全文获取类型
收费全文 | 149篇 |
免费 | 37篇 |
出版年
2022年 | 2篇 |
2021年 | 2篇 |
2020年 | 1篇 |
2019年 | 5篇 |
2018年 | 1篇 |
2017年 | 8篇 |
2016年 | 6篇 |
2015年 | 9篇 |
2014年 | 10篇 |
2013年 | 7篇 |
2012年 | 13篇 |
2011年 | 11篇 |
2010年 | 6篇 |
2009年 | 2篇 |
2008年 | 8篇 |
2007年 | 5篇 |
2006年 | 6篇 |
2005年 | 2篇 |
2004年 | 1篇 |
2003年 | 2篇 |
2002年 | 1篇 |
2001年 | 3篇 |
2000年 | 6篇 |
1999年 | 5篇 |
1998年 | 8篇 |
1997年 | 4篇 |
1996年 | 2篇 |
1995年 | 1篇 |
1994年 | 3篇 |
1993年 | 1篇 |
1992年 | 6篇 |
1991年 | 6篇 |
1989年 | 3篇 |
1988年 | 2篇 |
1987年 | 2篇 |
1986年 | 1篇 |
1981年 | 3篇 |
1979年 | 3篇 |
1977年 | 1篇 |
1976年 | 1篇 |
1975年 | 7篇 |
1973年 | 1篇 |
1972年 | 1篇 |
1971年 | 2篇 |
1970年 | 1篇 |
1969年 | 2篇 |
1967年 | 2篇 |
1954年 | 1篇 |
排序方式: 共有186条查询结果,搜索用时 203 毫秒
1.
In a variety of tumour systems, individuals carrying progressively growing neoplasms have lymphoid cells with a specific cytotoxic effect on cultured tumour cells from the same individual1–4. Since the sera of tumour-bearing individuals have been shown to prevent tumour cell destruction by immune lymphocytes in vitro2,5–8 and since this serum blocking activity appears early in primary and transplant tumour development5,7, it has been suggested that the appearance of this serum blocking activity might be responsible for the progressive growth of tumours in individuals having cytotoxic lymphocytes. Counteraction of this blocking activity would thus be of primary importance in facilitating the function of an already existing or bolstered cell-mediated immunity. The serum blocking activity might be inhibited in various ways, by preventing the formation of blocking antibody or by interfering with its action (“unblocking”), as demonstrated in Moloney sarcoma regressor sera9. This type of serum also has a therapeutic effect on Moloney sarcomas in vivo10,11, which has been tentatively attributed to its unblocking activity8,9 or, possibly, to a complement-dependent cytotoxicity10. Tumour growth in the Moloney sarcoma system, however, might be due in part to continuous recruitment of neoplastic cells by virus-induced transformation and so the therapeutic effect could be due to a virus-neutralizing serum activity9,10. 相似文献
2.
Kinetics of Induction of the Lactose Operon on an Episome in Salmonella typhimurium 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)
A. Ballesteros-Olmo John S. Kovach Peter Van Knippenberg Robert F. Goldberger 《Journal of bacteriology》1969,98(3):1232-1238
A kinetic study of induction of the enzymes of the lactose operon was carried out under conditions known to affect the kinetics of derepression of the enzymes of the histidine operon. The results show that the lactose system is similar to the histidine system in its responsiveness to conditions thought to affect the formylating capacity of the cell. This was demonstrated in the following ways: (i) trimethoprim, which is known to reduce the formylating capacity of the cell, gives rise to a relatively long interval between the times of induction of beta-galactosidase and transacetylase; (ii) under conditions in which the histidine operon is derepressed, chloramphenicol causes a prolongation of the interval between the times of induction of the two enzymes, and this prolongation is reversed by adenine, methionine, and serine, compounds known to enrich the one-carbon pool of the cell; and (iii) 4-amino-5-imidazolcarboxamide ribonucleoside, a compound which may act as a drain for formyl groups, reverses the effect of the latter compounds. The finding that the interval between the times of induction of the two enzymes is shortened under conditions expected to maintain a relatively high intracellular fo rmylating capacity suggests that under certain conditions translation of the polycistronic messenger ribonucleic acid of the lactose operon may be initiated at more than one site or may proceed more rapidly from the operator end. 相似文献
3.
