全文获取类型
收费全文 | 12757篇 |
免费 | 1273篇 |
国内免费 | 8篇 |
专业分类
14038篇 |
出版年
2022年 | 101篇 |
2021年 | 170篇 |
2020年 | 101篇 |
2019年 | 131篇 |
2018年 | 165篇 |
2017年 | 156篇 |
2016年 | 264篇 |
2015年 | 490篇 |
2014年 | 520篇 |
2013年 | 654篇 |
2012年 | 802篇 |
2011年 | 832篇 |
2010年 | 561篇 |
2009年 | 470篇 |
2008年 | 673篇 |
2007年 | 709篇 |
2006年 | 694篇 |
2005年 | 675篇 |
2004年 | 657篇 |
2003年 | 630篇 |
2002年 | 689篇 |
2001年 | 136篇 |
2000年 | 106篇 |
1999年 | 156篇 |
1998年 | 188篇 |
1997年 | 145篇 |
1996年 | 132篇 |
1995年 | 125篇 |
1994年 | 121篇 |
1993年 | 122篇 |
1992年 | 102篇 |
1991年 | 103篇 |
1990年 | 119篇 |
1989年 | 105篇 |
1988年 | 103篇 |
1987年 | 105篇 |
1986年 | 90篇 |
1985年 | 97篇 |
1984年 | 116篇 |
1983年 | 120篇 |
1982年 | 136篇 |
1981年 | 117篇 |
1980年 | 96篇 |
1979年 | 66篇 |
1978年 | 89篇 |
1977年 | 78篇 |
1976年 | 71篇 |
1975年 | 59篇 |
1974年 | 69篇 |
1973年 | 57篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
201.
Mary W. Corrigan Christine L. Kerwin-Iosue Alexander S. Kuczmarski Kunj B. Amin Dennis D. Wykoff 《PloS one》2013,8(7)
In vivo assembly of plasmids has become an increasingly used process, as high throughput studies in molecular biology seek to examine gene function. In this study, we investigated the plasmid construction technique called gap repair cloning (GRC) in two closely related species of yeast – Saccharomyces cerevisiae and Candida glabrata. GRC utilizes homologous recombination (HR) activity to join a linear vector and a linear piece of DNA that contains base pair homology. We demonstrate that a minimum of 20 bp of homology on each side of the linear DNA is required for GRC to occur with at least 10% efficiency. Between the two species, we determine that S. cerevisiae is slightly more efficient at performing GRC. GRC is less efficient in rad52 deletion mutants, which are defective in HR in both species. In dnl4 deletion mutants, which perform less non-homologous end joining (NHEJ), the frequency of GRC increases in C. glabrata, whereas GRC frequency only minimally increases in S. cerevisiae, suggesting that NHEJ is more prevalent in C. glabrata. Our studies allow for a model of the fate of linear DNA when transformed into yeast cells. This model is not the same for both species. Most significantly, during GRC, C. glabrata performs NHEJ activity at a detectable rate (>5%), while S. cerevisiae does not. Our model suggests that S. cerevisiae is more efficient at HR because NHEJ is less prevalent than in C. glabrata. This work demonstrates the determinants for GRC and that while C. glabrata has a lower efficiency of GRC, this species still provides a viable option for GRC. 相似文献
202.
A checklist of identified oribatid mite taxa from riverine freshwater environments from six islands in Polynesia (New Caledonia, Tahiti, Moorea, Rurutu, Tubuai, Raiatea) is presented; 18 species, 16 genera and eight families were recorded. Trhypochthoniellus longisetus (Berlese, 1904) and Trimalaconothrus albulus Hammer, 1972 prevailed on distribution. Fortuynia smitisp. n. (Fortuyniidae) is described from New Caledonia. The new speciesis morphologically most similar to Fortuynia marina Hammen, 1960 from New Guinea, but it differs from the latter by the longer notogastral setae dm, lm, c2, p1, epimeral setae 3b and adanal setae ad1 and the presence of prodorsal lateral ridges. 相似文献
203.
204.
