全文获取类型
收费全文 | 3885篇 |
免费 | 127篇 |
国内免费 | 379篇 |
专业分类
4391篇 |
出版年
2024年 | 15篇 |
2023年 | 51篇 |
2022年 | 59篇 |
2021年 | 70篇 |
2020年 | 63篇 |
2019年 | 94篇 |
2018年 | 60篇 |
2017年 | 57篇 |
2016年 | 52篇 |
2015年 | 73篇 |
2014年 | 104篇 |
2013年 | 185篇 |
2012年 | 111篇 |
2011年 | 118篇 |
2010年 | 92篇 |
2009年 | 171篇 |
2008年 | 192篇 |
2007年 | 222篇 |
2006年 | 203篇 |
2005年 | 217篇 |
2004年 | 220篇 |
2003年 | 165篇 |
2002年 | 155篇 |
2001年 | 140篇 |
2000年 | 156篇 |
1999年 | 119篇 |
1998年 | 106篇 |
1997年 | 90篇 |
1996年 | 97篇 |
1995年 | 94篇 |
1994年 | 85篇 |
1993年 | 74篇 |
1992年 | 97篇 |
1991年 | 79篇 |
1990年 | 49篇 |
1989年 | 54篇 |
1988年 | 49篇 |
1987年 | 58篇 |
1986年 | 37篇 |
1985年 | 48篇 |
1984年 | 46篇 |
1983年 | 15篇 |
1982年 | 24篇 |
1981年 | 22篇 |
1980年 | 24篇 |
1979年 | 26篇 |
1978年 | 20篇 |
1977年 | 6篇 |
1976年 | 11篇 |
1974年 | 6篇 |
排序方式: 共有4391条查询结果,搜索用时 11 毫秒
31.
Dr. Hans-Joachim Gabius Katalin Vehmeyer Reinhild Engelhardt Gerd A. Nagel Friedrich Cramer 《Cell and tissue research》1986,246(3):515-521
Summary A tumor model system of clones of myeloproliferative sarcoma virus (MPV)-transformed rat fibroblasts (NRK) with different growth properties and metastatic potential was studied. The relationship between metastatic behavior and composition of carbohydrate-binding proteins (lectins) was analyzed by affinity chromatography. The metastatic variant differs qualitatively from its parental clone in the presence of galactoside-binding proteins at apparent molecular weights of 80 kDa, 70 kDa, 22 kDa, 18 kDa and 16 kDa and of a fucose-binding protein at apparent molecular weight of 42 kDa. The -glucosyl-binding proteins at apparent molecular weights of 67 kDa and 53 kDa and a galactoside-binding protein of apparent molecular weight of 34 kDa, however, are not detectable in the metastatic variant in comparison to its parental clone. In this respect the parental clone shows closer resemblance to the clone 5–8#1 with different growth properties and low metastatic potential than to its own metastatic variant. Furthermore, only the parental clone has a melibiose- and a mannan-binding protein of an apparent molecular weight of 64 kDa and 14 kDa, respectively. Rosette formation as model system for intercellular interaction reveals differences in the inhibition pattern with sugar between the two clones 5–8#1 and 5–20#20, whereas the metastatic variant 5–20#20 (s) exhibits drastically reduced capability to form rosettes. Initial experiments demonstrate the feasibility of drug targeting to transformed fibroblasts via carbohydrate-binding proteins. 相似文献
32.
Summary Polyps of mature colonies of Hydractinia echinata obey the rule of distal transformation by regenerating heads but not stolons. However, this rule is not valid for young polyps as these regenerate stolons from proximal cut ends. Also, small cell aggregates and even small fragments excised from full-grown polyps are capable of stolon formation. Aggregates produced from dissociated cells undergo either distal or proximal transformation depending on their size, speed of head regeneration in the donor used for dissociation and the positional derivation of the cells. The latent capability of stolon formation is released under conditions that cause loss of morphogens and depletion of their sources. However, internal regulative processes can also lead to gradual proximal transformation: regenerating segments of polyps sometimes form heads at both ends and the distal pattern is duplicated. Subsequently, all sets of proximal structures, including stolons, are intercalated. In contrast to distal transformation, proximal transformation is a process the velocity of which declines with the age and size of the cell community. 相似文献
33.
