全文获取类型
收费全文 | 2387篇 |
免费 | 209篇 |
国内免费 | 1篇 |
出版年
2023年 | 11篇 |
2022年 | 22篇 |
2021年 | 70篇 |
2020年 | 28篇 |
2019年 | 56篇 |
2018年 | 51篇 |
2017年 | 45篇 |
2016年 | 70篇 |
2015年 | 115篇 |
2014年 | 124篇 |
2013年 | 192篇 |
2012年 | 215篇 |
2011年 | 211篇 |
2010年 | 145篇 |
2009年 | 99篇 |
2008年 | 152篇 |
2007年 | 140篇 |
2006年 | 133篇 |
2005年 | 127篇 |
2004年 | 116篇 |
2003年 | 105篇 |
2002年 | 97篇 |
2001年 | 14篇 |
2000年 | 9篇 |
1999年 | 18篇 |
1998年 | 13篇 |
1997年 | 10篇 |
1996年 | 13篇 |
1995年 | 16篇 |
1994年 | 9篇 |
1993年 | 20篇 |
1990年 | 4篇 |
1988年 | 3篇 |
1987年 | 7篇 |
1986年 | 7篇 |
1985年 | 11篇 |
1984年 | 6篇 |
1983年 | 12篇 |
1982年 | 11篇 |
1981年 | 4篇 |
1980年 | 9篇 |
1979年 | 6篇 |
1978年 | 6篇 |
1977年 | 7篇 |
1976年 | 9篇 |
1975年 | 5篇 |
1974年 | 6篇 |
1971年 | 5篇 |
1966年 | 3篇 |
1965年 | 5篇 |
排序方式: 共有2597条查询结果,搜索用时 15 毫秒
991.
Chen C Ridzon DA Broomer AJ Zhou Z Lee DH Nguyen JT Barbisin M Xu NL Mahuvakar VR Andersen MR Lao KQ Livak KJ Guegler KJ 《Nucleic acids research》2005,33(20):e179
A novel microRNA (miRNA) quantification method has been developed using stem-loop RT followed by TaqMan PCR analysis. Stem-loop RT primers are better than conventional ones in terms of RT efficiency and specificity. TaqMan miRNA assays are specific for mature miRNAs and discriminate among related miRNAs that differ by as little as one nucleotide. Furthermore, they are not affected by genomic DNA contamination. Precise quantification is achieved routinely with as little as 25 pg of total RNA for most miRNAs. In fact, the high sensitivity, specificity and precision of this method allows for direct analysis of a single cell without nucleic acid purification. Like standard TaqMan gene expression assays, TaqMan miRNA assays exhibit a dynamic range of seven orders of magnitude. Quantification of five miRNAs in seven mouse tissues showed variation from less than 10 to more than 30,000 copies per cell. This method enables fast, accurate and sensitive miRNA expression profiling and can identify and monitor potential biomarkers specific to tissues or diseases. Stem-loop RT-PCR can be used for the quantification of other small RNA molecules such as short interfering RNAs (siRNAs). Furthermore, the concept of stem-loop RT primer design could be applied in small RNA cloning and multiplex assays for better specificity and efficiency. 相似文献
992.
Methane production was studied in an Italian rice field over two consecutive years (1998, 1999) by measuring the rates of total and acetate-dependent methanogenesis in soil and root samples. Population dynamics of methanogens were followed by terminal restriction fragment length polymorphism and real-time PCR targeting archaeal SSU rRNA genes. Rates of total and acetate-dependent methanogenesis in soil increased during the season, reached a maximum at about 70-80 days after flooding and then decreased again. In contrast, the size of the archaeal community remained relatively constant. Therefore, the seasonal changes in the methanogenic processes were probably not caused by changes in the size of the methanogenic community but in its activity. During the 1998/1999 winter period, a slight decrease in archaeal cell numbers was found. In both years, the dominant groups were methanogens affiliated with Rice cluster I, Methanosaetaceae, Methanosarcinaceae and Methanobacteriaceae. Correspondence analysis showed, however, that the archaeal community structure was different in 1998 and 1999. Methanogens with potential acetoclastic activity made up a larger fraction of the total archaeal community in 1999 (32-53%) than in 1998 (20-32%). Furthermore, the frequency of Methanosaetaceae relative to Methanosarcinaceae was significantly higher in 1999 than in 1998. This difference could be explained by the much lower soil acetate concentrations in 1999, to which Methanosaetaceae are physiologically better adapted than Methanosarcinaceae. Over the season, however, the composition of the archaeal community remained relatively constant and thus did not reflect the observed seasonal change in CH(4) production activity. The analysis of rice root samples in 1999 showed that the archaeal community structure on the roots was similar to that in soil but with acetoclastic methanogens being relatively less common. This observation is in agreement with domination of CH(4) production by H(2)/CO(2)-dependent methanogenesis on roots. Our study provided a link between size, structure and function of the methanogenic community in an Italian rice field. 相似文献
993.
