codon usage, amino acid composition, frequency of Dcm sites, and contrast vocabularies. Group-I tetramers have in common that they are depleted by very-short-patch repair (VSP), while group-II tetramers are favored by VSP activity. The VSP system repairs G:T mismatches to G:C, thereby increasing the overall G+C content of the genome; for this reason the CTAG-rich sample has a lower G+C content than the CTAG-poor sample. This compositional heterogeneity can be tentatively explained by a low level of VSP activity on the CTAG-rich sample. A negative correlation is found between the frequency of group-I tetramers and the level of gene expression, as measured by the Codon Adaptation Index (CAI). A possible link between the rate of VSP activity and the level of gene expression is considered.Correspondence to: A. Marine 相似文献
85.
86.
Antonio Balas Felix Garcia-Sanchez Fernando Gomez-Reino Jose L. Vicario 《Immunogenetics》1994,39(6):452-452
Correspondence to: J. L. Vicario. 相似文献
87.
Molecular characterization of a fourth isoform of the catalytic subunit of protein phosphatase 2A from Arabidopsis thaliana 总被引:1,自引:0,他引:1
Antonio Casamayor Encarna Pérez-Callejón Gemma Pujol Joaquín Ariño Albert Ferrer 《Plant molecular biology》1994,26(1):523-528
We have recently reported the existence of multiple isoforms of the catalytic subunit of protein phosphatase 2A (PP2A) in Arabidopsis thaliana and the molecular cloning of cDNAs encoding three of these proteins (PP2A-1, PP2A-2, PP2A-3). The reported cDNA encoding PP2A-3 was truncated at the 5 terminus, lacking a short fragment of the N-terminal coding sequence. We have now isolated a near full-length cDNA encoding the entire PP2A-3 protein (313 residues). The clone includes 188 nucleotides of 5-untranslated region, where a 44 bp long poly(GA) track is found. We also describe the cloning of a cDNA encoding a fourth isoform of PP2A (PP2A-4). The polypeptide contains 313 residues being 98% identical to PP2A-3 and only 80% identical to both PP2A-1 and PP2A-2. The mRNA for PP2A-4 is 1.4 kb in length and, although predominantly expressed in roots, it is also found in other organs. It is concluded that in A. thaliana the isoforms of PP2A can be grouped in two extremely conserved subfamilies. 相似文献
88.
José M. Lora Jésus de la Cruz Tahía Benítez Antonio Llobell José A. Pintor-Toro 《Molecular & general genetics : MGG》1994,242(4):461-466
A cDNA clone encoding a putative cell wall protein (Qid3) was isolated from a library prepared from chitin-induced mRNA in cultures of the mycoparasitic fungus Trichoderma harzianum. The predicted 14 kDa protein shows a potential signal peptide, several hydrophobic domains and certain motifs that are structurally similar to proline-rich and glycine-rich plant cell wall proteins. Expression of the qid3 gene is derepressed in the absence of glucose. When introduced in yeast, qid3 expression causes cell division arrest into cytokinesis and cell separation, probably due to its cell wall localization. 相似文献
89.
N. Durán R. V. Antonio M. Haun R. A. Pilli 《World journal of microbiology & biotechnology》1994,10(6):686-690
Radio-isotope studies indicated not only that l-tryptophan can serve as carbon source for synthesis of the trypanocide, violacein by Chromobacterium violaceum (BB-78 strain) but also that isatin and indole 3-acetic acid are both important metabolic intermediates. Using 3-indolyl [2-14C] and [1-14C] acetic acid, it was found that the carboxylic carbon was not eliminated and that indole-3-acetic acid was incorporated intact into the pigment structure. N-Ethyl(5-hydroxy-indol-3-yl)-2-indolylethylamide is also an important metabolic intermediate in the violacein biosynthesis. This is the first report of a metabolic scheme for violacein synthesis which includes an intermediate other than l-tryptophan. 相似文献
90.
Gianfranco Tucci Marco Cosimo Simeone Carlo Gregori Fabio Maggini 《Plant Systematics and Evolution》1994,190(3-4):187-193
Intergenic spacers of the rRNA genes of three species of theCynareae tribe:Cynara cardunculus subsp.scolymus (artichoke),Onopordum acanthium, andO. illyricum were cloned in the plasmid pGEM-7zf(+). Detailed restriction mapping and partial sequencing of the IGSs were carried out. The structural analysis showed a clear diversity betweenCynara andOnopordum, while a high degree of homology was found between the twoOnopordum spp. In all three species a fragment of about 450 bp from the 5 end of 18S to the Acc I site with a high sequence homology was present. Nucleotide sequences upstream from the above mentioned Acc I site show a gradual decrease of homology betweenCynara andOnopordum. 相似文献