Images of the DNA double helix in water

The scanning tunneling microscope can image uncoated DNA submerged in water. The grooves of the double helix were clearly resolved in images of the 146-base pair fragment extracted from calf thymus nucleosome. In contrast to images obtained with dry DNA, the helix pitch varied only a small amount (36 +/- 5 angstroms). The path of the helix shows considerable variation. It is quite straight when the molecules are densely packed, but it curves and bends in isolated molecules.     

An amylose antiparallel double helix at atomic resolution

In the crystal structure of the polyiodide complex (p-nitrophenyl-alpha-maltohexaose(2)) . Ba(I(3))(2) . 22H(2)O, the maltohexaose units form an antiparallel, left-handed double helix with O-2 ... O-3 and O-6 ... O-6 hydrogen bonding and a central cavity that encloses two triiodide units. This structure contrasts with the parallel, left-handed double helix with no central cavity proposed for the A-and B-starch helix and the left-handed single helix in V-amylose and may be relevant for the stabilization of glycogen Structure.     

A cosmic double helix in the archetypical quasar 3C273

Finding direct evidence for plasma instability in extragalactic jets is crucial for understanding the nature of relativistic outflows from active galactic nuclei. Our radio interferometric observations of the quasar 3C273 made with the orbiting radio telescope, HALCA, and an array of ground telescopes have yielded an image in which the emission across the jet is resolved, revealing two threadlike patterns that form a double helix inside the jet. This double helical structure is consistent with a Kelvin-Helmholtz instability, and at least five different instability modes can be identified and modeled by a light jet with a Lorentz factor of 2 and Mach number of 3.5. The model reproduces in detail the internal structure of the jet on scales of up to 30 milli-arc seconds ( approximately 300 parsecs) and is consistent with the general morphology of the jet on scales of up to 1 kiloparsec.     

Time-resolved X-ray diffraction studies of the B in equilibrium D structural transition in the DNA double helix

Because of the relation between topology and function, there has been much interest in the structural transitions of the various conformations of DNA polymers. The x-ray fiber diffraction analysis system at the Daresbury Synchrotron Radiation Source was used to study the reversible transition between the B and D forms of the synthetic DNA poly[d(A-T)].poly[d(A-T)]. The gradual progression of conformations between these two forms indicates that the DNA double helix does not undergo a change of handedness during this transition.     

莲藕施肥机无轴双螺旋供肥装置性能分析与试验

针对藕田施肥过程中存在的劳动强度高、施肥稳定性及均匀性较低的问题,设计了一种水力喷射式莲藕施肥机,其主要分为供肥装置和水肥混合装置,主要对无轴双螺旋供肥装置进行了设计与分析。通过理论分析确定影响供肥性能参数主要为螺旋转速、直径和导程,结合藕田施肥农艺要求确定上述3项参数的取值范围。利用EDEM对无轴双螺旋供肥性能进行离散元仿真分析,以供肥量均值和供肥量稳定性变异系数为评价指标分别进行单因素试验和回归正交旋转组合试验。单因素试验结果表明：供肥量均值随着螺旋转速、直径和导程的增大而增大,并且具有良好的线性关系,因此可通过调节供肥螺旋转速、直径和导程来改变供肥量均值;供肥量稳定性变异系数随着螺旋转速和导程的增大呈现先降低后升高的趋势,而随着直径的增大而减小。运用Design-Expert软件对回归正交旋转组合试验结果进行分析优化,分别开展仿真试验和台架试验对优化后的参数进行验证。结果表明,当螺旋转速为160 r·min^-1,螺旋直径为78 mm,螺旋导程为53 mm时,仿真结果供肥量均值为329.43 g·s^-1,供肥量稳定性变异系数为1.43%,...     

Entropically driven helix formation

The helix is a ubiquitous motif for biopolymers. We propose a heuristic, entropically based model that predicts helix formation in a system of hard spheres and semiflexible tubes. We find that the entropy of the spheres is maximized when short stretches of the tube form a helix with a geometry close to that found in natural helices. Our model could be directly tested with wormlike micelles as the tubes, and the effect could be used to self-assemble supramolecular helices.     

