全文获取类型
收费全文 | 111479篇 |
免费 | 1970篇 |
国内免费 | 817篇 |
专业分类
114266篇 |
出版年
2023年 | 78篇 |
2022年 | 143篇 |
2021年 | 269篇 |
2020年 | 178篇 |
2019年 | 214篇 |
2018年 | 12037篇 |
2017年 | 10864篇 |
2016年 | 7850篇 |
2015年 | 1308篇 |
2014年 | 1082篇 |
2013年 | 1240篇 |
2012年 | 5477篇 |
2011年 | 13951篇 |
2010年 | 12699篇 |
2009年 | 8907篇 |
2008年 | 10693篇 |
2007年 | 12299篇 |
2006年 | 1174篇 |
2005年 | 1419篇 |
2004年 | 1852篇 |
2003年 | 1877篇 |
2002年 | 1579篇 |
2001年 | 593篇 |
2000年 | 429篇 |
1999年 | 349篇 |
1998年 | 276篇 |
1997年 | 206篇 |
1996年 | 192篇 |
1995年 | 181篇 |
1994年 | 176篇 |
1993年 | 195篇 |
1992年 | 243篇 |
1991年 | 253篇 |
1990年 | 175篇 |
1989年 | 185篇 |
1988年 | 199篇 |
1987年 | 164篇 |
1986年 | 132篇 |
1985年 | 153篇 |
1984年 | 141篇 |
1983年 | 148篇 |
1982年 | 125篇 |
1981年 | 97篇 |
1979年 | 116篇 |
1978年 | 102篇 |
1977年 | 86篇 |
1976年 | 74篇 |
1974年 | 102篇 |
1972年 | 312篇 |
1971年 | 319篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Dynamic viral populations in hypersaline systems as revealed by metagenomic assembly 总被引:1,自引:0,他引:1
JB Emerson BC Thomas K Andrade EE Allen KB Heidelberg JF Banfield 《Applied and environmental microbiology》2012,78(17):6309-6320
Viruses of the Bacteria and Archaea play important roles in microbial evolution and ecology, and yet viral dynamics in natural systems remain poorly understood. Here, we created de novo assemblies from 6.4 Gbp of metagenomic sequence from eight community viral concentrate samples, collected from 12 h to 3 years apart from hypersaline Lake Tyrrell (LT), Victoria, Australia. Through extensive manual assembly curation, we reconstructed 7 complete and 28 partial novel genomes of viruses and virus-like entities (VLEs, which could be viruses or plasmids). We tracked these 35 populations across the eight samples and found that they are generally stable on the timescale of days and transient on the timescale of years, with some exceptions. Cross-detection of the 35 LT populations in three previously described haloviral metagenomes was limited to a few genes, and most previously sequenced haloviruses were not detected in our samples, though 3 were detected upon reducing our detection threshold from 90% to 75% nucleotide identity. Similar results were obtained when we applied our methods to haloviral metagenomic data previously reported from San Diego, CA: 10 contigs that we assembled from that system exhibited a variety of detection patterns on a timescale of weeks to 1 month but were generally not detected in LT. Our results suggest that most haloviral populations have a limited or, possibly, a temporally variable global distribution. This study provides high-resolution insight into viral biogeography and dynamics and it places "snapshot" viral metagenomes, collected at a single time and location, in context. 相似文献
992.
Hasler U Greasley PJ von Heijne G Geering K 《The Journal of biological chemistry》2000,275(37):29011-29022
The structural and molecular determinants that govern the correct membrane insertion and folding of membrane proteins are still ill-defined. By following the addition of sugar chains to engineered glycosylation sites (glycosylation mapping) in Na,K-ATPase beta isoforms expressed in vitro and in Xenopus oocytes, in combination with biochemical techniques, we have defined the C-terminal end of the transmembrane domain of these type II proteins. N-terminal truncation and the removal of a single charged residue at the N-terminal start of the putative transmembrane domain influence the proper positioning of the transmembrane domain in the membrane as reflected by a repositioning of the transmembrane domain, the exposure of a putative cryptic signal peptidase cleavage site, and the production of protein species unable to insert into the membrane. Glycosylation mapping in vivo revealed that the degree of glycosylation at acceptor sites located close to the membrane increases with the time proteins spend in the endoplasmic reticulum. Furthermore, core sugars added to such acceptor sites cannot be processed to fully glycosylated species even when the protein is transported to the cell surface. Thus, the glycosylation mapping strategy applied in intact cells is a useful tool for the study of determinants for the correct membrane insertion of type II and probably other membrane proteins, as well as for the processing of sugar chains in glycoproteins. 相似文献
993.
Summary Three closely affiliated species of Heterospathe Scheff. (H. elegans (Becc.) Becc., H. humilis Becc. and H. versteegiana Becc.) from New Guinea are revised. They are reduced to a single species which is divided into two subspecies, and the new
combination H. elegans subsp. humilis (Becc.) M. S. Trudgen & W. J. Baker is made. The subspecies can be readily distinguished by their growth habit. Epitypes
are designated for the three previously published names, as informative material on the habit is not included in the existing
type specimens. A new, potentially related species is described as H. pullenii M. S. Trudgen & W. J. Baker. 相似文献
994.
