全文获取类型
收费全文 | 11357篇 |
免费 | 1311篇 |
国内免费 | 2篇 |
出版年
2022年 | 83篇 |
2021年 | 174篇 |
2020年 | 103篇 |
2019年 | 140篇 |
2018年 | 182篇 |
2017年 | 147篇 |
2016年 | 262篇 |
2015年 | 374篇 |
2014年 | 409篇 |
2013年 | 562篇 |
2012年 | 685篇 |
2011年 | 594篇 |
2010年 | 379篇 |
2009年 | 357篇 |
2008年 | 507篇 |
2007年 | 488篇 |
2006年 | 446篇 |
2005年 | 435篇 |
2004年 | 416篇 |
2003年 | 376篇 |
2002年 | 344篇 |
2001年 | 242篇 |
2000年 | 261篇 |
1999年 | 237篇 |
1998年 | 140篇 |
1997年 | 127篇 |
1996年 | 133篇 |
1995年 | 119篇 |
1994年 | 124篇 |
1993年 | 103篇 |
1992年 | 218篇 |
1991年 | 218篇 |
1990年 | 186篇 |
1989年 | 180篇 |
1988年 | 179篇 |
1987年 | 168篇 |
1986年 | 163篇 |
1985年 | 174篇 |
1984年 | 161篇 |
1983年 | 124篇 |
1982年 | 133篇 |
1981年 | 81篇 |
1980年 | 98篇 |
1979年 | 135篇 |
1978年 | 118篇 |
1977年 | 93篇 |
1976年 | 95篇 |
1975年 | 83篇 |
1974年 | 94篇 |
1973年 | 101篇 |
排序方式: 共有10000条查询结果,搜索用时 20 毫秒
261.
262.
Sandra Irene Pitta-Alvarez Ana María Giulietti 《In vitro cellular & developmental biology. Plant》1995,31(4):215-220
Summary
Brugmansia candida hairy roots, obtained by infection withAgrobacterium rhizogenes LBA 9402, exhibit, after subculturing in liquid media, a tendency towards dedifferentiation. It has been found that the following
strategies can be applied to inhibit this dedifferentiation and preserve normal root morphology: (a) lowering both the mineral
and sucrose concentration in the media employed so as to diminish osmotic stress (a condition to which these roots appear
to be particularly susceptible); (b) employing antiauxins in appropriate concentrations; and (c) maintaining the hairy roots
on solid media prior to use in production processes in liquid media. The first strategy suggested does not favor alkaloid
productivity, but in this case a two-step method could be attempted: biomass with normal root morphology could be obtained
in a first stage using low sucrose concentrations, and in a second stage, sucrose could be increased in order to achieve higher
productivity. In all the clones ofB. candida obtained, alkaloid production was biased towards scopolamine. 相似文献
263.
Expression of soybean nodulin 26 in transgenic tobacco. Targeting to the vacuolar membrane and effects on floral and seed development. 总被引:2,自引:0,他引:2
下载免费PDF全文
![点击此处可从《Molecular biology of the cell》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Nodulin 26 is an integral membrane protein of the symbiosome membrane of nitrogen-fixing soybean nodules. We expressed a nodulin 26 cDNA in transgenic tobacco (TN26 tobacco) under the control of the cauliflower mosaic virus 35S promoter to study subcellular targeting and the physiological effect(s) of its expression. Based on Northern and Western blots, the expression of nodulin 26 mRNA and protein in transgenic plants is high in apical shoot sections, flowers, and stems, low in mature leaves, and absent in roots. Western blot analysis revealed high levels of transgenic nodulin 26 protein in tonoplast membranes. In contrast, nodulin 26 protein was not found in isolated plasma membranes, the soluble fraction, nor in chloroplast and mitochondria-enriched membrane fractions. About 50-60% of the flowers and pods from TN26 tobacco plants abscised prematurely. Seed capsule size and seed fill per capsule from the remainder of surviving flowers were about 50% of that of control plants. Pollen viability was found to be normal, but flowers from TN26 tobacco plants showed shorter anther filaments compared with control plants. Normal seed production and capsule size was restored by manually crossing the stigmas from TN26 plants with isolated pollen from either transgenic or control plants. Thus, the aberrant filament growth could have resulted in the reproductive defects associated with the plants. 相似文献
264.
Gill JH Molloy CA Shoesmith KJ Bayly AC Roberts RA 《Cell death and differentiation》1995,2(3):211-217
The responses of a series of rat hepatoma cell lines (FaO, HTC, RH1) to the rodent non-genotoxic hepatocarcinogen and per-oxisome proliferator (PP) Nafenopin were studied to determine if this PP acts with EGF, a naturally occurring liver growth regulator, to perturb the balance between mitosis and apoptosis. EGF enhanced the growth of FaO cells (well differentiated) and HTC cells (intermediate differentiation) but not of the poorly differentiated RH1 cell line. Nafenopin also increased FaO cell growth but, surprisingly, retarded the growth of both HTC and RH1 cells. Since population expansion kinetics result from mitosis and death, replicative DNA synthesis (RDS) and apoptotic cell death were measured in HTC cells. As expected, EGF elevated RDS and suppressed cell death. However, Nafenopin depressed HTC net population expansion via a suppression of cell death coupled to a more marked inhibition of RDS. This apparent paradox was investigated using soft agar cloning. This revealed sub-populations with differing growth kinetics suggesting selective clonal expansion via an alteration in the balance between mitosis and apoptotic cell death. At later stages, cells are refractory to EGF and Nafenopin, suggesting that genetic changes may have superseded such factor-dependence. 相似文献
265.