Erkel C Kemnitz D Kube M Ricke P Chin KJ Dedysh S Reinhardt R Conrad R Liesack W 《FEMS microbiology ecology》2005,53(2):187-204
We report first insights into a representative genome of rice cluster I (RC-I), a major group of as-yet uncultured methanogens. The starting point of our study was the methanogenic consortium MRE50 that had been stably maintained for 3 years by consecutive transfers to fresh medium and anaerobic incubation at 50 degrees C. Process-oriented measurements provided evidence for hydrogenotrophic CO(2)-reducing methanogenesis. Assessment of the diversity of consortium MRE50 suggested members of the families Thermoanaerobacteriaceae and Clostridiaceae to constitute the major bacterial component, while the archaeal population was represented entirely by RC-I. The RC-I population amounted to more than 50% of total cells, as concluded from fluorescence in situ hybridization using specific probes for either Bacteria or Archaea. The high enrichment status of RC-I prompted construction of a large insert fosmid library from consortium MRE50. Comparative sequence analysis of internal transcribed spacer (ITS) regions revealed that three different RC-I rrn operon variants were present in the fosmid library. Three, approximately 40-kb genomic fragments, each representative for one of the three different rrn operon variants, were recovered and sequenced. Computational analysis of the sequence data resulted in two major findings: (i) consortium MRE50 most likely harbours only a single RC-I genotype, which is characterized by multiple rrn operon copies; (ii) seven genes were identified to possess a strong phylogenetic signal (eIF2a, dnaG, priA, pcrA, gatD, gatE, and a gene encoding a putative RNA-binding protein). Trees exemplarily computed for the deduced amino acid sequences of eIF2a, dnaG, and priA corroborated a specific phylogenetic association of RC-I with the Methanosarcinales. 相似文献
994.
Wonsettler DM Chang WW Wenger SL 《Journal of the Association of Genetic Technologists》2005,31(2):55-58
A 59-year-old hypertensive white male was diagnosed with acute myelogenous leukemia (AML), M4. A bone marrow aspirate showed a karyotype of 46,XY,del(20)(q11.2q13.3)[12]/ 47,XY,del(20)(q11.2q13.3)x2[8]. The majority of cases with 20q deletion are associated with myeloid disorders; however, an extra copy of the 20q deletion has rarely been reported. The patient expired seven days after admission to the hospital. At autopsy hepatosplenomegaly was present. Many foamy macrophages with bubbling cytoplasm in the spleen, liver, bone marrow and lymph nodes were suggestive of Niemann-Pick disease, type E. AML has not previously been reported with Niemann-Pick disease. 相似文献
995.
Sherwood RJ Hlusko LJ Duren DL Emch VC Walker A 《Human biology; an international record of research》2005,77(6):735-759
The hominoid mandibular symphysis has received a great deal of attention from anatomists, human biologists, and paleontologists. Much of this research has focused on functional interpretations of symphyseal shape variation. Here, we examine the two-dimensional cross-sectional shape of the adult mandibular symphysis for 45 humans, 42 chimpanzees, 37 gorillas, and 51 orangutans using eigenshape analysis, an outline-based morphometric approach. Our results demonstrate that a large proportion of the variation described by the first eigenshape correlates with proposed functional adaptations to counteract stresses at the mandibular midline during mastication. Subsequent eigenshapes describe subtle aspects of shape variation in the mandibular symphysis. The morphology associated with these eigenshapes does not conform with functional predictions, nor does it show a relationship with sexual dimorphism. However, eigenshapes provide for considerable taxonomic discrimination between the four taxa studied and may consequently prove useful in the analysis of fossil material. Comparison with elliptical Fourier analysis of the mandibular symphysis identifies eigenshape analysis as providing superior taxonomic discrimination. The results presented here demonstrate that the cross-sectional shape of the mandibular symphysis results from a complex interplay of functional and nonfunctional influences and for the first time identifies and quantifies the specific aspects of variation attributable to these factors. 相似文献
996.
The formation of protein disulfide bonds in the Escherichia coli periplasm by the enzyme DsbA is an inaccurate process. Many eukaryotic proteins with nonconsecutive disulfide bonds expressed in E. coli require an additional protein for proper folding, the disulfide bond isomerase DsbC. Here we report studies on a native E. coli periplasmic acid phosphatase, phytase (AppA), which contains three consecutive and one nonconsecutive disulfide bonds. We show that AppA requires DsbC for its folding. However, the activity of an AppA mutant lacking its nonconsecutive disulfide bond is DsbC-independent. An AppA homolog, Agp, a periplasmic acid phosphatase with similar structure, lacks the nonconsecutive disulfide bond but has the three consecutive disulfide bonds found in AppA. The consecutively disulfide-bonded Agp is not dependent on DsbC but is rendered dependent by engineering into it the conserved nonconsecutive disulfide bond of AppA. Taken together, these results provide support for the proposal that proteins with nonconsecutive disulfide bonds require DsbC for full activity and that disulfide bonds are formed predominantly during translocation across the cytoplasmic membrane. 相似文献
997.
