Human T-cell gamma genes contain N segments and have marked junctional variability

The gamma-chain genes are encoded by immunoglobulin-like gene segments in germline DNA which rearrange during the somatic development of T cells to form an active gene. The protein produced by these genes has not been identified and the diversity of the proteins that the genes can express has not been determined. We expect that the diversity of expressed gamma-chains is produced by the same three mechanisms that produce diversity of other immunoglobulin-like genes: (1) germline variable (V) and joining (J) region repertoires; (2) somatic mutation; and (3) junctional diversity. To define the contribution of each of these mechanisms to the generation of gamma-chain diversity, several gamma-chain complementary clones and rearranged gamma-chain genes have been characterized. Most of these clones seem to encode a defective gamma-chain, the variable- and constant-region portions being joined such that they would not be translated in the same reading frame. Here we report that the germline J-region diversity of the human T-cell gamma-chain is very limited and that somatic mutation does not contribute to the diversity of the gamma-chains encoded by the cloned segments. However, the junctional diversity of these gamma-chain genes is extensive. We suggest that N sequences (template-independent sequences) have been inserted enzymatically into all of the gamma-chain genes characterized.     

A family of candidate taste receptors in human and mouse

The gustatory system of mammals can sense four basic taste qualities, bitter, sweet, salty and sour, as well as umami, the taste of glutamate. Previous studies suggested that the detection of bitter and sweet tastants by taste receptor cells in the mouth is likely to involve G-protein-coupled receptors. Although two putative G-protein-coupled bitter/sweet taste receptors have been identified, the chemical diversity of bitter and sweet compounds leads one to expect that there is a larger number of different receptors. Here we report the identification of a family of candidate taste receptors (the TRBs) that are members of the G-protein-coupled receptor superfamily and that are specifically expressed by taste receptor cells. A cluster of genes encoding human TRBs is located adjacent to a Prp gene locus, which in mouse is tightly linked to the SOA genetic locus that is involved in detecting the bitter compound sucrose octaacetate. Another TRB gene is found on a human contig assigned to chromosome 5p15, the location of a genetic locus (PROP) that controls the detection of the bitter compound 6-n-propyl-2-thiouracil in humans.     

植物与根际微生物关系的研究进展

农业生产中大面积应用化肥、农药提高作物产量的同时,也产生了极大的环境问题,并且使生产成本提高。随着现代生物科学技术的进步,人们对应用微生物来代替化肥和农药寄予了厚望,从而对植物与根际微生物关系的研究给予了高度重视。本文依据近30年国内外对植物与根际微生物关系研究的成果,对根际微生物的多样性、植物与根际微生物的相互作用、根际微生物与连作障碍的关系三个方面进行了综述,并提出了今后在植物与根际微生物方面需要深入研究的几个问题。     

Ionic colloidal crystals of oppositely charged particles

Colloidal suspensions are widely used to study processes such as melting, freezing and glass transitions. This is because they display the same phase behaviour as atoms or molecules, with the nano- to micrometre size of the colloidal particles making it possible to observe them directly in real space. Another attractive feature is that different types of colloidal interactions, such as long-range repulsive, short-range attractive, hard-sphere-like and dipolar, can be realized and give rise to equilibrium phases. However, spherically symmetric, long-range attractions (that is, ionic interactions) have so far always resulted in irreversible colloidal aggregation. Here we show that the electrostatic interaction between oppositely charged particles can be tuned such that large ionic colloidal crystals form readily, with our theory and simulations confirming the stability of these structures. We find that in contrast to atomic systems, the stoichiometry of our colloidal crystals is not dictated by charge neutrality; this allows us to obtain a remarkable diversity of new binary structures. An external electric field melts the crystals, confirming that the constituent particles are indeed oppositely charged. Colloidal model systems can thus be used to study the phase behaviour of ionic species. We also expect that our approach to controlling opposite-charge interactions will facilitate the production of binary crystals of micrometre-sized particles, which could find use as advanced materials for photonic applications.     

