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   150篇
  免费   11篇
  2021年   2篇
  2020年   2篇
  2017年   2篇
  2016年   2篇
  2015年   5篇
  2014年   9篇
  2013年   4篇
  2012年   3篇
  2011年   5篇
  2010年   3篇
  2009年   6篇
  2008年   10篇
  2007年   7篇
  2006年   5篇
  2005年   6篇
  2004年   5篇
  2003年   6篇
  2002年   3篇
  2001年   7篇
  2000年   7篇
  1999年   7篇
  1998年   12篇
  1997年   2篇
  1996年   4篇
  1995年   8篇
  1994年   2篇
  1993年   4篇
  1992年   6篇
  1990年   1篇
  1989年   2篇
  1986年   2篇
  1984年   3篇
  1983年   1篇
  1982年   1篇
  1981年   2篇
  1977年   1篇
  1975年   2篇
  1973年   1篇
  1970年   1篇
排序方式: 共有161条查询结果,搜索用时 15 毫秒
1.
2.
Immunoglobulins are encoded by a large multigene system that undergoes somatic rearrangement and additional genetic change during the development of immunoglobulin-producing cells. Inducible antibody and antibody-like responses are found in all vertebrates. However, immunoglobulin possessing disulfide-bonded heavy and light chains and domain-type organization has been described only in representatives of the jawed vertebrates. High degrees of nucleotide and predicted amino acid sequence identity are evident when the segmental elements that constitute the immunoglobulin gene loci in phylogenetically divergent vertebrates are compared. However, the organization of gene loci and the manner in which the independent elements recombine (and diversify) vary markedly among different taxa. One striking pattern of gene organization is the "cluster type" that appears to be restricted to the chondrichthyes (cartilaginous fishes) and limits segmental rearrangement to closely linked elements. This type of gene organization is associated with both heavy- and light-chain gene loci. In some cases, the clusters are "joined" or "partially joined" in the germ line, in effect predetermining or partially predetermining, respectively, the encoded specificities (the assumption being that these are expressed) of the individual loci. By relating the sequences of transcribed gene products to their respective germ-line genes, it is evident that, in some cases, joined-type genes are expressed. This raises a question about the existence and/or nature of allelic exclusion in these species. The extensive variation in gene organization found throughout the vertebrate species may relate directly to the role of intersegmental (V<==>D<==>J) distances in the commitment of the individual antibody-producing cell to a particular genetic specificity. Thus, the evolution of this locus, perhaps more so than that of others, may reflect the interrelationships between genetic organization and function.   相似文献   
3.
Summary Opioid peptides are thought to interact with the cell membrane in their biological journey to the membrane-bound receptor. Both organic solvents and model membranes have been used previously to determine the stable solution conformations of peptide hormones. Leucine enkephalin has been studied in a number of different environments, but with limited resolution. Here it is shown that leucine enkephalin forms a stable type IV -turn structure in dodecylphosphocholine micelles. We have observed a highly solvent-shielded amide proton with no evidence for a complementary hydrogen bond acceptor. The structural details of the peptide as determined by NMR spectroscopy in solution are described.  相似文献   
4.
5.
Recently, a novel 87-amino acid influenza A virus protein with proapoptotic properties, PB1-F2, has been reported that originates from an alternative reading frame in the PB1 polymerase gene and is encoded in most known human influenza A virus isolates. Here we characterize the molecular structure of a biologically active synthetic version of the protein (sPB1-F2). Western blot analysis, chemical cross-linking, and NMR spectroscopy afforded direct evidence of the inherent tendency of sPB1-F2 to undergo oligomerization mediated by two distinct domains located in the N and C termini, respectively. CD and (1)H NMR spectroscopic analyses indicate that the stability of structured regions in the molecule clearly depends upon the hydrophobicity of the solvent. In aqueous solutions, the behavior of sPB1-F2 is typical of a largely random coil peptide that, however, adopts alpha-helical structure upon the addition of membrane mimetics. (1)H NMR analysis of three overlapping peptides afforded, for the first time, direct experimental evidence of the presence of a C-terminal region with strong alpha-helical propensity comprising amino acid residues Ile(55)-Lys(85) connected via an essentially random coil structure to a much weaker helix-like region, located in the N terminus between residues Trp(9) and Lys(20). The C-terminal helix is not a true amphipathic helix and is more compact than previously predicted. It corresponds to a positively charged region previously shown to include the mitochondrial targeting sequence of PB1-F2. The consequences of the strong oligomerization and helical propensities of the molecule are discussed and used to formulate a hypothetical model of its interaction with the mitochondrial membrane.  相似文献   
6.

