A four-variable model for the pattern-forming mechanism in Hydra

A generalized Gierer-Meinhardt model has been used to account for the transplantation experiments in Hydra. In this model, a cross inhibition between the two organizing centres (namely, head and foot) are assumed to be the only mode of interaction in setting up a stable morphogen distribution for the pattern formation in Hydra.

Is Human-X Chromosome Inactivation A Sex-Determining Device

The evolutionary function of X chromosome inactivation is thought to be dosage compensation. However, there is, at present, little evidence to suggest that most X chromosome-linked genes require such compensation. Another view--that X chromosome inactivation may be related to sex determination--is examined here. Consider a hypothetical DNA sequence regulating a major structural gene concerned with the determination of maleness. If this regulatory sequence occurs in both X and Y chromosomes and if its copy number in the Y chromosome is significantly greater than in the X chromosome, then the male-determining properties of the Y chromosome could be attributed to this higher copy number. On the other hand, if the Y chromosome has the same copy number of this sequence as the X chromosome, it is difficult to see how determination of two sexes would occur under such circumstances because XX and XY genomes would then be indistinguishable in this regard. Such a situation seems to occur in the human species with respect to the banded krait minor satellite, a repetitious DNA sequence associated with sex determination. This apparent difficulty may be resolved if X chromosome inactivation renders regulatory as well as structural genes nonfunctional and thereby brings about a significant reduction in the effective copy number of X chromosome-linked DNA sequences concerned with sex determination. It is suggested that X chromosome inactivation brings about, in this manner, a critical inequality between XX and XY embryos and that sex determination in humans is a consequence of this inequality. An analogous situation appears to exist in certain insects in which inactivation of a haploid set of chromosomes (and presumably, therefore, a 50% reduction in the effective copy number of most genes) is associated with maleness. If this line of reasoning is correct, it would suggest that sex determination may be the primary function of X chromosome inactivation.

Design of single variable controllers for ammonia reactors

Abstract is not available.

Inelastic Postbuckling of Columns of Variable Flexural Rigidity

Effects of large deformation and inelasticity are considered in formulating the behavior of columns of variable cross section subjected to an axial compressive load. Simple, approximate methods are used to obtain numerical results. The combined effect of the nonlinearities is shown to be of a hardening type for small column deflections

Non-linear bending of beams of variable cross-section

For the non-linear bending of cantilever beams of variable cross-section, the effect of large deformations, but with linear elasticity, is considered. The governing integral equation is solved by a numerical iterative procedure. Results for some typical cases are obtained and compared with some of those available in the literature.

Planar pursuit-evasion with variable speeds, part 1, extremal trajectory maps

Pursuit evasion in a plane is formulated with both players allowed to vary their speeds between fixed limits. A suitable choice of real-space coordinates confers open-loop optimality on the game. The solution in the small is described in terms of the individual players'' extremal trajectory maps (ETM). Each map is independent of role, adversary, and capture radius. An ETM depicts the actual real-space trajectories. A template method of generating constant control arcs is described. Examples of ETM for an aircraft flying at a constant altitude with fixed and varying speeds are presented.

Planar pursuit-evasion with variable speeds, part 2, barrier sections

An iterative method of constructing sections of the game surfaces from the players'' extremal trajectory maps is discussed. Barrier sections are presented for aircraft pursuit-evasion at constant altitude, with one aircraft flying at sustained speed and the other varying its speed.

Systematic search for optical Barker codes with minimum length

A method that yields optical Barker codes of smallest known lengths for given discrimination is described.

A model for mammalian male determination based on a passive Y chromosome

A model is suggested for mammalian male determination based on interactions postulated to occur among an autosomal repressor gene, an X-linked male-determining gene termed Tdx, and multiple copies of certain DNA sequences on the Y chromosome that do not code for any protein. The repressor, synthesised in limited amounts, has higher affinity for the Y-linked sequences than for Tdx and its affinity for Tdx is greater than that of RNA polymerase. In XY cells the Y effectively binds all available repressor, permitting transcription of Tdx to occur. In XX cells, since competition from the Y-linked high-affinity sequences is absent, the repressor binds to Tdx and prevents transcription. As a result of this competition between Tdx and the Y-linked high-affinity sites for limiting concentrations of the autosomal repressor, the product of the Tdx gene (TDX) is synthesized in the male but not in the female. It is suggested that in determination of the male sex, the role of the Y chromosome is to serve as a sink for the Tdx repressor. The proposed interactions provide a plausible explanation for the genetic properties of several anomalies of sexual development in mouse, man, and other mammals. The model suggests that the postulated multiple, highaffinity sequences on the Y chromosome of the mouse are included among the DNA sequences referred to as the Sxr-Bkm sequences.

A thymine-related mutation system in immunoglobulin variable regions

Abstract is not available.

Comments on "Diffuse sound reflection by maximum length sequences"

Some improvements are suggested to Schroeder's scheme [J. Acoust. Soc. Am. 57, 149–150 (1975)] of achieving diffuse sound reflection in concert halls.Subject Classification

Influence of adhesive non-linearities on the performance and optimal length of adhesive bonded joints

An analytical study for the static strength of adhesive lap joints is presented. The earlier solutions of Volkersen [i], DeBruyne[2] and others were limited to linear adhesives. The influence of adhesive non-linearity was first considered by Grimes' et al[3] and Dickson et al [4]. Recently Hart-Smith[5] successfully introduced elastic-plastic behaviour of the adhesive. In the present study the problem is formulated for general non-linear adhesive behaviour and an efficient numerical algorithm is written for the solution. Bilinear and trilinear models for the nonlinearity yield closed form analytical solutions.

On the stabilization of uniformly decoupled systems by state variable feedback

Conditions under which the asymptotic stabilization of uniformly decoupled time-varying multivariate systems is possible are explored. This is accomplished by developing a canonical form for integrator uniformly decoupled system in which the coefficient matrices have a simple structure. The procedures developed rely on certain conditions on the given system and yield explicit expressions for the stabilization compensators.

A Finite Variable Difference Relaxation Scheme for hyperbolic-parabolic equations

Using the framework of a new relaxation system, which converts a nonlinear viscous conservation law into a system of linear convection-diffusion equations with nonlinear source terms, a finite variable difference method is developed for nonlinear hyperbolic-parabolic equations. The basic idea is to formulate a finite volume method with an optimum spatial difference, using the Locally Exact Numerical Scheme (LENS), leading to a Finite Variable Difference Method as introduced by Sakai [Katsuhiro Sakai, A new finite variable difference method with application to locally exact numerical scheme, journal of Computational Physics, 124 (1996) pp. 301-308.], for the linear convection-diffusion equations obtained by using a relaxation system. Source terms are treated with the well-balanced scheme of Jin [Shi Jin, A steady-state capturing method for hyperbolic systems with geometrical source terms, Mathematical Modeling Numerical Analysis, 35 (4) (2001) pp. 631-645]. Bench-mark test problems for scalar and vector conservation laws in one and two dimensions are solved using this new algorithm and the results demonstrate the efficiency of the scheme in capturing the flow features accurately.