网络广告定向技术综述

近几年来网络广告异军突起,形式也逐渐多样化.随着网络广告市场的急剧增加,越来越多的广告主希望在推出某产品或服务后,能够用更少的支出吸引更多对该产品或服务感兴趣的用户与广告进行交互(看到广告,点击广告,注册、下订单、购买产品等等后继行为).网络广告定向技术因此应运而生.本文介绍了定向技术的分类,并对每种定向技术具体阐述了其应用场景;总结和对比了近几年网络定向技术中常用的方法和模型.     

Neural mechanisms mediating optimism bias

Humans expect positive events in the future even when there is no evidence to support such expectations. For example, people expect to live longer and be healthier than average, they underestimate their likelihood of getting a divorce, and overestimate their prospects for success on the job market. We examined how the brain generates this pervasive optimism bias. Here we report that this tendency was related specifically to enhanced activation in the amygdala and in the rostral anterior cingulate cortex when imagining positive future events relative to negative ones, suggesting a key role for areas involved in monitoring emotional salience in mediating the optimism bias. These are the same regions that show irregularities in depression, which has been related to pessimism. Across individuals, activity in the rostral anterior cingulate cortex was correlated with trait optimism. The current study highlights how the brain may generate the tendency to engage in the projection of positive future events, suggesting that the effective integration and regulation of emotional and autobiographical information supports the projection of positive future events in healthy individuals, and is related to optimism.     

基于ADAMS与MATLAB的车辆跨障综合交互仿真

利用ADAMS软件对车辆进行三维实体运动仿真,仿真结果对研究车辆结构的动态问题提供了一种很好的实验手段,本文给出了在ADAMS软件中结合MATLAB综合交互来实现跨越障碍运动仿真的一种方法.利用ADAMS构建车辆模型来实现对台阶状障碍的跨越运动,并结合MATLAB对其运动进行综合交互仿真.在对输出数据进行分析与研究后,得到预期效果.通过两者的结合,我们实现了机电一体化的联合分析,为我们以后的物理样机试验提供了更为可靠的依据.     

信息论中的概率统计

本文简要叙述了概率统计在信息论中的一些重要应用,期望揭示出信息论的基本特点,以利于信息论的教学与学习。     

信息论中的概率统计

本文简要叙述了概率统计在信息论中的一些重要应用,期望揭示出信息论的基本特点,以利于信息论的教学与学习.     

预测线虫和酵母基因组中内含子、外显子及基因间序列的离散增量方法

把基因组中的内含子、外显子和基因间序列分为三类序列，从这些序列中取64种三核苷的重复出现次数作为离散源的状态参数，计算三类序列的标准离散量，比较待测序列的离散量与三个标准离散量之间的离散增量值，由离散增量的最小值决定待测序列属于哪一类．本文用离散增量方法对线虫和酵母序列进行预测，结果表明，对酵母内含子预测的敏感性为81．5％，对外显子预测的敏感性为88．5％、特异性为99．6％，对基因间序列预测的敏感性为65．4％、特异性为87．5％；若以在相同长度区间的序列为标准离散源，预测相应长度区间的序列，对线虫内含子的预测的敏感性为82．2％、特异性为94．2％，对外显子预测的敏感性为91．1％、特异性为97．5％，对基因间序列预测的敏感性为78．8％。     

南充秋季农田景观中无脊椎动物的生物多样性初步研究

对四川省南充市郊农田景观各元素中脊椎动物的生物多样性进行初步研究，结果显示：1.池塘边缘生物的多样性指数和丰富度最高，但生物数量很低．2．蔬菜地物种均匀度最高，且数量也最多.并通过各个景观元素生物群落结构的比较，得出有利于提高生物多样性指数和丰富度的因素，为农田景观的合理设计，保证农业的持续发展提供参考．     

The dynamics of chromosome evolution in birds and mammals

Comparative mapping, which compares the location of homologous genes in different species, is a powerful tool for studying genome evolution. Comparative maps suggest that rates of chromosomal change in mammals can vary from one to ten rearrangements per million years. On the basis of these rates we would expect 84 to 600 conserved segments in a chicken comparison with human or mouse. Here we build comparative maps between these species and estimate that numbers of conserved segments are in the lower part of this range. We conclude that the organization of the human genome is closer to that of the chicken than the mouse and by adding comparative mapping results from a range of vertebrates, we identify three possible phases of chromosome evolution. The relative stability of genomes such as those of the chicken and human will enable the reconstruction of maps of ancestral vertebrates.     

