首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   177篇
  免费   7篇
  2022年   1篇
  2021年   1篇
  2020年   2篇
  2019年   2篇
  2018年   4篇
  2017年   5篇
  2016年   3篇
  2015年   8篇
  2014年   8篇
  2013年   5篇
  2012年   13篇
  2011年   9篇
  2010年   9篇
  2009年   5篇
  2008年   5篇
  2007年   4篇
  2006年   9篇
  2005年   4篇
  2004年   2篇
  2003年   11篇
  2002年   4篇
  2001年   12篇
  2000年   4篇
  1999年   3篇
  1998年   3篇
  1997年   2篇
  1996年   3篇
  1992年   4篇
  1991年   5篇
  1990年   4篇
  1989年   2篇
  1988年   5篇
  1986年   2篇
  1985年   3篇
  1984年   2篇
  1983年   3篇
  1980年   2篇
  1979年   2篇
  1978年   2篇
  1977年   1篇
  1975年   2篇
  1973年   2篇
  1971年   1篇
  1969年   1篇
排序方式: 共有184条查询结果,搜索用时 31 毫秒
121.
122.
The aim of the current study was to characterize Polish Black-and-White cattle by morphological study of the Y chromosome. A total of 14 Y-linked microsatellites from UMN and INRA group were genotyped and assessed for polymorphism in a total 22 bulls. Cytogenetic studies in Polish Black-and-White bulls showed the existence of two morphological forms of Y chromosome. Among the 22 karyotypic analyzed bulls, 12 had submetacentric and 10 metacentric Y chromosome. The centromeric index of Y chromosome measured as percentage length of the p arm to total length ratio in the first case was 28 ± 3.97% and in the second 47 ± 7.28%, whereas the relative size of these chromosomes remained within the same range. Morphology and G- and C-banding patterns of both forms of Y chromosome were typical for other cattle breeds originating from Bos taurus. Out of a total of 14 microsatellite loci examined, 13 showed specific alleles for two forms of Y chromosome. In a pool of 62 alleles, 43 (69.3%) were common in the two groups of cattle, 19 (30.7%) can be considered as specific for the group; among them 8 were typical for metacentric group of Y chromosome and 11 for submetacentric.  相似文献   
123.
Because our previous studies (Snell, W.J., and W.S. Moore, 1980, J. Cell Biol. 84:203- 210) on the mating reaction of chlamydomonas reinhardtii showed that there was an adhesion-induced turnover of proteins whose synthesis is induced during aggregation. Analysis by SDS PAGE and autoradiography showed that proteins of 220,000 M(r) and 165, 000 M(r) (designated A(1) and A(2) respectively) consistently showed a high rate of synthesis only in flagella or flagellar membrane-enriched fractions prepared from aggregating gametes. Since the two proteins were soluble in the non-ionic detergent NP-40 and were removed from intact cells by a brief pronase treatment, it is likely that A(1) and A(2) are membrane proteins expose on the cell surface. A(1) and A(2) were each synthesized by gametes of both mating types (mt(-) and mt(+)) and synthesis of these two proteins could be detected in the normal mating reaction (wild type mt(-) and mt(+)), in mixtures of mt(-) and impotent mt(+) gametes (which could aggregate but not fuse), and in mixtures of gametes of a single mating type with isolated flagella of the opposite mating type. Cells aggregating in tunicamycin, an inhibitor of protein glycosylation, lost their adhesiveness during aggregation and did not synthesize the 220,000 M(r) protein but instead produced a protein (possibly an underglycosylated form of A(1)) of slightly lower mol wt. The 220,000 and 165,000 M(R) proteins appeared to be flagellar proteins and not cell wall proteins because A(1) and A(2) did not co-migrate with previously identified cell wall proteins, and synthesis of the two proteins could not be detected in flagella-less (bald-2) mutant cells. Analysis of the adhesive activity of sucrose gradient fraction of detergent (octyl glucoside)-solubilized flagellar membranes revealed that fractions containing A(1) and A(2) did not have detectable adhesive activity. The possibility remains that A(1) and A(2) are adhesion molecules whose activity could not be measured in the assay we used. Alternatively, the 220,000 and 165,000 M(r) proteins may be inactivated adhesion molecules or else they may be flagellar surface proteins involved only indirectly in the adhesion process.  相似文献   
124.
