全文获取类型
收费全文 | 26543篇 |
免费 | 2250篇 |
国内免费 | 18篇 |
出版年
2023年 | 79篇 |
2021年 | 393篇 |
2020年 | 263篇 |
2019年 | 332篇 |
2018年 | 411篇 |
2017年 | 346篇 |
2016年 | 596篇 |
2015年 | 1031篇 |
2014年 | 1208篇 |
2013年 | 1449篇 |
2012年 | 1930篇 |
2011年 | 1952篇 |
2010年 | 1260篇 |
2009年 | 1090篇 |
2008年 | 1702篇 |
2007年 | 1754篇 |
2006年 | 1605篇 |
2005年 | 1565篇 |
2004年 | 1580篇 |
2003年 | 1519篇 |
2002年 | 1403篇 |
2001年 | 278篇 |
2000年 | 175篇 |
1999年 | 292篇 |
1998年 | 339篇 |
1997年 | 232篇 |
1996年 | 194篇 |
1995年 | 221篇 |
1994年 | 235篇 |
1993年 | 228篇 |
1992年 | 183篇 |
1991年 | 176篇 |
1990年 | 150篇 |
1989年 | 133篇 |
1988年 | 159篇 |
1987年 | 125篇 |
1986年 | 129篇 |
1985年 | 167篇 |
1984年 | 167篇 |
1983年 | 169篇 |
1982年 | 191篇 |
1981年 | 184篇 |
1980年 | 158篇 |
1979年 | 107篇 |
1978年 | 114篇 |
1977年 | 91篇 |
1976年 | 91篇 |
1975年 | 72篇 |
1974年 | 78篇 |
1973年 | 51篇 |
排序方式: 共有10000条查询结果,搜索用时 187 毫秒
41.
Kimberly A. Aeling Kirsty A. Salmon José M. Laplaza Ling Li Jennifer R. Headman Alex H. Hutagalung Stephen Picataggio 《Journal of industrial microbiology & biotechnology》2012,39(11):1597-1604
We have integrated and coordinately expressed in Saccharomyces cerevisiae a xylose isomerase and cellobiose phosphorylase from Ruminococcus flavefaciens that enables fermentation of glucose, xylose, and cellobiose under completely anaerobic conditions. The native xylose isomerase was active in cell-free extracts from yeast transformants containing a single integrated copy of the gene. We improved the activity of the enzyme and its affinity for xylose by modifications to the 5′-end of the gene, site-directed mutagenesis, and codon optimization. The improved enzyme, designated RfCO*, demonstrated a 4.8-fold increase in activity compared to the native xylose isomerase, with a Km for xylose of 66.7?mM and a specific activity of 1.41?μmol/min/mg. In comparison, the native xylose isomerase was found to have a Km for xylose of 117.1?mM and a specific activity of 0.29?μmol/min/mg. The coordinate over-expression of RfCO* along with cellobiose phosphorylase, cellobiose transporters, the endogenous genes GAL2 and XKS1, and disruption of the native PHO13 and GRE3 genes allowed the fermentation of glucose, xylose, and cellobiose under completely anaerobic conditions. Interestingly, this strain was unable to utilize xylose or cellobiose as a sole carbon source for growth under anaerobic conditions, thus minimizing yield loss to biomass formation and maximizing ethanol yield during their fermentation. 相似文献
42.