Theory predicts the uneven distribution of genetic diversity within species

Global efforts to conserve species have been strongly influenced by the heterogeneous distribution of species diversity across the Earth. This is manifest in conservation efforts focused on diversity hotspots. The conservation of genetic diversity within an individual species is an important factor in its survival in the face of environmental changes and disease. Here we show that diversity within species is also distributed unevenly. Using simple genealogical models, we show that genetic distinctiveness has a scale-free power law distribution. This property implies that a disproportionate fraction of the diversity is concentrated in small sub-populations, even when the population is well-mixed. Small groups are of such importance to overall population diversity that even without extrinsic perturbations, there are large fluctuations in diversity owing to extinctions of these small groups. We also show that diversity can be geographically non-uniform--potentially including sharp boundaries between distantly related organisms--without extrinsic causes such as barriers to gene flow or past migration events. We obtained these results by studying the fundamental scaling properties of genealogical trees. Our theoretical results agree with field data from global samples of Pseudomonas bacteria. Contrary to previous studies, our results imply that diversity loss owing to severe extinction events is high, and focusing conservation efforts on highly distinctive groups can save much of the diversity.     

Metabolic trade-offs and the maintenance of the fittest and the flattest

How is diversity maintained? Environmental heterogeneity is considered to be important, yet diversity in seemingly homogeneous environments is nonetheless observed. This, it is assumed, must either be owing to weak selection, mutational input or a fitness advantage to genotypes when rare. Here we demonstrate the possibility of a new general mechanism of stable diversity maintenance, one that stems from metabolic and physiological trade-offs. The model requires that such trade-offs translate into a fitness landscape in which the most fit has unfit near-mutational neighbours, and a lower fitness peak also exists that is more mutationally robust. The 'survival of the fittest' applies at low mutation rates, giving way to 'survival of the flattest' at high mutation rates. However, as a consequence of quasispecies-level negative frequency-dependent selection and differences in mutational robustness we observe a transition zone in which both fittest and flattest coexist. Although diversity maintenance is possible for simple organisms in simple environments, the more trade-offs there are, the wider the maintenance zone becomes. The principle may be applied to lineages within a species or species within a community, potentially explaining why competitive exclusion need not be observed in homogeneous environments. This principle predicts the enigmatic richness of metabolic strategies in clonal bacteria and questions the safety of lethal mutagenesis as an antimicrobial treatment.     

Disturbance and diversity in experimental microcosms

External agents of mortality (disturbances) occur over a wide range of scales of space and time, and are believed to have large effects on species diversity. The "intermediate disturbance hypothesis", which proposes maximum diversity at intermediate frequencies of disturbance, has received support from both field and laboratory studies. Coexistence of species at intermediate frequencies of disturbance is thought to require trade-offs between competitive ability and disturbance tolerance, and a metapopulation structure, with disturbance affecting only a few patches at any given time. However, a unimodal relationship can also be generated by global disturbances that affect all patches simultaneously, provided that the environment contains spatial niches to which different species are adapted. Here we report the results of tests of this model using both isogenic and diverse populations of the bacterium Pseudomonas fluorescens. In both cases, a unimodal relationship between diversity and disturbance frequency was generated in heterogeneous, but not in homogeneous, environments. The cause of this relationship is competition among niche-specialist genotypes, which maintains diversity at intermediate disturbance, but not at high or low disturbance. Our results show that disturbance can modulate the effect of spatial heterogeneity on biological diversity in natural environments.     