Measurements of the transepithelial electrical resistance correlated with freeze-fracture observations have been used to study the process of tight junction formation under various experimental conditions in monolayers of the canine kidney epithelial cell line MDCK. Cells derived from previously confluent cultures and plated immediately after trypsin- EDTA dissociation develop a resistance that reaches its maximum value of several hundred ohms-cm(2) after approximately 24 h and falls to a steady-state value of 80-150 ohms- cm(2) by 48 h. The rise in resistance and the development of tight junctions can be completely and reversibly prevented by the addition of 10 μg/ml cycloheximide at the time of plating, but not when this inhibitor is added more than 10 h after planting. Thus tight junction formation consists of separable synthetic and assembly phases. These two phases can also be dissociated and the requirement for protein synthesis after plating eliminated if, following trypsinization, the cells are maintained in spinner culture for 24 h before plating. The requirement for protein synthesis is restored, however, if cells maintained in spinner culture are treated with trypsin before plating. Actinomycin D prevents development of resistance only in monolayers formed from cells derived from sparse rather than confluent cultures, but new mRNA synthesis is not required if cells obtained from sparse cultures are maintained for 24 h in spinner culture before plating. Once a steady-state resistance has been reached, its maintenance does not require either mRNA or protein synthesis; in fact, inhibition of protein synthesis causes a rise in the resistance over a 30-h period. Following treatments that disrupt the junctions in steady- state monolayers recovery of resistance also does not require protein synthesis. These observations suggest that proteins are involved in tight junction formation. Such proteins, which do not turn over rapidly under steady-state conditions, are destroyed by trypsinization and can be resynthesized in the absence of stable cell-cell or cell-substratum contact. Messenger RNA coding for proteins involved in tight junction formation is stable except when cells are sparsely plated, and can also be synthesized without intercellular contacts or cell-substratum attachment.  相似文献   
125.
The effect of seasonal changes and ecological aspects of Afrodiplozoon polycotyleus (Paperna, 1973) collected on Labeobarbus marequensis (Smith, 1841), Enteromius trimaculatus (Peters, 1852) and Enteromius radiatus (Peters, 1853) was investigated from January to October 2008. Fish were collected at the Nwanedi-Luphephe dams, Limpopo River System, South Africa using gill, cast and seine nets and electrofishing gear. Enteromius radiatus and E. trimaculatus were the most infested compared with L. marequensis. Seasonal changes had an influence on the intensity of A. polycotyleus, with infestation rates higher in spring, summer and autumn. The adults predominantly infected the medial region of the second gill arch, whereas diporpae were mainly found on the first gill. Neither sex nor water quality changes had an influence on the intensity of the parasite. The health condition of the hosts was not affected by the parasite. The different infestation rates of the parasite between the species could be attributed to host characteristics, behaviour and habitat preferences. The selection of a particular gill/region could be attributed to space for attachment organs and food supply, as well as flow and velocity of the water over the gills. The occurrence of A. polycotyleus on E. radiatus constitutes a new host record.  相似文献   
126.
Non-uniform stress and strain fields are prevalent in many tissues in vivo, and often exacerbated by disease or injury. These mechanical gradients potentially play a role in contributing to pathological conditions, presenting a need for experimental tools to allow investigation of cell behavior within non-uniformly stimulated environments. Herein, we employ two in vitro cell-stretching devices (one previously published; one newly presented) capable of subjecting cells to cyclic, non-uniform stretches upon the surface of either a circular elastomeric membrane or a cylindrical PDMS tube. After 24 hours of cyclic stretch, 10T1/2 cells on both devices showed marked changes in long-axis orientation, with tendencies to align parallel to the direction of minimal deformation. The degree of this response varied depending on location within the stretch gradients. These results demonstrated the feasibility of conducting cell mechanobiology investigations with the two novel devices, while also highlighting the experimental capabilities of non-uniform mechanical environments for these types of studies. Such capabilities include robust data collection for developing mechanobiological dose-response curves, signal threshold identification, and potential spatial targeting for drug delivery.  相似文献   
127.