Previous work has shown that bacterial isolates from the sheep rumen are capable of detoxifying 2,4,6-trinitrotoluene (TNT)
into polar constituents. In this study, the dietary effects of TNT on the sheep rumen microbial community were evaluated using
molecular microbiology ecology tools. Rumen samples were collected from sheep fed with and without TNT added to their diet,
genomic DNA was extracted, and the 16S rRNA-V3 gene marker was used to quantify changes in the microbial population in the
rumen. Control and treatment samples yielded 533 sequences. Phylogenetic analyses were performed to determine the microbial
changes between the two conditions. Results indicated the predominant bacterial populations present in the rumen were comprised
of the phyla Firmicutes and Bacteroidetes, irrespective of presence/absence of TNT in the diet. Significant differences (P < 0.001) were found between the community structure of the bacteria under TNT (−) and TNT (+) diets. Examination of the TNT
(+) diet showed an increase in the clones belonging to family Ruminococcaceae, which have previously been shown to degrade
TNT in pure culture experiments. 相似文献
995.
Konishi MA Fukuoka T Shimane Y Mori K Nagano Y Ohta Y Kitamoto D Hatada Y 《Biotechnology letters》2011,33(1):139-145
Purpose of work
To explore a novel glycolipid, we performed biochemical reactions using a recombinant α-glucosidase from Geobacillus sp. which shows excellent transglycosylation reaction to hydroxyl groups in a variety of compounds. 相似文献996.
997.
Li Sun Quan Yuan Tianhua Xu Li Yao Jiangmin Feng Jianfei Ma Lining Wang Changlong Lv Danan Wang 《Biotechnology letters》2017,39(8):1159-1166
Objectives
To investigate the potential of interleukin (IL)-15 as a novel adjuvant for Mycobacterium tuberculosis (Mtb) antigen 85A (Ag85A) vaccine.Results
C57BL/6 mice were intramuscularly immunized three times with a plasmid expressing the Ag85A-IL-15 fusion protein (pcDNA3.1-Ag85A-IL-15), with the empty pcDNA3.1 vector and the pcDNA3.1-Ag85A as control. Mice vaccinated with pcDNA3.1-Ag85A-IL-15 generated more secretory IgA (sIgA) into their lung (209 ± 21 μg/ml) and acquired an enhanced serum IgG response to Ag85A. IgG2a/IgG1 ratios were upregulated, natural killer cell activity was augmented and Ag85A-specific splenic T cell proliferation was enhanced in these mice as well. Vaccination with pcDNA3.1-Ag85A-IL-15 promoted the polarization of CD4+ T cells towards a Th1 type in the spleen, and significantly upregulated the serum level of interferon (IFN)-γ (458 ± 98 pg/ml), a typical Th1 cytokine. IFN-γ-expressing CD8+ cells were also increased in the spleen after pcDNA3.1-Ag85A-IL-15 immunization.Conclusions
A superior immune type I response in mice vaccinated with plasmid Ag85A-IL-15 has been achieved.998.
Differentiation of embryonic stem cell (ESC)-derived embryoid bodies (EBs) is a heterogeneous process. ESCs can differentiate
in vitro into different cell types including beating cardiomyocytes. The main aim of the present study was to develop an improved
preparation method for scanning electron microscopic study of ESC-derived cardiac bundles and to investigate the fine structural
characteristics of mouse ESCs-derived cardiomyocytes using electron microscopy. The mouse ESCs differentiation was induced
by EBs’ development through hanging drop, suspension and plating stages. Cardiomyocytes appeared in the EBs’ outgrowth as
beating clusters that grew in size and formed thick branching bundles gradually. Cardiac bundles showed cross striation even
when they were observed under an inverted microscope. They showed a positive immunostaining for cardiac troponin I and α-actinin.
Transmission and scanning electron microscopy (TEM & SEM) were used to study the structural characteristics of ESC-derived
cardiomyocytes. Three weeks after plating, differentiated EBs showed a superficial layer of compact fibrous ECM that made
detailed observation of cardiac bundles impossible. We tried several preparation methods to remove unwanted cells and fibers,
and finally we revealed the branching bundles of cardiomyocytes. In TEM study, most cardiomyocytes showed parallel arrays
of myofibrils with a mature sarcomeric organization marked by H-bands, M-lines and numerous T-tubules. Cardiomyocytes were
connected to each other by intercalated discs composed of numerous gap junctions and fascia adherences. 相似文献
999.
P450 BM3 mutant can catalyze indole to indoxyl, and indoxyl can dimerize to form indigo. But the reaction catalyzed by P450
BM3 requires NADPH, as coenzyme regeneration is very important in this system. As we know, when glucose dehydrogenase oxidizes
glucose to glucolactone, NADH or NADPH can be formed, which can contribute to NADPH regeneration in the reaction catalyzed
by P450 BM3. In this paper, a recombinant Escherichia coli BL21 (pET28a (+)-P450 BM3-gdh0310) was constructed to co-express both P450 BM3 gene and glucose dehydrogenase (GDH) gene.
To improve the expression level of P450 BM3 and GDH in E. coli and to avoid the complex and low-efficiency refolding operation in the purification procedure, the expression conditions
were optimized. Under the optimized conditions, the maximum P450 BM3 and GDH activities amounted to 8173.13 and 0.045 U/mg
protein, respectively. Then bioconversion of indole to indigo was carried out by adding indole and glucose to the culture
after improved expression level was obtained under optimized conditions, and 2.9 mM (760.6 mg/L) indigo was formed with an
initial indole concentration of 5 mM. 相似文献
1000.
Diel vertical migration of zooplankton in the Northeast Atlantic 总被引:7,自引:0,他引:7
Acoustic Doppler current profiler (ADCP) data collected duringAugustSeptember 1991 reveal the diel migration of zooplanktonin the northeast Atlantic (5060 相似文献