Producer-decomposer co-dependency influences biodiversity effects

Producers, such as plants and algae, acquire nutrients from inorganic sources that are supplied primarily by decomposers whereas decomposers, mostly fungi and bacteria, acquire carbon from organic sources that are supplied primarily by producers. This producer-decomposer co-dependency is important in governing ecosystem processes, which implies that the impacts of declining biodiversity on ecosystem functioning should be strongly influenced by this process. Here we show, by simultaneously manipulating producer (green algal) and decomposer (heterotrophic bacterial) diversity in freshwater microcosms, that algal biomass production varies considerably among microcosms (0.0-0.67 mg ml(-1)), but that neither algal nor bacterial diversity by itself can explain this variation. Instead, production is a joint function of both algal and bacterial diversity. Furthermore, the range in algal production in microscosms in which bacterial diversity was manipulated was nearly double (1.82 times) that of microcosms in which bacterial diversity was not manipulated. Measures of organic carbon use by bacteria in these microcosms indicate that carbon usage is the mechanism responsible for these results. Because both producer and microbial diversity respond to disturbance and habitat modification, the main causes of biodiversity loss, these results suggest that ecosystem response to changing biodiversity is likely to be more complex than other studies have shown.     

Dscam diversity is essential for neuronal wiring and self-recognition

Neurons are thought to use diverse families of cell-surface molecules for cell recognition during circuit assembly. In Drosophila, alternative splicing of the Down syndrome cell adhesion molecule (Dscam) gene potentially generates 38,016 closely related transmembrane proteins of the immunoglobulin superfamily, each comprising one of 19,008 alternative ectodomains linked to one of two alternative transmembrane segments. These ectodomains show isoform-specific homophilic binding, leading to speculation that Dscam proteins mediate cell recognition. Genetic studies have established that Dscam is required for neural circuit assembly, but the extent to which isoform diversity contributes to this process is not known. Here we provide conclusive evidence that Dscam diversity is essential for circuit assembly. Using homologous recombination, we reduced the entire repertoire of Dscam ectodomains to just a single isoform. Neural circuits in these mutants are severely disorganized. Furthermore, we show that it is crucial for neighbouring neurons to express distinct isoforms, but that the specific identity of the isoforms expressed in an individual neuron is unimportant. We conclude that Dscam diversity provides each neuron with a unique identity by which it can distinguish its own processes from those of other neurons, and that this self-recognition is essential for wiring the Drosophila brain.     

General patterns of taxonomic and biomass partitioning in extant and fossil plant communities

A central goal of evolutionary ecology is to identify the general features maintaining the diversity of species assemblages. Understanding the taxonomic and ecological characteristics of ecological communities provides a means to develop and test theories about the processes that regulate species coexistence and diversity. Here, using data from woody plant communities from different biogeographic regions, continents and geologic time periods, we show that the number of higher taxa is a general power-function of species richness that is significantly different from randomized assemblages. In general, we find that local communities are characterized by fewer higher taxa than would be expected by chance. The degree of taxonomic diversity is influenced by modes of dispersal and potential biotic interactions. Further, changes in local diversity are accompanied by regular changes in the partitioning of community biomass between taxa that are also described by a power function. Our results indicate that local and regional processes have consistently regulated community diversity and biomass partitioning for millions of years.     

分形理论发展及在混凝过程中的应用

分形是组成部分以某种方式与整体相似的形 ,它研究的对象是具有自相似性的无序系统 ,其维数的变化是连续的 .据此总结了分形理论的发展过程 ,特别是在化学领域中的应用进展 ,并介绍了分形理论在混凝技术中的发展趋势 .通过对分形理论在絮体结构研究、混凝机理研究和分形动力学模型研究中的全面综述 ,以期对混凝过程进行深入研究     

教案设计及实施

随着我院新教师数量的迅速增加 ,关注教学质量和教学效果更显得重要。该文论述了教案设计与实施过程中的几个重要原则、技巧及必须注意的几个问题 ,期望对新教师的成长有所帮助。