Alyssa Schermel Julio Mendoza Spencer Henson Steven Dukeshire Laura Pasut Teri E. Emrich Wendy Lou Ying Qi Mary R. L’Abbé 《PloS one》2014,9(1)
Background
Poor nutrition is harmful to one’s health as it can lead to overweight and obesity and a number of chronic diseases. Understanding consumer perceptions toward diet and nutrition is critical to advancing nutrition-related population health interventions to address such issues. The purpose of this paper was to examine Canadians’ perceived health and diet status, compared to their actual health status, and general concern about their own diet and beliefs about health. Also analyzed were some of the perceived barriers to eating “healthy” foods, with a focus on the availability of “healthy” processed foods.Methods
Two surveys were administered online to a group of Canadian panelists from all ten provinces during May 2010 to January 2011. Thirty thousand were invited; 6,665 completed the baseline survey and 5,494 completed the second survey. Panelists were selected to be nationally representative of the Canadian adult population by age, sex, province and education level, according to 2006 census data.Results
Approximately one third of Canadians perceived their health or diet to be very good while very few Canadians perceived their health or diet to be very poor. While the majority of Canadians believed food and nutrition to be very important for improving one’s health, fewer Canadians were concerned about their own diets. The majority of Canadians reported difficulty finding “healthy” processed foods (low in salt and sugar and with sufficient vitamins and minerals). Many also reported difficulty finding healthy foods that are affordable.Conclusion
Although consumers believe that nutrition is one of the most important factors for maintaining health, there are still a number of attitudinal and perceived environmental barriers to healthy eating. 相似文献205.
Structure and expression of the overlapping ND4L and ND5 genes of Neurospora crassa mitochondria 总被引:1,自引:0,他引:1
Summary Genes homologous to the mammalian mitochondrial NADH dehydrogenase subunit genes ND4L and ND5 were identified in the mitochondrial genome of the filamentous fungus Neurospora crassa, and the structure and expression of these genes was examined. The ND4L gene (interrupted by one intervening sequence) potentially encodes an 89 residue long hydrophobic protein that shares about 26% homology (or 41% homology if conservative amino acid substitutions are allowed) with the analogous human mitochondrial protein. The ND5 gene (which contains two introns) encodes a 715 residue polypeptide that shares 23% homology with the human analogue; a 300 amino acid long region is highly conserved (50% homology) in the two ND5 proteins. The stop codon of the ND4L gene overlaps the initiation codon of the downstream ND5 gene, and the two genes are contranscribed and probably cotranslated. A presumed mature dicistronic (ND4L plus ND5) RNA was detected. The postulated mRNA (about 3.2 kb) contains 5 and 3 non-coding regions of about 86 and 730 nucleotides, respectively; this species is generated from very large precursor RNAs by a complex processing pathway. The ND4L and ND5 introns are all stable after their excision from the precursor species.Abbreviations bp
base pairs
- rRNA
ribosomal RNA
- ND
NADH dehydrogenase
- URF
unidentified reading frame
- kDal
kilodaltons; a.a., amino acid 相似文献
206.
Phylogenetics of Perissodactyla and Tests of the Molecular Clock 总被引:3,自引:0,他引:3
Two mitochondrial genes, the protein-coding cytochrome c oxidase subunit II (COII) gene and a portion of the 12S rRNA gene, were used for phylogenetic investigation of the mammalian
order Perissodactyla. The primary objective of the study was to utilize the extensive fossil record of perissodactyls for
calibrating molecular clocks and comparing estimates of divergence times using both genes and two fossil calibration points.
Secondary objectives included clarification of previously unresolved relationships within Tapiridae and comparison of the
results of separate and combined analyses of two genes. Analyses included several perissodactyl lineages representing all
three families (Tapiridae, Equidae, and Rhinocerotidae), most extant genera, all four species of tapirs, two to four species
of rhinoceros, and two species of Equus. The application of a relatively recent fossil calibration point and a relatively ancient calibration point produced greatly
different estimates of evolutionary rates and divergence times for both genes, even though a relative rates test did not find
significant rate differences among taxa. A likelihood-ratio test, however, rejected a molecular clock for both genes. Neither
calibration point produced estimates of divergence times consistent with paleontological evidence over a range of perissodactyl
radiations. The combined analysis of both genes produces a well-resolved phylogeny with Perissodactyla that conforms to traditional
views of interfamilial relationships and supports monophyly of neotropical tapirs. Combining the data sets increases support
for most nodes but decreases the support for a neotropical tapir clade because the COII and 12S rRNA data sets are in conflict
for tapir relationships.
Received: 6 January 1999 / Accepted: 2 August 1999 相似文献
207.