Examination of dispersional characteristics of Pratylenchus scribneri and Hoplolaimus galeatus indicated that there were patches within soybean fields in which both survival and reproduction wexe enhanced in spite of apparent homogeneity of soil type and topography. Treatment with carbofuran reduced the patchiness (or increased the dispersion) for H. galeatus while it had the opposite effect for P. scribneri. P. scribneri was less highly dispersed in conventional tillage plots than in the zero tillage plots. Populations from quadrats contained entirely within the patches could be described by the normal distribution (in the case of P. scribneri) or by the Poisson distribution (in the case of H. galeatus), while populations from quadrats contained entirely outside the patches could be described by the Poisson distribution for both nematodes. None of the distributions tested (Poisson, normal, negative binomial, Neyman''s) gave an adequate fit when populations from both inside and outside the patches were considered together. In all instances, log₁₀ and ln transformations reduced the goodness of fit of the data to all of the distributions tested. Even with logarithmic transformations, parametric statistics were not appropriate for analysis of data in most instances. 相似文献
34.
The greening of the upper part of the outerAllium cepa L. bulb scales, in particular along the vascular regions, is limited to the hypodermal cells in which typical leucoplasts are transformed to normal and functional chloroplasts. This process is light dependent and cannot afterwards be reversed or modified by darkness. The changes in fine structure are described and briefly discussed.Dedicated to Prof. DrLothar Geitler on the occasion of his 90th birthday and 55 years after the publication of his Grundriß der Cytologie. 相似文献
35.
Plant transformation by microinjection techniques 总被引:4,自引:0,他引:4
Several techniques have been developed for introducing cloned genes into plant cells. Vectorless delivery systems such as PEG-mediated direct DNA uptake (e.g. Pasz-kowski et al. 1984), electroporation (e.g. Shillito et al. 1985), and fusion of protoplasts with liposomes (Deshayes et al. 1985) are routinely used in many experiments (see several chapters of this issue). A wide range of plant species, dicotyledonous as well as monocotyledonous, has been transformed by these vectorless DNA transfer systems. However, the availability of an efficient protoplast regeneration system is a prerequisite for the application of these techniques. For cells with intact cell walls and tissue explants the biological delivery system of virulent Agrobacterium species has been routinely used (for review see Fraley et al. 1986). However, the host range of Agrobacterium restricts the plant species, which can be transformed using this vector system. In addition, all these methods depend on selection systems for recovery of transformants. Therefore a selection system has to be established first for plant species to be transformed. The microinjection technique is a direct physical approach, and therefore host-range independent, for introducing substances under microscopical control into defined cells without damaging them. These two facts differentiate this technique from other physical approaches, such as biolistic transformation and macroinjection (see chapters in this issue). In these other techniques, damaging of cells and random manipulation of cells without optical control cannot be avoided so far. In recent years microinjection technology found its application in plant sciences, whereas this technique has earlier been well established for transformation of animal tissue culture cells (Capecchi 1980) and the production of transgenic animals (Brin-ster et al. 1981, Rusconi and Schaffner 1981). Furthermore, different parameters affecting the DNA transfer via microinjection, such as the nature of microinjected DNA, and cell cycle stage, etc, have been investigated extensively in animal cells (Folger et al. 1982, Wong and Capecchi 1985), while analogous experiments on plant cells are still lacking. 相似文献
36.
Elizabeth E. Hood David H. Clapham Inger Ekberg Thomas Johannson 《Plant molecular biology》1990,14(2):111-117
The hypervirulent Agrobacterium tumefaciens strain A281 formed frequent tumors (31%) on Picea abies (Norway spruce), an economically important tree species in Swedish forests. Three-month-old seedlings were inoculated and tumors were established that grew hormone-independently in culture. Tumors contained agropine and mannopine/mannopinic acid as determined by acid pH paper electrophoresis. In addition, DNA hybridization studies showed that the DNA from these tumor lines contained sequences homologous to Ti plasmid T-DNA, whereas wild-type spruce seedling DNA did not. These results suggest that Agrobacterium vectors can be used for gene transfer into this important forest species. 相似文献
37.