Developmental regulation of alpha beta T cell antigen receptor expression results from differential stability of nascent TCR alpha proteins within the endoplasmic reticulum of immature and mature T cells. 总被引:5,自引:2,他引:3
下载免费PDF全文
![点击此处可从《The EMBO journal》网站下载免费的PDF全文](/ch/ext_images/free.gif)
K P Kearse J L Roberts T I Munitz D L Wiest T Nakayama A Singer 《The EMBO journal》1994,13(19):4504-4514
The alpha beta T cell antigen receptor (TCR) that is expressed on most T lymphocytes is a multisubunit transmembrane complex composed of at least six different proteins (alpha, beta, gamma, delta, epsilon and zeta) that are assembled in the endoplasmic reticulum (ER) and then transported to the plasma membrane. Expression of the TCR complex is quantitatively regulated during T cell development, with immature CD4+CD8+ thymocytes expressing only 10% of the number of surface alpha beta TCR complexes that are expressed on mature T cells. However, the molecular basis for low TCR expression in developing alpha beta T cells is unknown. In the present study we report the unexpected finding that assembly of nascent component chains into complete TCR alpha beta complexes is severely impaired in immature CD4+CD8+ thymocytes relative to their mature T cell progeny. In particular, the initial association of TCR alpha with TCR beta proteins, which occurs relatively efficiently in mature T cells, is markedly inefficient in immature CD4+CD8+ thymocytes, even for a matched pair of transgenic TCR alpha and TCR beta proteins. Inefficient formation of TCR alpha beta heterodimers in immature CD4+CD8+ thymocytes was found to result from the unique instability of nascent TCR alpha proteins within the ER of immature CD4+CD8+ thymocytes, with nascent TCR alpha proteins having a median survival time of only 15 min in CD4+CD8+ thymocytes, but > 75 min in mature T cells. Thus, these data demonstrate that stability of TCR alpha proteins within the ER is developmentally regulated and provide a molecular basis for quantitative differences in alpha beta TCR expression on immature and mature T cells. In addition, these results provide the first example of a receptor complex whose expression is quantitatively regulated during development by post-translational limitations on receptor assembly. 相似文献
266.
Richard Wales Hazel C. Gorham Khalid Hussain Lynne M. Roberts J. Michael Lord 《Glycoconjugate journal》1994,11(4):274-281
Deleted forms of ricin B chain (RTB) containing only one of the two galactose binding sites were produced inE. coli and targeted to the periplasm by fusion to theompA orompF signal sequences. The proteins were then isolated from the periplasm and their sugar binding properties assessed. Previous studies investigating the properties of such proteins produced inXenopus laevis oocytes suggested that deleted forms of RTB, when not glycosylated, retain their ability to bind simple sugars, unlike the full-length unglycosylated proteins. When produced inE. coli however we found that only one, EB733, of a number of deleted forms of RTB closely related to those previously produced inXenopus laevis oocytes, bound to simple sugars. All of the deletion forms of RTB were found to bind in the asialofetuin binding assay; an assay which has been previously utilized to measure binding of lectins to the terminal galactose residues of glycoprotein oligosaccharides. However, in contrast to glycosylated RTB, binding of the deletion mutants could be competed to only a small degree or not at all with galactose. The only deletion mutant observed to bind to free galactose when produced inE. coli corresponded closely to the complete domain 2 of RTB. It is assumed that this mutant forms a stable structure similar to that of the C-terminal domain in the full-length protein. The structural integrity of EB733 was not only suggested by its sugar binding properties and solubility but also by its consistently higher level of expression and the absence of any apparent susceptibility toE. coli proteases.Abbreviations RTA
ricin toxin A chain
- RTB
ricin toxin B chain
- ER
endoplasmic reticulum
- SDS-PAGE
sodium dodecyl sulphate-polyacrylamide gel electrophoresis
- IPTG
isopropyl -d-thiogalactopyranoside 相似文献
267.
REBASE is a comprehensive database of information about restriction enzymes and their associated methylases, including their recognition and cleavage sites and their commercial availability. Information from REBASE is available via monthly electronic mailings as well as via WAIS and anonymous ftp. Specialized files are available that can be used directly by many software packages. 相似文献
268.