Changes in the morphology of dendritic spines are prominent during learning and in different neurological and neuropsychiatric diseases, including those in which glycogen synthase kinase-3β (GSK-3β) has been implicated. Despite much evidence of the involvement of GSK-3β in functional synaptic plasticity, it is unclear how GSK-3β controls structural synaptic plasticity (i.e., the number and shape of dendritic spines). In the present study, we used two mouse models overexpressing and lacking GSK-3β in neurons to investigate how GSK-3β affects the structural plasticity of dendritic spines. Following visualization of dendritic spines with DiI dye, we found that increasing GSK-3β activity increased the number of thin spines, whereas lacking GSK-3β increased the number of stubby spines in the dentate gyrus. Under conditions of neuronal excitation, increasing GSK-3β activity caused higher activity of extracellularly acting matrix metalloproteinase-9 (MMP-9), and MMP inhibition normalized thin spines in GSK-3β overexpressing mice. Administration of the nonspecific GSK-3β inhibitor lithium in animals with active MMP-9 and animals lacking MMP-9 revealed that GSK-3β and MMP-9 act in concert to control dendritic spine morphology. Altogether, our data demonstrate that the dysregulation of GSK-3β activity has dramatic consequences on dendritic spine morphology, implicating MMP-9 as a mediator of GSK-3β-induced synaptic alterations.  相似文献   
128.
The lipid envelope of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is an essential component of the virus; however, its molecular composition is undetermined. Addressing this knowledge gap could support the design of antiviral agents as well as further our understanding of viral-host protein interactions, infectivity, pathogenicity, and innate immune system clearance. Lipidomics revealed that the virus envelope comprised mainly phospholipids (PLs), with some cholesterol and sphingolipids, and with cholesterol/phospholipid ratio similar to lysosomes. Unlike cellular membranes, procoagulant amino-PLs were present on the external side of the viral envelope at levels exceeding those on activated platelets. Accordingly, virions directly promoted blood coagulation. To investigate whether these differences could enable selective targeting of the viral envelope in vivo, we tested whether oral rinses containing lipid-disrupting chemicals could reduce infectivity. Products containing PL-disrupting surfactants (such as cetylpyridinium chloride) met European virucidal standards in vitro; however, components that altered the critical micelle concentration reduced efficacy, and products containing essential oils, povidone-iodine, or chlorhexidine were ineffective. This result was recapitulated in vivo, where a 30-s oral rinse with cetylpyridinium chloride mouthwash eliminated live virus in the oral cavity of patients with coronavirus disease 19 for at least 1 h, whereas povidone-iodine and saline mouthwashes were ineffective. We conclude that the SARS-CoV-2 lipid envelope i) is distinct from the host plasma membrane, which may enable design of selective antiviral approaches; ii) contains exposed phosphatidylethanolamine and phosphatidylserine, which may influence thrombosis, pathogenicity, and inflammation; and iii) can be selectively targeted in vivo by specific oral rinses.  相似文献   
129.
The crystals of a 4:1 mixture of 6-(N-benzyl-N-tert-butoxycarbonylamino)-2,3,6,7-tetradeoxy-a-DL-er ythro- and -beta-DL-threo-hept-2-enopyranos-4-ulose were monoclinic, space group P2(1)/c, with cell dimensions: a = 9.490(2), b = 21.516(5), c = 10.279(2) A, beta = 115.31(1) degrees, Z = 4. The ulose ring had a half-chair conformation deformed towards the sofa (envelope) form.  相似文献   
130.

Background  

This article addresses the problem of interoperation of heterogeneous bioinformatics databases.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号