全文获取类型
收费全文 | 15374篇 |
免费 | 820篇 |
国内免费 | 763篇 |
专业分类
16957篇 |
出版年
2024年 | 22篇 |
2023年 | 268篇 |
2022年 | 430篇 |
2021年 | 548篇 |
2020年 | 486篇 |
2019年 | 707篇 |
2018年 | 542篇 |
2017年 | 342篇 |
2016年 | 369篇 |
2015年 | 507篇 |
2014年 | 1103篇 |
2013年 | 1184篇 |
2012年 | 865篇 |
2011年 | 985篇 |
2010年 | 808篇 |
2009年 | 658篇 |
2008年 | 805篇 |
2007年 | 749篇 |
2006年 | 581篇 |
2005年 | 560篇 |
2004年 | 494篇 |
2003年 | 405篇 |
2002年 | 371篇 |
2001年 | 213篇 |
2000年 | 191篇 |
1999年 | 217篇 |
1998年 | 196篇 |
1997年 | 184篇 |
1996年 | 194篇 |
1995年 | 237篇 |
1994年 | 216篇 |
1993年 | 147篇 |
1992年 | 171篇 |
1991年 | 116篇 |
1990年 | 108篇 |
1989年 | 85篇 |
1988年 | 59篇 |
1987年 | 56篇 |
1986年 | 58篇 |
1985年 | 80篇 |
1984年 | 102篇 |
1983年 | 91篇 |
1982年 | 96篇 |
1981年 | 53篇 |
1980年 | 65篇 |
1979年 | 56篇 |
1978年 | 38篇 |
1977年 | 36篇 |
1976年 | 24篇 |
1973年 | 21篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
21.
22.
Summary A complete ribosomal DNA (rDNA) repeat unit has been cloned from the genome of Pisum sativum (garden pea) and used to construct a map containing a total of 58 cleavage sites for 23 different restriction enzymes. Regions encoding 18s and 25s ribosomal RNA (rRNA) were identified by R-loop analysis. A 180 bp sequence element is repeated eight times in the intergenic nontranscribed spacer (NTS) region, as defined by eight evenly spaced RsaI cleavage sites. Sequence heterogeneity among these elements (subrepeats) is indicated by the presence of an NcoI site within the five RsaI subrepeats distal to the 25s rRNA gene but not in the three subrepeats proximal to this gene, and also by the presence of an additional RsaI cleavage site in one subrepeat.The approximately 4000 copies of the rDNA repeat in the pea nuclear genome show considerable heterogeneity with respect to the length of the NTS region, and differences are also frequently observed between different genotypes. In both cases the length variation appears to be due primarily to differences in the number of subrepeat elements.Comparison of rDNA restriction maps for two pea genotypes separated for hundreds or perhaps thousands of generations reveals that they contain many rDNA identical repeat units. This data is consistent with the view that new rDNA variants are fixed only infrequently in the evolution of a species.Differences also exist between the rDNA repeats of a single genotype with respect to the degree of base modification at certain restriction sites. A large number of sites known to exist in the pea rDNA clone are not cleaved at all in genomic rDNA, or are cleaved in only some copies of the rDNA repeat. We believe these examples of incomplete cleavage results mostly from methylation, although it is difficult to rule out the possibility of sequence variation in all cases. Most putative modifications are best interpreted in terms of cytosine methylation in CG and CXG sequences, but at least one example is more consistent with adenine methylation.We also have constructed a more detailed restriction map of the wheat rDNA clone pTA71 and present a comparison of this map to our map of pea, pumpkin, and wheat in order to assess the amount of useful evolutionary information that can be obtained by comparison of such maps. 相似文献
23.
M. V. Kashlev A. I. Gragerov V. G. Nikiforov 《Molecular & general genetics : MGG》1989,216(2-3):469-474
Summary
Escherichia coli cells, carrying a rifampicin sensitive RNA polymerase -subunit gene in the chromosome and a rifampicin resistant -subunit gene placed under the control of a strong promoter in a multicopy plasmid, are unable to grow in the presence of rifampicin, despite the accumulation of large quantities of the resistant subunit. A major portion of the overproduced subunit is found in an insoluble form. Conditions known to induce the heat shock proteins (hsps), e.g. elevated temperature or the presence of ethanol in the growth medium, increase the amount of the plasmid-borne -subunit which apparently assembles into active RNA polymerase and makes the plasmid bearing cells rifampicin resistant. Alternatively, plasmid-borne subunits assemble into RNA polymerase with low efficiency in rpoH mutant cells known to have reduced level of hsps. We suggest that the plasmid-borne subunit is poorly assembled into RNA polymerase and that hsps promote the assembly by interfering with -subunit aggregation. 相似文献
24.