Osteogenesis imperfecta type I: molecular heterogeneity for COL1A1 null alleles of type I collagen. 总被引:11,自引:4,他引:7
下载免费PDF全文
![点击此处可从《American journal of human genetics》网站下载免费的PDF全文](/ch/ext_images/free.gif)
M. C. Willing S. P. Deschenes D. A. Scott P. H. Byers R. L. Slayton S. H. Pitts H. Arikat E. J. Roberts 《American journal of human genetics》1994,55(4):638-647
Osteogenesis imperfecta (OI) type I is the mildest form of inherited brittle-bone disease. Dermal fibroblasts from most affected individuals produce about half the usual amount of type I procollagen, as a result of a COL1A1 "null" allele. Using PCR amplification of genomic DNA from affected individuals, followed by denaturing gradient gel electrophoresis (DGGE) and SSCP, we identified seven different COL1A1 gene mutations in eight unrelated families with OI type I. Three families have single nucleotide substitutions that alter 5' donor splice sites; two of these unrelated families have the same mutation. One family has a point mutation, in an exon, that creates a premature termination codon, and four have small deletions or insertions, within exons, that create translational frameshifts and new termination codons downstream of the mutation sites. Each mutation leads to both marked reduction in steady-state levels of mRNA from the mutant allele and a quantitative decrease in type I procollagen production. Our data demonstrate that different molecular mechanisms that have the same effect on type I collagen production result in the same clinical phenotype. 相似文献
269.
Temporal and spatial patterns of palisade mesophyll cell expansion in Zinnia elegans were characterized as a basis for developing a suspension culture model for mesophyll cell expansion. Our objectives were to 1) identify the leaf regions from which cells in various stages of expansion could be selectively isolated for culture, and 2) develop a basis for comparison of rate and extent of mesophyll cell expansion in culture with that in the leaf. Palisade mesophyll cells were isolated from expanding leaves by gentle physical maceration without the use of enzymes. Isolated cells from leaves in different stages of expansion were then measured by computer image analysis. Analysis of size frequency distributions showed that unexpanded cells can be isolated from the entire blade of small leaves or the basal regions of partially expanded leaves. Fully expanded cells can be obtained from the apical and middle regions of partially expanded leaves. Within the leaf, Zinnia mesophyll cells expanded from about 400 μm2 to about 2.300 μm2 at an estimated rate of 160 μm2 d-1. The percent increase in cell length exceeded the percent increase in cell width. Expansion of mesophyll cells continued for 6–8 d after epidermal expansion ceased. This difference in the timing of cell expansion in epidermal and mesophyll cells indicates that different regulatory factors may be operating in these adjacent tissues and underscores the importance of investigating the regulation of mesophyll cell expansion at the cellular level. 相似文献
270.
Halotolerance of Methanobacterium thermoautotrophicum delta H and Marburg. 总被引:1,自引:1,他引:0
下载免费PDF全文
![点击此处可从《Journal of bacteriology》网站下载免费的PDF全文](/ch/ext_images/free.gif)
R Ciulla C Clougherty N Belay S Krishnan C Zhou D Byrd M F Roberts 《Journal of bacteriology》1994,176(11):3177-3187
Methanobacterium thermoautotrophicum delta H and Marburg were adapted to grow in medium containing up to 0.65 M NaCl. From 0.01 to 0.5 M NaCl, there was a lag before cell growth which increased with increasing external NaCl. The effect of NaCl on methane production was not significant once the cells began to grow. Intracellular solutes were monitored by nuclear magnetic resonance (NMR) spectroscopy as a function of osmotic stress. In the delta H strain, the major intracellular small organic solutes, cyclic-2,3-diphosphoglycerate and glutamate, increased at most twofold between 0.01 and 0.4 M NaCl and decreased when the external NaCl was 0.5 M. M. thermoautotrophicum Marburg similarly showed a decrease in solute (cyclic-2,3-diphosphoglycerate, 1,3,4,6-tetracarboxyhexane, and L-alpha-glutamate) concentrations for cells grown in medium containing > 0.5 M NaCl. At 0.65 M NaCl, a new organic solute, which was visible in only trace amounts at the lower NaCl concentrations, became the dominant solute. Intracellular potassium in the delta H strain, detected by atomic absorption and 39K NMR, was roughly constant between 0.01 and 0.4 M and then decreased as the external NaCl increased further. The high intracellular K+ was balanced by the negative charges of the organic osmolytes. At the higher external salt concentrations, it is suggested that Na+ and possibly Cl- ions are internalized to provide osmotic balance. A striking difference of strain Marburg from strain delta H was that yeast extract facilitated growth in high-NaCl-containing medium. The yeast extract supplied only trace NMR-detectable solutes (e.g., betaine) but had a large effect on endogenous glutamate levels, which were significantly decreased. Exogenous choline and glycine, instead of yeast extract, also aided growth in NaCl-containing media. Both solutes were internalized with the choline converted to betaine; the contribution to osmotic balance of these species was 20 to 25% of the total small-molecule pool. These results indicate that M. thermoautotrophicum shows little changes in its internal solutes over a wide range of external NaCl. Furthermore, they illustrate the considerable differences in physiology in the delta H and Marburg strains of this organism. 相似文献