全文获取类型
收费全文 | 4684篇 |
免费 | 479篇 |
专业分类
5163篇 |
出版年
2022年 | 34篇 |
2021年 | 55篇 |
2020年 | 44篇 |
2019年 | 55篇 |
2018年 | 56篇 |
2017年 | 79篇 |
2016年 | 88篇 |
2015年 | 146篇 |
2014年 | 190篇 |
2013年 | 204篇 |
2012年 | 278篇 |
2011年 | 259篇 |
2010年 | 179篇 |
2009年 | 172篇 |
2008年 | 230篇 |
2007年 | 208篇 |
2006年 | 191篇 |
2005年 | 205篇 |
2004年 | 212篇 |
2003年 | 202篇 |
2002年 | 166篇 |
2001年 | 180篇 |
2000年 | 150篇 |
1999年 | 148篇 |
1998年 | 81篇 |
1997年 | 85篇 |
1996年 | 62篇 |
1995年 | 47篇 |
1994年 | 66篇 |
1993年 | 46篇 |
1992年 | 86篇 |
1991年 | 74篇 |
1990年 | 72篇 |
1989年 | 73篇 |
1988年 | 57篇 |
1987年 | 41篇 |
1986年 | 41篇 |
1985年 | 67篇 |
1984年 | 43篇 |
1983年 | 35篇 |
1982年 | 39篇 |
1981年 | 28篇 |
1980年 | 25篇 |
1979年 | 42篇 |
1978年 | 23篇 |
1977年 | 26篇 |
1976年 | 24篇 |
1974年 | 29篇 |
1973年 | 23篇 |
1972年 | 22篇 |
排序方式: 共有5163条查询结果,搜索用时 0 毫秒
141.
Bruno Studer Torben Asp Ursula Frei Stephan Hentrup Helena Meally Aurélie Guillard Susanne Barth Hilde Muylle Isabel Roldán-Ruiz Philippe Barre Carole Koning-Boucoiran Gerda Uenk-Stunnenberg Oene Dolstra Leif Skøt Kirsten P. Skøt Lesley B. Turner Mervyn O. Humphreys Roland Kölliker Niels Roulund Klaus K. Nielsen Thomas Lübberstedt 《Molecular breeding : new strategies in plant improvement》2008,21(4):533-548
An expressed sequence tag (EST) library of the key grassland species perennial ryegrass (Lolium perenne L.) has been exploited as a resource for microsatellite marker development. Out of 955 simple sequence repeat (SSR) containing
ESTs, 744 were used for primer design. Primer amplification was tested in eight genotypes of L. perenne and L. multiflorum representing (grand-) parents of four mapping populations and resulted in 464 successfully amplified EST-SSRs. Three hundred
and six primer pairs successfully amplified products in the mapping population VrnA derived from two of the eight genotypes
included in the original screening and revealed SSR polymorphisms for 143 ESTs. Here, we report on 464 EST-derived SSR primer
sequences of perennial ryegrass established in laboratory assays, providing a dedicated tool for marker assisted breeding
and comparative mapping within and among forage and turf grasses.
Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. 相似文献
142.
Turner KK Nielsen BD O'Connor-Robison CI Nielsen FH Orth MW 《Biological trace element research》2008,121(2):134-148
The objective was to determine the effects of sodium zeolite A (SZA) on mineral metabolism and tissue mineral composition
in calves. Twenty calves were placed on study at 3 days of age and were placed into one of two groups: SS, which received
0.05% BW SZA added to their milk replacer, and CO, which received only milk replacer. Blood samples were taken on days 0,
30, and 60 for mineral analysis. Urine and feces were collected on day 30 for mineral metabolism, and on day 60, the calves
were euthanized, and samples were taken from numerous organs for mineral analyses. Aluminum retention was increased in the
SS calves (p = 0.001). Silicon concentrations were increased in the aorta, spleen, lung, muscle, and kidney of the SS calves, and aluminum
was increased in all SS tissues (p < 0.05). Calcium concentrations were increased in aorta, liver, muscle, and tendon; phosphorus concentrations were increased
in aorta, but decreased in plasma; magnesium concentrations were increased in aorta, heart, kidney, liver, and pancreas, but
decreased in plasma; and iron concentrations were decreased in kidney and liver (p < 0.05). The accumulation of tissue aluminum and therefore potential adverse consequences may preclude any benefits of using
SZA as a dietary supplement. 相似文献
143.