Development of Desiccation Tolerance during Embryogenesis in Rice (Oryza sativa) and Wild Rice (Zizania palustris) (Dehydrin Expression,Abscisic Acid Content,and Sucrose Accumulation) 总被引:3,自引:1,他引:2
下载免费PDF全文
![点击此处可从《Plant physiology》网站下载免费的PDF全文](/ch/ext_images/free.gif)
The ability of seeds to withstand desiccation develops during embryogenesis and differs considerably among species. Paddy rice (Oryza sativa L.) grains readily survive dehydration to as low as 2% water content, whereas North American wild rice (Zizania palustris var interior [Fasset] Dore) grains are not tolerant of water contents below 6% and are sensitive to drying and imbibition conditions. During embryogenesis, dehydrin proteins, abscisic acid (ABA), and saccharides are synthesized, and all have been implicated in the development of desiccation tolerance. We examined the accumulation patterns of dehydrin protein, ABA, and soluble saccharides (sucrose and oligosaccharides) of rice embryos and wild rice axes in relation to the development of desiccation tolerance during embryogenesis. Dehydrin protein was detected immunologically with an antibody raised against a conserved dehydrin amino acid sequence. Both rice and wild rice embryos accumulated a 21-kD dehydrin protein during development, and an immunologically related 38-kD protein accumulated similarly in rice. Dehydrin protein synthesis was detected before desiccation tolerance had developed in both rice embryos and wild rice axes. However, the major accumulation of dehydrin occurred after most seeds of both species had become desiccation tolerant. ABA accumulated in wild rice axes to about twice the amount present in rice embryos. There were no obvious relationships between ABA and the temporal expression patterns of dehydrin protein in either rice or wild rice. Wild rice axes accumulated about twice as much sucrose as rice embryos. Oligosaccharides were present at only about one-tenth of the maximum sucrose concentrations in both rice and wild rice. We conclude that the desiccation sensitivity displayed by wild rice grains is not due to an inability to synthesize dehydrin proteins, ABA, or soluble carbohydrates. 相似文献
4.
On the mechanism of ATP-induced shape changes in the human erythrocyte membranes: the role of ATP
下载免费PDF全文
![点击此处可从《The Journal of cell biology》网站下载免费的PDF全文](/ch/ext_images/free.gif)
In the preceding paper (Sheetz, M. and S.J. Singer. 1977. J Cell Biol. 73:638-646) it was shown that erythrocyte ghosts undergo pronounced shape changes in the presence of mg-ATP. The biochemical effects of the action of ATP are herein examined. The biochemical effects of the action of ATP are herein examined. Phosphorylation by ATP of spectrin component 2 of the erythrocyte membrane is known to occur. We have shown that it is only membrane protein that is significantly phosphorylated under the conditions where the shape changes are produced. The extent of this phosphorylation rises with increasing ATP concentration, reaching nearly 1 mol phosphoryle group per mole of component 2 at 8mM ATP. Most of this phosphorylation appears to occur at a single site on the protein molecule, according to cyanogen bromide peptide cleavage experiments. The degree of phosphorylation of component 2 is apparently also regulated by a membrane-bound protein phosphatase. This activity can be demonstrated in erythrocyte ghosts prepared from intact cells prelabeled with [(32)P]phosphate. In addition to the phosphorylation of component 2, some phosphorylation of lipids, mainly of phosphatidylinositol, is also known to occur. The ghost shape changes are, however, shown to be correlated with the degree of phosphorylation of component 2. In such experiment, the incorporation of exogenous phosphatases into ghosts reversed the shape changes produced by ATP, or by the membrane-intercalating drug chlorpromazine. The results obtained in this and the preceding paper are consistent with the proposal that the erythrocyte membrane possesses kinase and phosphates activities which produce phosphorylation and dephosphorylation of a specific site on spectrin component 2 molecules; the steady-state level of this phosphorylation regulates the structural state of the spectrin complex on the cytoplasmic surface of the membrane, which in turn exerts an important control on the shape of the cell. 相似文献
5.
Irwin A. Schafer Maureen Kovach Robert L. Price Richard B. Fratianne 《Experimental cell research》1991,195(2)
Single cell suspensions of human keratinocytes when seeded onto floating three-dimensional gels constructed with type I collagen form a tissue resembling epidermis. These morphogenetic events occur in a serum-free environment in the absence of fibroblasts. Light and transmission electron microscopy show that cells form a basal layer plus suprabasilar cell layers corresponding to the stratum spinosum, stratum granulosum, and stratum corneum. The suprabasilar keratinocyte layers show morphologies which resemble intact skin in which cells are connected by desmosomes and contain intermediate filaments and keratohyalin-fillagrin granules. The basal cell layer differs from skin in vivo in that there is no connection to a basement membrane via hemidesmosomes. Cells in the basal layers are polarized as evidenced by the secretion of type IV collagen, heparan sulfate proteoglycans, and laminin at the cell membrane interface with the collagen gel. These proteins are not organized into a cytological basement membrane. Bullous pemphigoid antigen, a protein component of hemidesmosomes, is synthesized by basal keratinocytes, but like the basement membrane proteins it is not incorporated into a definable cytological structure. Keratinocytes in the basal and suprabasilar layers also synthesize α2β1 integrins. The mechanisms of keratinocyte adhesion to the gel may be through the interactions of this cell surface receptor with laminin and type IV collagen synthesized by the cell and/or direct interactions between the receptor and type I collagen within the gel. This in vitro experimental system is a useful model for defining the molecular events which control the formation and turnover of basement membranes and the mechanisms by which keratinocytes adhere to type I collagen when sheets of keratinocytes are used clinically for wound coverage. 相似文献
6.
