全文获取类型
收费全文 | 118652篇 |
免费 | 20229篇 |
国内免费 | 651篇 |
专业分类
139532篇 |
出版年
2023年 | 673篇 |
2022年 | 1067篇 |
2021年 | 2212篇 |
2020年 | 3394篇 |
2019年 | 5035篇 |
2018年 | 6051篇 |
2017年 | 5911篇 |
2016年 | 6874篇 |
2015年 | 8128篇 |
2014年 | 7994篇 |
2013年 | 9866篇 |
2012年 | 8913篇 |
2011年 | 8152篇 |
2010年 | 7138篇 |
2009年 | 5110篇 |
2008年 | 5739篇 |
2007年 | 5057篇 |
2006年 | 4799篇 |
2005年 | 4263篇 |
2004年 | 4067篇 |
2003年 | 3713篇 |
2002年 | 3438篇 |
2001年 | 2455篇 |
2000年 | 2288篇 |
1999年 | 1875篇 |
1998年 | 886篇 |
1997年 | 705篇 |
1996年 | 652篇 |
1995年 | 628篇 |
1994年 | 502篇 |
1993年 | 466篇 |
1992年 | 906篇 |
1991年 | 799篇 |
1990年 | 705篇 |
1989年 | 715篇 |
1988年 | 656篇 |
1987年 | 583篇 |
1986年 | 542篇 |
1985年 | 567篇 |
1984年 | 522篇 |
1983年 | 372篇 |
1982年 | 363篇 |
1981年 | 300篇 |
1980年 | 260篇 |
1979年 | 360篇 |
1978年 | 328篇 |
1975年 | 306篇 |
1974年 | 329篇 |
1973年 | 324篇 |
1972年 | 265篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
1.
The desulfurization activity of the resting cells of Gordona sp. CYKS1 was strongly depended on harvest time and the highest value when the cells had been harvested in the early growth phase (0.12 mg sulfur g–1 cell–1 h–1). For the model oil, hexadecane containing dibenzothiophene, the specific desulfurization rate decreased as the reaction proceeded. Both the specific and the volumetric desulfurization rates were not significantly affected by the aqueous-to-oil phase ratio. The diesel oils, light gas oil and a middle distillate unit feed were desulfurized at higher rates (ca. 0.34 mg sulfur g–1 cell–1 h–1) than the model oil (0.12 mg sulfur g–1 cell–1 h–1). 相似文献
2.
Juan Viruel Pilar Catalán José Gabriel Segarra‐Moragues 《Journal of Biogeography》2012,39(6):1073-1085
Aim The Chilean endemic Dioscorea biloba (Dioscoreaceae) is a dioecious geophyte that shows a remarkable 600 km north–south disjunction in the peripheral arid area of the Atacama Desert. Its restricted present‐day distribution and probable Neogene origin indicate that its populations have a history linked to that of the Atacama Desert, making this an ideal model species with which to investigate the biogeography of the region. Location Chile, Atacama Desert and peripheral arid area. Methods Two hundred and seventy‐five individuals from nine populations were genotyped for seven nuclear microsatellite loci, and plastid trnL–F and trnT–L sequences were obtained for a representative subset of these. Analyses included the estimation of genetic diversity and population structure through clustering, Bayesian and analysis of molecular variance analyses, and statistical parsimony networks of chloroplast haplotypes. Isolation by distance was tested against alternative dispersal hypotheses. Results Microsatellite markers revealed moderate to high levels of genetic diversity within populations, with those from the southern Limarí Valley showing the highest values and northern populations showing less exclusive alleles. Bayesian analysis of microsatellite data identified three genetic groups that corresponded to geographical ranges. Chloroplast phylogeography revealed no haplotypes shared between northern and southern ranges, and little haplotype sharing between the two neighbouring southern valleys. Dispersal models suggested the presence of extinct hypothetical populations between the southern and northern ranges. Main conclusions Our results are consistent with prolonged isolation of the northern and southern groups, mediated by the life‐history traits of the species. Significant isolation was revealed at both large and moderate distances as gene flow was not evident even between neighbouring valleys. Bayesian analyses of microsatellite and chloroplast haplotype diversity identified the southern area of Limarí as the probable area of origin of the species. Our data do not support recent dispersal of D. biloba from the southern range into Antofagasta, but indicate the fragmentation of an earlier wider range, concomitant with the Pliocene–Pleistocene climatic oscillations, with subsequent extinctions of the Atacama Desert populations and the divergence of the peripheral ones as a consequence of genetic drift. 相似文献
3.