Quantitative Aspects of the in Vivo Regulation of Pyrophosphate:Fructose-6-Phosphate 1-Phosphotransferase by Fructose-2,6-Bisphosphate 下载免费PDF全文
Pyrophosphate:fructose-6-phosphate 1-phosphotransferase (PFP) was quantified in developing barley (Hordeum vulgare) leaves by immunostaining on western blots using a purified preparation of barley leaf PFP as standard. Fructose-2,6-bisphosphate (Fru-2,6-bisP) was quantified in the same tissues. Depending on age and tissue development, the concentration of PFP varied between 11 and 80 [mu]g PFP protein g-1 fresh weight, which corresponds to 0.09 to 0.65 nmol g-1 fresh weight of each of the [alpha] and [beta] PFP subunits. The level depends primarily on the maturity of the tissue. In the same tissues the concentration of Fru-2,6-bisP varied between 0.07 and 0.46 nmol g-1 fresh weight. Thus, the concentrations of PFP subunits and Fru-2,6-bisP were of the same order of magnitude. In young leaf tissues the concentration of PFP subunits may exceed the concentration of Fru-2,6-bisP. This means that the amount of Fru-2,6-bisP present will be too low to occupy all the allosteric binding sites on PFP even though the concentration of Fru-2,6-bisP exceeds the Ka(Fru-2,6-bisP) by several orders of magnitude. These results are discussed in relation to Fru-2,6-bisP as a regulator of enzyme activities under in vivo conditions. 相似文献
144.
Reactive oxygen species are important mediators of taurine release from skeletal muscle cells 总被引:3,自引:0,他引:3
Ørtenblad N Young JF Oksbjerg N Nielsen JH Lambert IH 《American journal of physiology. Cell physiology》2003,284(6):C1362-C1373
The present study illustrates elements ofthe signal cascades involved in the activation of taurine effluxpathways in myotubes derived from skeletal muscle cells. Exposingprimary skeletal muscle cells, loaded with 14C-taurine, to1) hypotonic media, 2) the phospholipaseA2 (PLA2) activator melittin, 3)anoxia, or 4) lysophosphatidyl choline (LPC) causes anincrease in 14C-taurine release and a concomitantproduction of reactive oxygen species (ROS). The antioxidants butulatedhydroxy toluene and vitamin E inhibit the taurine efflux after cellswelling, anoxia, and addition of LPC. The muscle cells possess twoseparate taurine efflux pathways, i.e., a swelling- andmelittin-induced pathway that requires 5-lipoxygenase activity foractivation and a LPC-induced pathway. The two pathways aredistinguished by their opposing sensitivity toward the anion channelblocker DIDS and cholesterol. These data provide evidence forPLA2 products and ROS as key mediators of the signalcascade leading to taurine efflux in muscle. 相似文献
145.
S. V. Nielsen J. L. Banks R. E. Diaz Jr P. A. Trainor T. Gamble 《Journal of evolutionary biology》2018,31(4):484-490
Much of our current state of knowledge concerning sex chromosome evolution is based on a handful of ‘exceptional’ taxa with heteromorphic sex chromosomes. However, classifying the sex chromosome systems of additional species lacking easily identifiable, heteromorphic sex chromosomes is indispensable if we wish to fully understand the genesis, degeneration and turnover of vertebrate sex chromosomes. Squamate reptiles (lizards and snakes) are a potential model clade for studying sex chromosome evolution as they exhibit a suite of sex‐determining modes yet most species lack heteromorphic sex chromosomes. Only three (of 203) chameleon species have identified sex chromosome systems (all with female heterogamety, ZZ/ZW). This study uses a recently developed method to identify sex‐specific genetic markers from restriction site‐associated DNA sequence (RADseq) data, which enables the identification of sex chromosome systems in species lacking heteromorphic sex chromosomes. We used RADseq and subsequent PCR validation to identify an XX/XY sex chromosome system in the veiled chameleon (Chamaeleo calyptratus), revealing a novel transition in sex chromosome systems within the Chamaeleonidae. The sex‐specific genetic markers identified here will be essential in research focused on sex‐specific, comparative, functional and developmental evolutionary questions, further promoting C. calyptratus’ utility as an emerging model organism. 相似文献
146.