Background  

In addition to known protein-coding genes, large amounts of apparently non-coding sequence are conserved between the human and mouse genomes. It seems reasonable to assume that these conserved regions are more likely to contain functional elements than less-conserved portions of the genome.  相似文献   
7.
Leptin is a peptide released by adipocytes that has profound effects on central regulation of body metabolism. The present study represents an investigation into leptin effects on hypothalamic control of reproductive function, specifically on GnRH release. Adult male rats (gonadectomized or sham-operated) were used as donors of hypothalamic tissue that was used as intact hemihypothalami or as enzymatically dispersed hemihypothalami in a perifusion culture system. Continuous samples were collected at 10-min intervals for 8 to 10 hr and were assayed to measure temporal changes in GnRH release in response to various doses of leptin infused into the perifusion chambers. Leptin at the highest dose (10(-8) M) resulted in consistent and significant stimulation of GnRH release. There were no effects of treatment for surgical preparation (gonadectomy versus sham) or tissue preparation (intact versus dispersed hemihypothalami). The results of this study support the hypothesis that leptin plays a direct stimulatory role in the regulation of GnRH release. This study describes an important step in our understanding of the mechanism that connects changes in basal metabolism with reproductive function. These results indicate an intact interneuronal network is unnecessary for these leptin effects, but does not exclude a role for interneuronal networks in this regulatory pathway.  相似文献   
8.
To select a Saccharomyces cerevisiae reference strain amenable to experimental techniques used in (molecular) genetic, physiological and biochemical engineering research, a variety of properties were studied in four diploid, prototrophic laboratory strains. The following parameters were investigated: 1) maximum specific growth rate in shake-flask cultures; 2) biomass yields on glucose during growth on defined media in batch cultures and steady-state chemostat cultures under controlled conditions with respect to pH and dissolved oxygen concentration; 3) the critical specific growth rate above which aerobic fermentation becomes apparent in glucose-limited accelerostat cultures; 4) sporulation and mating efficiency; and 5) transformation efficiency via the lithium-acetate, bicine, and electroporation methods. On the basis of physiological as well as genetic properties, strains from the CEN.PK family were selected as a platform for cell-factory research on the stoichiometry and kinetics of growth and product formation.  相似文献   
9.
Regulation of fermentative capacity was studied in chemostat cultures of two Saccharomyces cerevisiae strains: the laboratory strain CEN.PK113-7D and the industrial bakers’ yeast strain DS28911. The two strains were cultivated at a fixed dilution rate of 0.10 h−1 under various nutrient limitation regimes: aerobic and anaerobic glucose limitation, aerobic and anaerobic nitrogen limitation on glucose, and aerobic ethanol limitation. Also the effect of specific growth rate on fermentative capacity was compared in glucose-limited, aerobic cultures grown at dilution rates between 0.05 h−1 and 0.40 h−1. Biomass yields and metabolite formation patterns were identical for the two strains under all cultivation conditions tested. However, the way in which environmental conditions affected fermentative capacity (assayed off-line as ethanol production rate under anaerobic conditions) differed for the two strains. A different regulation of fermentative capacity in the two strains was also evident from the levels of the glycolytic enzymes, as determined by in vitro enzyme assays. With the exception of phosphofructokinase and pyruvate decarboxylase in the industrial strain, no clear-cut correlation between the activities of glycolytic enzymes and the fermentative capacity was found. These results emphasise the need for controlled cultivation conditions in studies on metabolic regulation in S. cerevisiae and demonstrate that conclusions from physiological studies cannot necessarily be extrapolated from one S. cerevisiae strain to the other.  相似文献   
10.
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