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Biomedical subjects

A D Nazarea

Publications and source records attributed to A D Nazarea.

10 recordsLinked to original sources
PubMed1987

Dynamics and stability in coevolutionary ecological systems. I. Community stability and coevolutionarily stable states.

An extension of J. Roughgarden's [1979, Theor. Pop. Biol. 9, 388; 1979, "An Introduction to Evolutionary Ecology and Population Genetic Theory," Macmillan, New York] formalism for investigating the effects of coevolution on community structure is presented. The extension assumes the result that a coevolved community is asymptotically stable when coevolution takes place at a genetically noninvasible boundary. This is proved for the general case of n interacting species. From this a community persistence function, phi (P), is defined that allows measuring the domain of attraction for the community as well as the resilience time, that is, the time taken for a perturbation to decay to 1-1/e (63%) of its initial value.

Animals↗
PubMed1985

Detection of a fundamental modular format common to transfer and ribosomal RNAs: second-order spectral analysis.

Second-order spectral analysis is used to detect rigorously and to characterize the principal periodicities in the positions of conserved sequences common to tRNAs and rRNAs. It is shown that the shared periodicity having the largest spectral amplitude is 9, followed by 8 and 10, thus forming a closed triad of significant multiplets centered at 9 bases. This conclusion is proposed to reflect a closed triadic set of fundamental tandem repeat lengths in a class of ancestral macromolecules possessing a restricted sequence symmetry. The terms "remanent" and "archeomodular" are used to describe a relic modular format, traces of which are shown here to persist despite the changes that have occurred in the primary structures of ribonucleic acids during the course of their evolution.

Animals↗
PubMed1978

Field-induced instabilities in two-component systems: the cell-free glycolytic system.

The effects of weak static electric field on a cell-free glycolytic system, which is known to exhibit oscillatory behavior, have been studied using an allosteric model (due to Goldbeter and Lefever). Linear stability analysis is used to determine the change in the nature of stability and the consequent appearance of dissipative structures, due to the electric field. The results show that for this system all the necessary conditions for a field induced instability are satisfied. An order of magnitude calculation of the field strength shows that field strength in the range 10-100 V/cm is required to produce observable change in the system's behavior.

Cell-Free System↗
PubMed1978

Spatiotemporal pattern formation in thin layers and membranes: critical focal size.

A theory for the determination of the focal nucleation size of oscillatory membrane (or thin layer) reactions is developed on the assumption that the focal domain is square planar. From this, one can estimate that, for a period of the oscillatory mode of, say, 5 min and lateral diffusion coefficient of approximately 10(-9) cm2/sec, the critical focal area would be of the order of 148 micrometer2, corresponding to a linear dimension of approximately 12.2 micrometer2. An alternative general expression for the critical focal size, involving explicitly the viscosity of the membrane, is also given.

Diffusion↗
PubMed1974

Critical length of the transport-dominated region for oscillating non-linear reactive processes.

Beyond an instability, non-linear processes can give rise to reactive modes exhibiting sustained oscillations in particle numbers. The coupling of such an oscillatory mode to diffusional transport in a system can cause coherent spatio-temporal structures to arise and persist indefinitely, far from thermal equilibrium, provided the system is above some critical size and is maintained open to mass and energy transfer.In the case of one spatial dimension, the critical size of the system, following Goldbeter [Proc. Nat. Acad. Sci. USA 70, 3255-3259 (1973)], can be defined as that size up to which there exists exactly one unique time-independent solution (which is completely transport dominated) to the macroscopic equation that characterizes the coupling of the reactive mode to diffusional transport and which is subject to inhomogeneous boundary conditions. A theoretical estimate of the critical size is derived (valid for arbitrary systems involving multicomponent non-linear reactive processes possessing an oscillatory mode) by making use only of the parameter-dependent period of the oscillatory mode and of the elements of the diffusion tensor. This estimate specifically takes cross-diffusion into account. In the special case of simple diagonal diffusion, an illustrative comparison is made with the prediction of a more model-specific estimate of Goldbeter that involves a model for glycolytic oscillations.

Journal Article↗
PubMed1971

The stability theory of nonpolynomial kinetics.

The results of an investigation of the formal stability behavior of equivalent classes of kinetic equations (generated by stability constraint specifications) that are general enough to include nonpolynomial kinetics are outlined. The immediate motivation for the study arose from the need for a comprehensive theory when steric interactions in biosynthetic networks are explicitly taken into account.

Catalysis↗