Gold salts and phenylbutazone selectively inhibit the synthesis of PGF2α and PGE2 respectively. Lowered production of one prostaglandin species is accompanied by an increased production of the other. Selective inhibition by these drugs was observed in the presence of adrenaline, reduced glutathione and copper sulphate under conditions when most anti-inflammatory compounds inhibited PGE2 and PGF2α syntheses equally. It is postulated that selective inhibitors may have a different mode of action
and beneficial effects may be related to the endogenous ratio of PGE to PGF required for normal function. 相似文献
7.
Christopher J Kavanagh Geoffrey W Swain Brett S Kovach Judith Stein Christina Darkangelo-Wood Kathryn Truby 《Biofouling》2013,29(6):381-390
Barnacle adhesion strength was used to screen seventy-seven polydimethylsiloxane elastomeric coatings for fouling-release properties. The test coatings were designed to investigate the effect on barnacle adhesion strength of silicone fluid additive type, additive location, additive molecular weight, additive loading level, mixtures of additives, coating matrix type and coating fillers. The type of silicone fluid additive was the primary controlling factor in barnacle fouling-release. The type of silicone matrix in which the fluid resided was found to alter the effect on fouling-release. Two PDMS fluids, DMSC15 and DBE224, significantly reduced the adhesion strength of barnacles compared to unmodified elastomers. Optimum fouling-release performance was dependent on the interaction of fluid type and elastomeric matrix. 相似文献
8.
Climate-induced phenological shifts can influence population, evolutionary, and ecological dynamics, but our understanding of these phenomena is hampered by a lack of long-term demographic data. We use a multi-decade census of 5 salmonid species representing 14 life histories in a warming Alaskan stream to address the following key questions about climate change and phenology: How consistent are temporal patterns and drivers of phenology for similar species and alternative life histories? Are shifts in phenology associated with changes in phenotypic variation? How do phenological changes influence the availability of resource subsidies? For most salmonid species, life stages, and life histories, freshwater temperature influences migration timing – migration events are occurring earlier in time (mean = 1.7 days earlier per decade over the 3–5 decades), and the number of days over which migration events occur is decreasing (mean = 1.5 days per decade). Temporal trends in migration timing were not correlated with changes in intra-annual phenotypic variation, suggesting that these components of the phenotypic distribution have responded to environmental change independently. Despite commonalities across species and life histories, there was important biocomplexity in the form of disparate shifts in migration timing and variation in the environmental factors influencing migration timing for alternative life history strategies in the same population. Overall, adult populations have been stable during these phenotypic and environmental changes (λ ≈1.0), but the temporal availability of salmon as a resource in freshwater has decreased by nearly 30 days since 1971 due to changes in the median date of migration timing and decreases in intra-annual variation in migration timing. These novel observations advance our understanding of phenological change in response to climate warming, and indicate that climate change has influenced the ecology of salmon populations, which will have important consequences for the numerous species that depend on this resource. 相似文献
9.
Genotype‐environment associations support a mosaic hybrid zone between two tidal marsh birds
下载免费PDF全文
![点击此处可从《Ecology and evolution》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Jennifer Walsh Rebecca J. Rowe Brian J. Olsen W. Gregory Shriver Adrienne I. Kovach 《Ecology and evolution》2016,6(1):279-294
Local environmental features can shape hybrid zone dynamics when hybrids are bounded by ecotones or when patchily distributed habitat types lead to a corresponding mosaic of genotypes. We investigated the role of marsh‐level characteristics in shaping a hybrid zone between two recently diverged avian taxa – Saltmarsh (Ammodramus caudacutus) and Nelson's (A. nelsoni) sparrows. These species occupy different niches where allopatric, with caudacutus restricted to coastal marshes and nelsoni found in a broader array of wetland and grassland habitats and co‐occur in tidal marshes in sympatry. We determined the influence of habitat types on the distribution of pure and hybrid sparrows and assessed the degree of overlap in the ecological niche of each taxon. To do this, we sampled and genotyped 305 sparrows from 34 marshes across the hybrid zone and from adjacent regions. We used linear regression to test for associations between marsh characteristics and the distribution of pure and admixed sparrows. We found a positive correlation between genotype and environmental variables with a patchy distribution of genotypes and habitats across the hybrid zone. Ecological niche models suggest that the hybrid niche was more similar to that of A. nelsoni and habitat suitability was influenced strongly by distance from coastline. Our results support a mosaic model of hybrid zone maintenance, suggesting a role for local environmental features in shaping the distribution and frequency of pure species and hybrids across space. 相似文献
10.