Triple-stranded polynucleotide helix containing only purine bases

The structure of the complex involving one polyadenylic acid and two polyinosinic acid chains has been determined by x-ray diffraction. The three coaxial, helical chains have conformations like conventional RNA double helices despite the absence of purine-pyrimidine pairing. Formation of hypoxanthine pairs in codon-anticodon interactions therefore requires only trivial changes in the conformation of a standard nucleotide. Evolution of the contemporary genetic code involving purine-pyrimidine complementarity from a primeval code with only adenine-hypoxanthine pairing would have been possible without major discontinuities in molecular geometry.     

Tubules of globoid leukodystrophy: a right-handed helix

Morphologic similarity between the cytoplasmic tubules of globoid leukodystrophy and Gaucher's disease (as demonstrated by thin sectioning, negative staining, and shadowing techniques) and their resemblance to negatively stained beef cerebroside are presented as evidence favoring the accumulation of cerebrosides in globoid cells. The tubules of globoid leukodystrophy have a 60-angstrom periodic banding similar to the tubules of Gaucher's disease. A right-handed helical twisting of the tubules is observed in both diseases.     

双宾语结构再研究

在前人有关双宾语结构与与格结构关系研究的基础上,从句法理论、句法结构、语义制约等方面,论述双宾语结构与与格结构的关系,认为二者之间无转换生成关系。     

A four-base paired genetic helix with expanded size

We describe a new molecular class of genetic-pairing system that has a native DNA backbone but has all four base pairs replaced by new, larger pairs. The base pairs include size-expanded analogs of thymine and of adenine, both extended by the width of a benzene ring (2.4 A). The expanded-diameter double helices are more thermodynamically stable than the Watson-Crick helix, likely because of enhanced base stacking. Structural data confirm a right-handed, double-stranded, and base-paired helical form. Because of the larger base size, all the pairs of this helical system are fluorescent, which suggests practical applications in detection of natural DNA and RNA. Our findings establish that there is no apparent structural or thermodynamic prohibition against genetic systems having sizes different from the natural one.     

影响常春藤生长的因素分析

采用室外试验观测的方法．研究了影响常春藤生长的因素。结果表明：影响常春藤生长的主要因子是：光、温、水、肥、栽培基质、生长激素、攀附介面．其次是栽培李节、栽培方法与管理措施。各种因素同时综合地作用于常春藤。     

小叶常春藤扦插繁殖试验初报

【目的】探讨不同种类外源激素、不同处理浓度、不同处理时间对小叶常春藤扦插生根、成活的效果,为小叶常春藤快速繁殖和大面积推广应用提供参考依据。【方法】以当年生、1年生、2年生健壮无病虫害的日本小叶常春藤为试验材料,采用4因素3水平正交试验,研究外源生长激素吲哚乙酸(IBA)、α-萘乙酸(NAA)和ABT生根粉1号(ABT1)不同浓度、不同处理时间和不同插穗类型对日本小叶常春藤扦插生根效果的影响。【结果】外源生长激素种类、浓度、处理时间及插穗类型对小叶常春藤扦插成活率、生根数、根鲜重和平均根长均有显著影响,其中S7处理(ABT1100 mg/L浸泡1年生小叶常春藤插穗12 h)的成活率最高达95.53%,与其他处理差异显著(P<0.05);对插穗扦插成活率的影响大小顺序是:插穗类型>激素种类>激素浓度>处理时间。【结论】影响小叶常春藤扦插成活率的主要因素是插穗类型,以1年生插穗扦插效果最好;选择ABT1100 mg/L浸泡1年生小叶常春藤插穗12 h最易于其生根和成活。     

Localized dispersal and recruitment in great barrier reef corals: the helix experiment

To examine the problem of how far coral larvae disperse from their natal reef, coral recruitment densities were experimentally determined at distances up to 5 kilometers from a small, relatively isolated platform reef, Helix Reef, on the central Great Barrier Reef for 7 months. High concentrations of recruits, accounting for up to 40 percent of all recruitment, were found downstream of the reef in areas of high water residence time, suggesting that near-field(proximal) circulation has a profound influence on dispersal and recruitment of coral larvae. Coral recruitment declined logarithmically with distance from the reef, decreasing by an order of magnitude at radial distances of only 600 to 1200 meters. On an ecological time scale, advective dispersal of semipassive marine larvae with relatively short planktonic lives(minimally days) may be extensive, but success of recruitment is highly limited. Through evolutionary time, sufficient dispersal occurs to ensure gene flow to reef tracts hundreds or possibly thousands of kilometers apart. In the short term, however, coral reefs appear to be primarily self-seeded with respect to coral larvae.