全文获取类型
收费全文 | 15097篇 |
免费 | 765篇 |
国内免费 | 579篇 |
专业分类
16441篇 |
出版年
2024年 | 30篇 |
2023年 | 147篇 |
2022年 | 218篇 |
2021年 | 236篇 |
2020年 | 301篇 |
2019年 | 402篇 |
2018年 | 455篇 |
2017年 | 319篇 |
2016年 | 308篇 |
2015年 | 322篇 |
2014年 | 997篇 |
2013年 | 1324篇 |
2012年 | 724篇 |
2011年 | 793篇 |
2010年 | 580篇 |
2009年 | 763篇 |
2008年 | 739篇 |
2007年 | 773篇 |
2006年 | 678篇 |
2005年 | 668篇 |
2004年 | 554篇 |
2003年 | 523篇 |
2002年 | 457篇 |
2001年 | 374篇 |
2000年 | 301篇 |
1999年 | 322篇 |
1998年 | 310篇 |
1997年 | 244篇 |
1996年 | 242篇 |
1995年 | 295篇 |
1994年 | 280篇 |
1993年 | 236篇 |
1992年 | 202篇 |
1991年 | 143篇 |
1990年 | 133篇 |
1989年 | 88篇 |
1988年 | 93篇 |
1987年 | 68篇 |
1986年 | 78篇 |
1985年 | 91篇 |
1984年 | 110篇 |
1983年 | 92篇 |
1982年 | 107篇 |
1981年 | 74篇 |
1980年 | 60篇 |
1979年 | 47篇 |
1978年 | 36篇 |
1977年 | 28篇 |
1976年 | 23篇 |
1973年 | 19篇 |
排序方式: 共有10000条查询结果,搜索用时 9 毫秒
991.
Young Jee Jeong Byung-Cheon Jeong Hyun Kyu Song 《Biochemical and biophysical research communications》2011,(1):2261
The ubiquitin-like (Ubl) system has been shown to be ubiquitous in all three kingdoms of life following the very recent characterization of ubiquitin-like small archaeal modifier proteins (SAMP1 and 2) from Haloferax volcanii. The ubiquitin (Ub) and Ubl molecules in eukaryotes have been studied extensively and their cellular functions are well established. Biochemical and structural data pertaining to prokaryotic Ubl protein (Pup) continue to be reported. In contrast to eukaryotes and prokaryotes, no structural information on the archaeal Ubl molecule is available. Here we determined the crystal structure of SAMP1 at 1.55 Å resolution and generated a model of SAMP2. These were then compared with other Ubl molecules from eukaryotes as well as prokaryotes. The structure of SAMP1 shows a β-grasp fold of Ub, suggesting that the archaeal Ubl molecule is more closely related to eukaryotic Ub and Ubls than to its prokaryotic counterpart. The current structure identifies the location of critical elements such a single lysine residue (Lys4), C-terminal di-glycine motif, hydrophobic patches near leucine 60, and uniquely inserted α-helical segments (α1 and α3) in SAMP1. Based on the structure of SAMP1, several Ub-like features of SAMPs such as poly-SAMPylation and non-covalent interactions have been proposed, which should provide the basis for further investigations concerning the molecular function of archaeal Ubls and the large super-family of β-grasp fold proteins in the archaeal kingdom. 相似文献
992.
993.
994.
Stéphanie Beaucourt Antonio V. Bordería Lark L. Coffey Nina F. Gn?dig Marta Sanz-Ramos Yasnee Beeharry Marco Vignuzzi 《Journal of visualized experiments : JoVE》2011,(52)
RNA viruses use RNA dependent RNA polymerases to replicate their genomes. The intrinsically high error rate of these enzymes is a large contributor to the generation of extreme population diversity that facilitates virus adaptation and evolution. Increasing evidence shows that the intrinsic error rates, and the resulting mutation frequencies, of RNA viruses can be modulated by subtle amino acid changes to the viral polymerase. Although biochemical assays exist for some viral RNA polymerases that permit quantitative measure of incorporation fidelity, here we describe a simple method of measuring mutation frequencies of RNA viruses that has proven to be as accurate as biochemical approaches in identifying fidelity altering mutations. The approach uses conventional virological and sequencing techniques that can be performed in most biology laboratories. Based on our experience with a number of different viruses, we have identified the key steps that must be optimized to increase the likelihood of isolating fidelity variants and generating data of statistical significance. The isolation and characterization of fidelity altering mutations can provide new insights into polymerase structure and function1-3. Furthermore, these fidelity variants can be useful tools in characterizing mechanisms of virus adaptation and evolution4-7. 相似文献
995.