We report here an efficient and highly reproducible delivery system, using an improved biolistic transformation device, that facilitates transient expression of -glucuronidase (GUS) in chloroplasts of cultured tobacco suspension cells. Cultured tobacco cells collected on filter papers were bombarded with tungsten particles coated with pUC118 or pBI101.3 (negative controls), pBI505 (positive nuclear control) or a chloroplast expression vector (pHD203-GUS), and were assayed for GUS activity. No GUS activity was detected in cells bombarded with pUC118 or pBI101.3. Cells bombarded with pBI505 showed high levels of expression with blue color being distributed evenly throughout the whole cytosol of the transformants. pHD203-GUS was expressed exclusively in chloroplasts. We base this conclusion on: i) the procaryotic nature of the promoter used in the chloroplast expression vector; ii) delayed GUS staining; iii) localization of blue color within subcellular compartments corresponding to plastids in both shape and size; and iv) confirmation of organelle-specific expression of pHD203-GUS using PEG-mediated protoplast transformation. Chloroplast transformation efficiencies increased dramatically (about 200-fold) using an improved helium-driven biolistic device, as compared to the more commonly used gun powder charge-driven device. Using GUS as a reporter gene and the improved biolistic device, optimal bombardment conditions were established, consistently producing several hundred transient chloroplast transformants per Petri plate. Chloroplast transformation efficiency was found to be increased further (20-fold) with supplemental osmoticum (0.55 M sorbitol and 0.55 M mannitol) in the bombardment and incubation medium. This system provides a highly effective mechanism for introducing and expressing plasmid DNA within higher-plant chloroplasts, and the fact that GUS functions as an effective marker gene now makes many genetic studies possible which were not possible before. 相似文献
38.
A wound-inducible proteinase Inhibitor I gene from tomato containing 725 bp of the 5 region and 2.5 kbp of the 3 region was stably incorporated into the genome of black nightshade plants (Solanum nigrum) using an Agrobacterium Ti plasmid-derived vector. Transgenic nightshade plants were selected that expressed the tomato Inhibitor I protein in leaf tissue. The leaves of the plants contained constitutive levels of the inhibitor protein of up to 60 g/g tissue. These levels increased by a factor of about two in response to severe wounding. Only leaves and petioles exhibited the presence of the inhibitor, indicating that the gene exhibited the same tissue specificity of expression found in situ in wounded tomato leaves. Inhibitor I was extracted from leaves of wounded transformed nightshade plants and was partially purified by affinity chromatography on a chymotrypsin-Sepharose column. The affinity-purified protein was identical to the native tomato Inhibitor I in its immunological reactivity and in its inhibitory activity against chymotrypsin. The protein exhibited the same M
r of 8 kDa as the native tomato Inhibitor I and its N-terminal amino acid sequence was identical to that of the native tomato inhibitor I, indicating that the protein was properly processed in nightshade plants. These expriments are the first report of the expression of a member of the wound-inducible tomato Inhibitor I gene family in transgenic plants. The results demonstrate that the gene contains elements that can be regulated in a wound-inducible, tissuespecific manner in nightshade plants. 相似文献
39.
A region (NS1) that acts like an enhancer is located approximately 300 bp upstream of the larval cap site in theAdh gene ofD. melanogaster. When this sequence is deleted (NS1), the gene fails to express ADH protein. Gene expression can be restored by placing a secondAdh gene with an intact enhancer elsewhere on the same plasmid. In these circumstances, both genes are expressed equally regardless of their orientation on the plasmid. In this report we further characterize the interactions that occur when a single enhancer activates expression from a proximal and distant promoter. We have made the following observations: (1) While the two genes are expressed equivalently, their expression relative to a plasmid carrying two intact genes is reduced by a factor of 2 to 6 depending on the orientation of the two genes. (2) The single enhancer drives expression of both genes on any given plasmid molecule. (3) The enhancer does not interact with theAdh gene from which the NS7 region (which spans the larval TATA box) is removed. (4) Expression of the NS1 gene can be restored by an intact gene when both are inserted together into theDrosophila genome via P element-mediated transformation. (5) Increasing the separation between the two genes on a plasmid by up to 15 kbp does not prevent the restoration of expression of the NS1 gene. We propose a model that explains how a single enhancer can stimulate equal expression from two genes. 相似文献
40.
Optimization of biolistic method for transient gene expression and production of agronomically useful transgenic Basmati rice plants 总被引:4,自引:0,他引:4
We have developed a reproducible biolistic procedure for the efficient transformation of embryogenic suspension cells of an improved aromatic Indica rice variety, Pusa Basmati 1. The -glucuronidase gene was used to assay transient transformation; other plasmids carrying either a potato protease inhibitor 2 (Pin2) gene, or a late embryogenesis-abundant protein (LEA3) gene from barley, were used for the optimization of biolistic process and transgenic plant production. After optimization of the procedure, over 600 transient transformants and at least five fertile plants showing integrative transformation were obtained per bombarded filter. At least 30% of the plants were derived from independent transformation events. The new improved procedure involves the use of a reporter gene or other useful genes driven by the strong rice actin 1 gene (Act1) promoter, osmotic pre-conditioning of cells for 24 h on medium supplemented with 0.25 M mannitol prior to bombardment, use of gold particles for DNA delivery, and use of plant regeneration medium with high (1.0%) agarose concentration. 相似文献