Michael I. Lerman Farida Latif Gladys M. Glenn Lambert N. Daniel Hiltrud Brauch Shigeto Hosoe Krista Hampsch John Delisio Mary Lou Orcutt O. Wesley McBride Karl-Heinz Grzeschik Takashi Takahashi John Minna Patrick Anglard W. Marston Linehan Berton Zbar 《Human genetics》1991,86(6):567-577
Summary A collection of 2,000 lambda phage-carrying human single-copy inserts (> 700 bp) were isolated from two chromosome-3 flow-sorted libraries. The single-copy DNA fragments were first sorted into 3p and 3q locations and about 700 3p fragments were regionally mapped using a deletion mapping panel comprised of two humanhamster and two-human-mouse cell hybrids, each containing a chromosome 3 with different deletions in the short arm. The hybrids were extensively mapped with a set of standard 3p markers physically localized or ordered by linkage. The deletion mapping panel divided the short arm into five distinct subregions (A-E). The 3p fragments were distributed on 3p regions as follows: region A, 26%; B, 31%; C, 4%; D, 4% and E, 35%. We screened 300 single-copy DNA fragments from the distal part of 3p (regions A and B) with ten restriction endonucleases for their ability to detect restriction fragment length polymorphisms (RFLPs). Of these fragments 110 (36%) were found to detect useful RFLPs: 35% detected polymorphisms with frequency of heterozygosity of 40% or higher, and 25% with frequency of 30% or higher. All polymorphisms originated from single loci and most of them were of the base pair substitution type. These RFLP markers make it possible to construct a fine linkage map that will span the distal part of chromosome 3p and encompasses the von Hippel-Lindau disease locus. The large number of single-copy fragments (2,000) spaced every 100–150 kb on chromosome 3 will make a significant contribution to mapping and sequencing the entire chromosome 3. The 300 conserved chromosome 3 probes will increase the existing knowledge of man-mouse homologies. 相似文献
208.
Synopsis The ability of embryos of the viviparous scorpaenidSebastes melanops to take up nutrients from an exogenous substrate was demonstrated by incubating embryos at various stages of development
(18–30 days after fertilization) in14C-labeled glycine for 24 h. Uptake was highest for embryos at the latest stages (28–30 days) and increased at a linear rate
during the incubation period. Nutrient uptake was not time dependent in embryos at the early stages (18–22 days). Nutrient
utilization byS. melanops embryos was measured by the oxidation of14C-labeled glycine to14CO2. The amount of respired14CO2 by the oldest embryos increased significantly at a linear rate over the 24 h incubation period. There was no evidence of
nutrient utilization by the youngest embryos. The developmental changes we observed in the uptake and utilization of exogenous
glycine are supported by our previous findings that the oldest embryos have fully developed mouths and guts, and require additional
nutrition from intraovarian sources at this stage of development. 相似文献
209.
R. Ashton Lavoie Jeffrey T. Zugates Andrew T. Cheeseman Matt A. Teten Srivatsan Ramesh Julia M. Freeman Summer Swango Jeremy Fitzpatrick Amod Joshi Bradley Hollers Zufan Debebe Tyler K. Lindgren Amber N. Kozak Vinay K. Kondeti Mary K. Bright Eric J. Yearley Alexander Tracy Jacob A. Irwin Michael Guerrero 《Biotechnology and bioengineering》2023,120(10):2953-2968
Adeno-associated virus-based gene therapies have demonstrated substantial therapeutic benefit for the treatment of genetic disorders. In manufacturing processes, viral capsids are produced with and without the encapsidated gene of interest. Capsids devoid of the gene of interest, or “empty” capsids, represent a product-related impurity. As a result, a robust and scalable method to enrich full capsids is crucial to provide patients with as much potentially active product as possible. Anion exchange chromatography has emerged as a highly utilized method for full capsid enrichment across many serotypes due to its ease of use, robustness, and scalability. However, achieving sufficient resolution between the full and empty capsids is not trivial. In this work, anion exchange chromatography was used to achieve empty and full capsid resolution for adeno-associated virus serotype 5. A salt gradient screen of multiple salts with varied valency and Hofmeister series properties was performed to determine optimal peak resolution and aggregate reduction. Dual salt effects were evaluated on the same product and process attributes to identify any synergies with the use of mixed ion gradients. The modified process provided as high as ≥75% AAV5 full capsids (≥3-fold enrichment based on the percent full in the feed stream) with near baseline separation of empty capsids and achieved an overall vector genome step yield of >65%. 相似文献
210.
Garne E Loane M Dolk H Barisic I Addor MC Arriola L Bakker M Calzolari E Matias Dias C Doray B Gatt M Melve KK Nelen V O'Mahony M Pierini A Randrianaivo-Ranjatoelina H Rankin J Rissmann A Tucker D Verellun-Dumoulin C Wiesel A 《Birth defects research. Part A, Clinical and molecular teratology》2012,94(3):134-140