Yi-Hua Wu Chia-Pei Chang Chin-I Chien Yi-Kuan Tseng Chien-Chia Wang 《Molecular and cellular biology》2013,33(17):3515-3523
The yeast Saccharomyces cerevisiae possesses two distinct glycyl-tRNA synthetase (GlyRS) genes: GRS1 and GRS2. GRS1 is dually functional, encoding both cytoplasmic and mitochondrial activities, while GRS2 is dysfunctional and not required for growth. The protein products of these two genes, GlyRS1 and GlyRS2, are much alike but are distinguished by an insertion peptide of GlyRS1, which is absent from GlyRS2 and other eukaryotic homologues. We show that deletion or mutation of the insertion peptide modestly impaired the enzyme''s catalytic efficiency in vitro (with a 2- to 3-fold increase in Km and a 5- to 8-fold decrease in kcat). Consistently, GRS2 can be conveniently converted to a functional gene via codon optimization, and the insertion peptide is dispensable for protein stability and the rescue activity of GRS1 at 30°C in vivo. A phylogenetic analysis further showed that GRS1 and GRS2 are paralogues that arose from a gene duplication event relatively recently, with GRS1 being the predecessor. These results indicate that GlyRS2 is an active enzyme essentially resembling the insertion peptide-deleted form of GlyRS1. Our study suggests that the insertion peptide represents a novel auxiliary domain, which facilitates both productive docking and catalysis of cognate tRNAs. 相似文献
4.
5.
6.
7.
8.
Sebastián Moran Miguel Vizoso Anna Martinez-Cardús Antonio Gomez Xavier Matías-Guiu Sebastián M Chiavenna Andrés G Fernandez Manel Esteller 《Epigenetics》2014,9(6):829-833
A formalin-fixed paraffin-embedded (FFPE) sample usually yields highly degraded DNA, which limits the use of techniques requiring high-quality DNA, such as Infinium Methylation microarrays. To overcome this restriction, we have applied an FFPE restoration procedure consisting of DNA repair and ligation processes in a set of paired fresh-frozen (FF) and FFPE samples. We validated the FFPE results in comparison with matched FF samples, enabling us to use FFPE samples on the Infinium HumanMethylation450 Methylation array. 相似文献
9.
Monica Driscoll 《Developmental neurobiology》1992,23(9):1327-1351
In C. elegans, cell death can be readily studied at the cellular, genetic, and molecular levels. Two types of death have been characterized in this nematode: (1) programmed cell death, which occurs as a normal component in development; and (2) pathological cell death which occurs aberrantly as a consequence of mutation. Analysis of mutations that disrupt programmed cell death in various ways has defined a genetic pathway for programmed cell death which includes genes that perform such functions as the determination of which cells die, the execution of cell death, the engulfment of cell corpses, and the digestion of DNA from dead cells. Molecular analysis is providing insightinto the nature of the molecules that function in these aspects of programmed cell death. Characterization of some genes that mutate to induce abnormal cell death has defined a novel gene family called degenerins that encode putative membrane proteins. Dominant alleles of at least two degenerin genes, mec-4 and deg-1, can cause cellular swelling and late onset neurodegeneration of specific groups of cells. © 1992 John Wiley & Sons, Inc. 相似文献