There is limited knowledge regarding how the RNA-dependent RNA polymerases of the nonsegmented negative-strand RNA viruses initiate genome replication. In a previous study of respiratory syncytial virus (RSV) RNA replication, we found evidence that the polymerase could select the 5'-ATP residue of the genome RNA independently of the 3' nucleotide of the template. To investigate if a similar mechanism is used during antigenome synthesis, a study of initiation from the RSV leader (Le) promoter was performed using an intracellular minigenome assay in which RNA replication was restricted to a single step, so that the products examined were derived only from input mutant templates. Templates in which Le nucleotides 1U, or 1U and 2G, were deleted directed efficient replication, and in both cases, the replication products were initiated at the wild-type position, at position -1 or -2 relative to the template, respectively. Sequence analysis of the RNA products showed that they contained ATP and CTP at the -1 and -2 positions, respectively, thus restoring the mini-antigenome RNA to wild-type sequence. These data indicate that the RSV polymerase is able to select the first two nucleotides of the antigenome and initiate at the correct position, even if the 3'-terminal two nucleotides of the template are missing. Substitution of positions +1 and +2 of the template reduced RNA replication and resulted in increased initiation at positions +3 and +5. Together these data suggest a model for how the RSV polymerase initiates antigenome synthesis. 相似文献
996.
997.
Black CC 《Photosynthesis research》2008,95(1):1-10
The very personal touch of Professor Martin Gibbs as a worldwide advocate for photosynthesis and plant physiology was lost
with his death in July 2006. Widely known for his engaging humorous personality and his humanitarian lifestyle, Martin Gibbs
excelled as a strong international science diplomat; like a personal science family patriarch encouraging science and plant
scientists around the world. Immediately after World War II he was a pioneer at the Brookhaven National Laboratory in the
use of 14C to elucidate carbon flow in metabolism and particularly carbon pathways in photosynthesis. His leadership on carbon metabolism
and photosynthesis extended for four decades of working in collaboration with a host of students and colleagues. In 1962,
he was selected as the Editor-in-Chief of Plant Physiology. That appointment initiated 3 decades of strong directional influences by Gibbs on plant research and photosynthesis. Plant Physiology became and remains a premier source of new knowledge about the vital and primary roles of plants in earth’s environmental
history and the energetics of our green-blue planet. His leadership and charismatic humanitarian character became the quintessence
of excellence worldwide. Martin Gibbs was in every sense the personification of a model mentor not only for scientists but
also shown in devotion to family. Here we pay tribute and honor to an exemplary humanistic mentor, Martin Gibbs. 相似文献
998.
Davide Zannoni 《Archives of microbiology》1982,133(4):267-273
Membrane fragments from the phytopathogenic bacteria Pseudomonas cichorii and Pseudomonas aptata have been examined. A branched respiratory chain is operative in P. cichorii whereas a linear electron transport system characterizes the related bacterium P. aptata. Both species contain several b type cytochromes resolved by redox titration analysis, but no a type components may be detected. In contrast, only P. cichorii is endowed with c type cytochromes and hence with cytochrome c oxidase activity. Among the b type cytochromes, two high-potential components, with Em7.0 at +250 mV and +380 mV, have been kinetically characterized and tentatively associated with cyanideresistant and cytochrome c oxidase activities, respectively. Cytochrome b-250 should correspond to the spectrally detectable cytochrome o whereas cytochrome b-380 is functionally similar to cytochrome b-410 described in Rhodopseudomonas capsulata. This conclusion seems to blur previous reported data on other obligate aerobes in which cytochrome o has been generally associated with cytochrome c oxidase and also suggests that a more accurate reconsideration of the actual physiological role of cyt. o in bacterial respiration is necessary. Furthermore the question arises whether cyt. b-410 like oxidases, i. e. high-potential b's similar to cyt. b-410 of R. capsulata, may be widely distributed among aerobes rather than restricted to facultative photosynthetic prokaryotes. 相似文献
999.
C.J.A. van Echteld B. de Kruijff A.J. Verkleij J. Leunissen-Bijvelt J. de Gier 《生物化学与生物物理学报:生物膜》1982,692(1):126-138
The hydrophobic peptide gramicidin is shown by 31P-NMR, freeze-fracture electron microscopy and small-angle X-ray diffraction, to induce a hexogonal HII-phase lipid organization when incorporated in liquid crystalline saturated and unsaturated synthetic and natural phosphatidylcholines if the length of the fatty acids exceeds a 16 carbon atoms chain. The amount of hexagonally organized lipid increases with increasing fatty acid chain length. With phosphatidylcholines possessing shorter fatty acid chains, as well as with the longer phosphatidylcholines in the gel state, a lamellar organization results. Of the various possible models to explain the induction of the hexagonal HII phase by gramicidin, one in which gramicidin dimers span adjacent cylinders of the hexagonal HII phase seems most plausible. In phosphatidylcholines with intermediary chain lengths gramicidin induces intermediary structures, such as lipidic particles and possibly cubic phases. These experiments suggest that the balance between the length of the hydrophobic domain of a peptide and the membrane thickness is of critical importance for the structure of the membrane. In relation to this observation, the possible involvement of non-bilayer lipid structures in insertion and anchoring of membrane proteins is discussed. 相似文献
1000.