Derren J. Heyes Basile Khara Michiyo Sakuma Samantha J. O. Hardman Ronan O'Cualain Stephen E. J. Rigby Nigel S. Scrutton 《PloS one》2012,7(12)
Phytochromes are dimeric photoreceptors that regulate a range of responses in plants and microorganisms through interconversion of red light-absorbing (Pr) and far-red light-absorbing (Pfr) states. Photoconversion between these states is initiated by light-driven isomerization of a bilin cofactor, which triggers protein structural change. The extent of this change, and how light-driven structural changes in the N-terminal photosensory region are transmitted to the C-terminal regulatory domain to initiate the signalling cascade, is unknown. We have used pulsed electron-electron double resonance (PELDOR) spectroscopy to identify multiple structural transitions in a phytochrome from Synechocystis sp. PCC6803 (Cph1) by measuring distances between nitroxide labels introduced into the protein. We show that monomers in the Cph1 dimer are aligned in a parallel ‘head-to-head’ arrangement and that photoconversion between the Pr and Pfr forms involves conformational change in both the N- and C-terminal domains of the protein. Cryo-trapping and kinetic measurements were used to probe the extent and temporal properties of protein motions for individual steps during photoconversion of Cph1. Formation of the primary photoproduct Lumi-R is not affected by changes in solvent viscosity and dielectric constant. Lumi-R formation occurs at cryogenic temperatures, consistent with their being no major structural reorganization of Cph1 during primary photoproduct formation. All remaining steps in the formation of the Pfr state are affected by solvent viscosity and dielectric constant and occur only at elevated temperatures, implying involvement of a series of long-range solvent-coupled conformational changes in Cph1. We show that signalling is achieved through ultrafast photoisomerization where localized structural change in the GAF domain is transmitted and amplified to cause larger-scale and slower conformational change in the PHY and histidine kinase domains. This hierarchy of timescales and extent of structural change orientates the histidine kinase domain to elicit the desired light-activated biological response. 相似文献
43.
Stephen D. Houston 《American anthropologist》1994,96(3):716-718
Mesoamerican Writing Systems: Propaganda, Myth, and History in Four Ancient Civilizations . Joyce Marcus 相似文献
44.
45.
46.
Stephen B. Vander Wall 《Animal behaviour》1982,30(1):84-94
Five hypotheses of cache recovery behaviour in Clark's nutcracker (Nucifraga columbiana) were examined experimentally. Most caches were made in soil within 5 cm of conspicuous large objects. Both seed-caching and non-seed-caching nutcrackers were able to locate caches. Seed-caching nutcrackers relocated caches using large objects as remembered visual cues. Soil microtopography and small (<2 cm diameter) objects may be used as cues to facilitate cache recovery but are not essential. Non-seed-caching nutcrackers located caches by using soil disturbances at cache sites as visual cues and by searching preferentially near objects where caches were concentrated. Success rates of seed-caching nutcrackers ranged from 52 to 78% and those of non-seed-caching nutcrackers ranged from 8 to 12%. Nutcrackers do not use random search or olfactory cues to locate caches. 相似文献
47.
48.
Stephen M. Reilly 《Journal of morphology》1987,191(2):205-214
Comparison of metamorphosis of skull and hyobranchial system in two species of neotenic salamanders reveals two different types of neoteny. Ambystoma talpoideum is completely neotenic owing to delayed metamorphosis. Notophthalmus viridescens exhibits limited neoteny as a result of incomplete metamorphosis. Morphological details of neoteny are compared to life history in both species in order to discuss the ecological morphology of the two neotenic strategies. Comparisons to Taricha granulosa, Triturus vulgaris, and Ambystoma gracile indicate that these two strategies are widely employed and may represent familial patterns. 相似文献
49.
Stephen R. Campion Elizabeth Elsasser Roger Chung 《Protein expression and purification》1997,10(3):331-339
AnEscherichia coliexpression system that exploits the bacterial alkaline phosphatase (PhoA) signal sequence to translocate recombinant human epidermal growth factor (hEGF) to the periplasm was used to evaluate how changes in the composition and sequence of amino acids near the PhoA–hEGF junction influence the periplasmic accumulation of recombinant protein. A series of chimeric structural genes was generated byin vitroreplacement of hEGF sequence with analogous segments from the EGF-like domain of human heregulin (HRG), significantly altering the electrostatic character of the amino-terminal region of the mature protein. Quantitation of HRG/EGF protein inE. coliperiplasmic extracts, by RP-HPLC, showed a fourfold decrease after one of two acidic residues located in the amino-terminal region of the mature hEGF, near the PhoA junction, was replaced. An additional threefold decrease was observed when the second acidic residue was replaced with a positively charged lysine. Further extension of the amino-terminal HRG sequence, beyond the first six residues, resulted in net neutralization of a more distant EGF acidic residue with no additional effect on protein yield. The importance of having a negatively charged group in the amino-terminal region of the mature protein was confirmed when insertion of an aspartic acid near the amino-terminus of two poorly expressed hybrid protein sequences resulted in a five- to eightfold increase in their recovery from the periplasm. This study demonstrates the importance of having negatively charged residues near the fusion junction of recombinant proteins expressed inE. coliusing the PhoA signal sequence for protein export. 相似文献
50.