Ubiquitination of p27Kip1 requires physical interaction with cyclin E and probable phosphate recognition by SKP2 总被引:6,自引:0,他引:6
p27Kip1 is an essential cell cycle inhibitor of Cyclin-dependent kinases. Ubiquitin-mediated proteolysis of p27Kip1 is an important mechanism for activation of Cyclin E-Cdk2 and facilitates G1/S transition. Ubiquitination of p27 is primarily catalyzed by a multisubunit E3 ubiquitin ligase, SCF(Skp2), and requires an adapter protein Cks1. In addition, phosphorylation of p27 at Thr187 by Cyclin E and Cdk2 is also essential for triggering substrate ubiquitination. Here we investigate the molecular mechanism of p27 ubiquitination. We show that Cyclin E-Cdk2 is essential for targeting the p27 substrate to SCF(Skp2). Direct physical contact between Cyclin E but not Cdk2 and p27 is required for p27 recruitment to SCF(Skp2). In a search for positively charged amino acid residues that may be involved in recognition of the Thr187 phosphate group, we found that Arg306 of Skp2 is required for association and ubiquitination of phosphorylated p27 but dispensable for ubiquitination of unphosphorylated p21. Thus, our data unravel the molecular organization of the ubiquitination complex that catalyzes p27 ubiquitination and provide unique insights into the specificity of substrate recognition by SCF(Skp2). 相似文献
998.
Travassos LH Carneiro LA Girardin SE Boneca IG Lemos R Bozza MT Domingues RC Coyle AJ Bertin J Philpott DJ Plotkowski MC 《The Journal of biological chemistry》2005,280(44):36714-36718
The mammalian innate immune system recognizes pathogen-associated molecular patterns through pathogen recognition receptors. Nod1 has been described recently as a cytosolic receptor that detects specifically diaminopimelate-containing muropeptides from Gram-negative bacteria peptidoglycan. In the present study we investigated the potential role of Nod1 in the innate immune response against the opportunistic pathogen Pseudomonas aeruginosa. We demonstrate that Nod1 detects the P. aeruginosa peptidoglycan leading to NF-kappaB activation and that this activity is diminished in epithelial cells expressing a dominant-negative Nod1 construct or in mouse embryonic fibroblasts from Nod1 knock-out mice infected with P. aeruginosa. Finally, we demonstrate that the cytokine secretion kinetics and bacterial killing are altered in Nod1-deficient cells infected with P. aeruginosa in the early stages of infection. 相似文献
999.
Girardin SE Jéhanno M Mengin-Lecreulx D Sansonetti PJ Alzari PM Philpott DJ 《The Journal of biological chemistry》2005,280(46):38648-38656
Nod1 is an intracellular pattern recognition molecule activated following bacterial infection, which senses a specific muropeptide (l-Ala-d-Glu-meso-DAP (diaminopimelic acid); "Tri(DAP)") from peptidoglycan. Here we investigated the molecular basis of Tri(DAP) sensing by human (h) Nod1. Our results identified the domain responsible for Tri(DAP) detection in the center of the concave surface of hNod1 leucine-rich repeat domain. Amino acid residues critical for sensing define a contiguous surface patch that is largely conserved in Nod1 proteins from different species. Accordingly, the distinct specificities of human versus murine Nod1 toward muropeptide detection were also found to lie in this central cleft. Several splicing variants of Nod1 lacking repeats 7-9 have been characterized recently, the relative balance of which is thought to correlate with the onset of asthma or inflammatory bowel disease. We demonstrated that these isoforms failed to transduce NF-kappaB activation upon muropeptide stimulation. This study provided insights into the molecular mechanisms responsible for the detection of bacterial peptidoglycan by Nod1 and suggested that defects in Nod1-dependent peptidoglycan sensing may contribute to elicit certain inflammatory disorders. 相似文献
1000.
The aim of the study was to test the hypothesis that expression of retinoid receptors (RARalpha, RARbeta, RARgamma), rexinoid receptors (RXRalpha, RXRbeta), thyroid hormone receptors (TRalpha, TRbeta), estrogen receptors (ERalpha, ERbeta), nuclear receptor coregulators (N-CoR, SRC-1, SMRT), and in addition type I iodothyronine 5'-deiodinase (5'-DI), EGFR and erb-B2/neu would be different in mammary postlactating tissue in comparison with that of nonlactating mammary gland. Using RT-PCR, we have shown that expression of RARalpha, RXRalpha,TRalpha, ERalpha,ERbeta,N-CoR, SRC-1, SMRT and EGFR in rat was significantly increased in postlactating mammary gland when compared to that of nonlactating mammary tissue. Postlactating mammary glands were found to express all RAR and RXR subtypes studied when compared to nonlactating mammary tissues that express exclusively RARalpha and RXRalpha subtypes. Enhanced expression of a number of nuclear hormone receptors, their coregulators in mammary tissue of postlactating rats in comparison with nonlactating animals identify a potential role for retinoid, thyroid and estrogen signalling pathways also after lactation period. 相似文献