Continuous signaling of CD79b and CD19 is required for the fitness of Burkitt lymphoma B cells 下载免费PDF全文
Xiaocui He Kathrin Kläsener Joseena M Iype Martin Becker Palash C Maity Marco Cavallari Peter J Nielsen Jianying Yang Michael Reth 《The EMBO journal》2018,37(11)
Expression of the B‐cell antigen receptor (BCR) is essential not only for the development but also for the maintenance of mature B cells. Similarly, many B‐cell lymphomas, including Burkitt lymphoma (BL), require continuous BCR signaling for their tumor growth. This growth is driven by immunoreceptor tyrosine‐based activation motif (ITAM) and PI3 kinase (PI3K) signaling. Here, we employ CRISPR/Cas9 to delete BCR and B‐cell co‐receptor genes in the human BL cell line Ramos. We find that Ramos B cells require the expression of the BCR signaling component Igβ (CD79b), and the co‐receptor CD19, for their fitness and competitive growth in culture. Furthermore, we show that in the absence of any other BCR component, Igβ can be expressed on the B‐cell surface, where it is found in close proximity to CD19 and signals in an ITAM‐dependent manner. These data suggest that Igβ and CD19 are part of an alternative B‐cell signaling module that use continuous ITAM/PI3K signaling to promote the survival of B lymphoma and normal B cells. 相似文献
147.
Secondary Metabolite- and Endochitinase-Dependent Antagonism toward Plant-Pathogenic Microfungi of Pseudomonas fluorescens Isolates from Sugar Beet Rhizosphere 总被引:4,自引:0,他引:4 下载免费PDF全文
Mette Neiendam Nielsen Jan Srensen Johannes Fels Hans Christian Pedersen 《Applied microbiology》1998,64(10):3563-3569
Forty-seven isolates representing all biovars of Pseudomonas fluorescens (biovars I to VI) were collected from the rhizosphere of field-grown sugar beet plants to select candidate strains for biological control of preemergence damping-off disease. The isolates were tested for in vitro antagonism toward the plant-pathogenic microfungi Pythium ultimum and Rhizoctonia solani in three different plate test media. Mechanisms of fungal inhibition were elucidated by tracing secondary-metabolite production and cell wall-degrading enzyme activity in the same media. Most biovars expressed a specific mechanism of antagonism, as represented by a unique antibiotic or enzyme production in the media. A lipopeptide antibiotic, viscosinamide, was produced independently of medium composition by P. fluorescens bv. I, whereas the antibiotic 2,4-diacetylphloroglucinol was observed only in glucose-rich medium and only in P. fluorescens bv. II/IV. Both pathogens were inhibited by the two antibiotics. Finally, in low-glucose medium, a cell wall-degrading endochitinase activity in P. fluorescens bv. I, III, and VI was the apparent mechanism of antagonism toward R. solani. The viscosinamide-producing DR54 isolate (bv. I) was shown to be an effective candidate for biological control, as tested in a pot experiment with sugar beet seedlings infested with Pythium ultimum. The assignment of different patterns of fungal antagonism to the biovars of P. fluorescens is discussed in relation to an improved selection protocol for candidate strains to be used in biological control. 相似文献
148.