RNA polymerase from the archaebacterium Sulfolobus acidocaldarius was chemically modified with AMP o-formylphenyl ester followed by reduction with borohydride. The modified protein catalyzes the labeling of its own largest subunit when incubated with [-33P]UTP in the presence of poly[d(A-T)]. On cleaving of the labeled protein using cyanogen bromide, hydroxylamine or amino acid-specific endoproteinases for a very brief period, the pattern and size of the radioactive fragments formed are best explained by attachment of the label between Gly843 and Met895 of the largest subunit. In this region there exists a highly conserved sequence which is also found in other archaebacterial, eukaryotic and prokaryotic RNA polymerases. This suggests that the binding site for the initiating substrate of RNA polymerases has been conserved during evolution. 相似文献
25.
Marie-Claude Pepin Serge Beaulieu Nicholas Barden 《Cellular and molecular neurobiology》1990,10(2):227-235
1. Differential regulation, by dexamethasone, of glucocorticoid receptor gene expression was studied in three different neuronal cultures derived from hypothalamus amygdala, and cerebral cortex. 2. Cellular glucocorticoid receptor (GR) mRNA concentration was measured by hybridization using a 32P-labeled RNA probe complementary to a 2.2-kb fragment of the glucocorticoid receptor mRNA. Changes in the amount of GR mRNA were evaluated in relation to the content of beta-actin mRNA. 3. In cells derived from either hypothalamus or cerebral cortex, we observed a complex pattern of GR mRNA concentrations which were characterized by cyclic variations of GR mRNA content during continuous treatment with dexamethasone for up to 72 hr. 4. In contrast to cells derived from the hypothalamus where a persistent 30-40% reduction in GR mRNA levels was seen for up to a least 72 hr, we observed, in cells derived from the cerebral cortex, a sustained increased (1.4-fold) of the GR mRNA at this same time interval. 相似文献
26.
Vicente Corbatón Patricio Fernández-Silva Manuel J. López-Pérez Dr. Julio Montoya 《Neurochemical research》1990,15(7):711-717
We have isolated RNA from sheep brain synaptosomes and mitochondria separated by an aqueous two-phase system composed of dextran and poly(ethylene glycol). RNA was fractionated through oligo(dT)-cellulose columns and analyzed by electrophoresis through agarose slab gels containing methylmercuric hydroxide and stained with ethidium bromide. The electrophoretic patterns of the poly(A)-containing RNA fraction from synaptosomes and mitochondria are very similar although some high molecular weight RNA species, clearly visible in the synaptosomal fraction, are scarcely detected in the mitochondrial preparations. The electrophoretic analysis of a cleaner RNA preparation from digitonin-treated free mitochondria (mitoplasts) showed that all the poly (A)-RNA species of the synaptosomal preparation are also present in mitoplast. These results strongly suggest that all the discrete poly(A)-RNA species identified in brain synaptosomes are of mitochondrial origin. 相似文献
27.
Tatsuo Nakahara Makoto Hirano Takashi Matsumoto Toshihide Kuroki Yoshinori Tatebayashi Tetsuyuki Tsutsumi Kouji Nishiyama Hiroaki Ooboshi Kaoru Nakamura Hiroshi Yao Akio Shiraishi Michinori Waki Hideyuki Uchimura 《Neurochemical research》1990,15(6):609-611
DNA and RNA contents in 20 brain regions or nuclei of the rat were determined by a highly sensitive method using high-performance liquid chromatography with electrochemical detection. The high DNA and RNA contents were found in the hypothalamic nuclei, especially the median eminence-arcuate nucleus. These results may be available for the preparation of nucleic acids as the regional control. 相似文献
28.
29.
30.