M. Chartrain G. Hunt L. Horn A. Kirpekar D. Mathre A. Powell L. Wassel J. Nielsen B. Buckland R. Greasham 《Journal of industrial microbiology & biotechnology》1991,7(4):293-299
Summary An efrotomycin fermentation was characterized through physical, chemical and biochemical studies. Growth of the actinomycete,Nocardia lactamdurans occurred during the first 50 h of the fermentation cycle at the expense of glucose, protein, and triglycerides. The initiation of efrotomycin biosynthesis was observed when glucose dropped to a low concentration. Upon glucose depletion, cell growth ceased and a switch in the respiratory quotient occurred. Efrotomycin biosynthesis was supported by the utilization of soybean oil and starch. Analysis of triglyceride metabolism showed that no diglycerides or monoglycerides accumulated during the fermentation. The activity of extracellular enzymes (lipase, protease, and amylase) increase during the cell growth phase and decreased significantly after 150 h. The concentrations of DNA, tetrahydro-vitamin K2 (a membrane component), and free amino acids in the supernatant increased dramatically late in the fermentation cycle (225 h), indicating massive cell lysis. During this same time period, a reduction in cellular respiratory activity and efrotomycin biosynthesis were observed. 相似文献
149.
Gerald
Ryan
R Aquino Nicolai Krogh Philipp Hackert Roman Martin Jimena
Davila Gallesio Robert W van
Nues Claudia Schneider Nicholas
J Watkins Henrik Nielsen Katherine E Bohnsack Markus
T Bohnsack 《Nucleic acids research》2021,49(7):4066
RNA helicases play important roles in diverse aspects of RNA metabolism through their functions in remodelling ribonucleoprotein complexes (RNPs), such as pre-ribosomes. Here, we show that the DEAD box helicase Dbp3 is required for efficient processing of the U18 and U24 intron-encoded snoRNAs and 2′-O-methylation of various sites within the 25S ribosomal RNA (rRNA) sequence. Furthermore, numerous box C/D snoRNPs accumulate on pre-ribosomes in the absence of Dbp3. Many snoRNAs guiding Dbp3-dependent rRNA modifications have overlapping pre-rRNA basepairing sites and therefore form mutually exclusive interactions with pre-ribosomes. Analysis of the distribution of these snoRNAs between pre-ribosome-associated and ‘free’ pools demonstrated that many are almost exclusively associated with pre-ribosomal complexes. Our data suggest that retention of such snoRNPs on pre-ribosomes when Dbp3 is lacking may impede rRNA 2′-O-methylation by reducing the recycling efficiency of snoRNPs and by inhibiting snoRNP access to proximal target sites. The observation of substoichiometric rRNA modification at adjacent sites suggests that the snoRNPs guiding such modifications likely interact stochastically rather than hierarchically with their pre-rRNA target sites. Together, our data provide new insights into the dynamics of snoRNPs on pre-ribosomal complexes and the remodelling events occurring during the early stages of ribosome assembly. 相似文献
150.
The generation of novel yeast cell factories for production of high-value industrial biotechnological products relies on three metabolic engineering principles: design, construction, and analysis. In the last two decades, strong efforts have been put on developing faster and more efficient strategies and/or technologies for each one of these principles. For design and construction, three major strategies are described in this review: (1) rational metabolic engineering; (2) inverse metabolic engineering; and (3) evolutionary strategies. Independent of the selected strategy, the process of designing yeast strains involves five decision points: (1) choice of product, (2) choice of chassis, (3) identification of target genes, (4) regulating the expression level of target genes, and (5) network balancing of the target genes. At the construction level, several molecular biology tools have been developed through the concept of synthetic biology and applied for the generation of novel, engineered yeast strains. For comprehensive and quantitative analysis of constructed strains, systems biology tools are commonly used and using a multi-omics approach. Key information about the biological system can be revealed, for example, identification of genetic regulatory mechanisms and competitive pathways, thereby assisting the in silico design of metabolic engineering strategies for improving strain performance. Examples on how systems and synthetic biology brought yeast metabolic engineering closer to industrial biotechnology are described in this review, and these examples should demonstrate the potential of a systems-level approach for fast and efficient generation of yeast cell factories. 相似文献