SUMMARYIn this paper the pressure method for incompressible fluid flow simulation is extended and applied to the numerical simulation of compressible fluid pow. The governing equations, obtained from the physical principles of conservation of momentum,mass and energy,are first studied from a characteristic point of view. Then they are discretized with a semi-implicit finite difference technique in such a fashion that stability is achieved independently of the speed of sound. The resulting algorithm is fast, accurate and particularly efficient in subsonic flow calculations. As an example, the computer simulation of the von Karman vortex street is described and discussed.
Summary. In Part I of this work, numerical methods were derived for the solution of the equations of motion of a single particle subject to a central force which conserved exactly the energy and momenta. In the present work, the methodology of Part I is extended, in part, to motion of a system of particles, in that the energy and linear momentum are conserved exactly. In addition, the angular momentum will be conserved to one more order of accuracy than in conventional methods. Exact conservation of the total angular momentum results only for the lowest order numerical approximation, which is equivalent to the "discrete mechanics" presented elsewhere.
Two lines of evidence derived from fusion gene constructs indicate that sequences residing in the 5'-nontranslated region of a cell cycle-dependent human H3 histone mRNA are involved in the selective destabilization that occurs when DNA synthesis is terminated. The experimental approach was to construct chimeric genes in which fragments of the mRNA coding regions of the H3 histone gene were fused with fragments of genes not expressed in a cell cycle-dependent manner. After transfection in HeLa S3 cells with the recombinant plasmids, levels of fusion mRNAs were determined by S1 nuclease analysis prior to and following DNA synthesis inhibition. When the first 20 nucleotides of an H3 histone mRNA leader were replaced with 89 nucleotides of the leader from a Drosophila heat-shock (hsp70) mRNA, the fusion transcript remained stable during inhibition of DNA synthesis, in contrast to the rapid destabilization of the endogenous histone mRNA in these cells. In a reciprocal experiment, a histone-globin fusion gene was constructed that produced a transcript with the initial 20 nucleotides of the H3 histone mRNA substituted for the human f3-globin mRNA leader. In HeLa cells treated with inhibitors of DNA synthesis and/or protein synthesis, cellular levels of this histone-globin fusion mRNA appeared to be regulated in a manner similar to endogenous histone mRNA levels. These results suggest that the first 20 nucleotides of the leader are sufficient to couple histone mRNA stability with DNA replication.The human histone genes are a moderately repeated gene family that encode the major structural proteins ofchromatin. It has been well established that histone gene expression and DNA replication are temporally and functionally coupled. The synthesis of most histone proteins (1-6) and the steadystate levels of histone mRNAs (7-12) are closely correlated with DNA synthesis in the S phase of the cell cycle. At the natural end of S phase or following inhibitor-induced termination of DNA synthesis, there is a coordinate and stoichiometric decrease in histone mRNA levels and histone protein synthesis (8)(9)(10)(11)(12)(13). The rapid loss of histone mRNA under these conditions is in contrast to minimal changes in nonhistone mRNA levels. The selective destabilization of histone mRNA during DNA synthesis inhibition is posttranscriptionally mediated; destabilization is not dependent on transcription (11) but requires protein synthesis (11-15). The cellular and molecular basis for histone mRNA turnover, however, remains unresolved.To address molecular mechanisms operative in the selective destabilization of histone mRNAs, we are attempting to identify regions of a cloned, cell cycle-dependent human H3 histone gene (16,17) that are involved in the destabilization of its transcripts. Our approach is to construct fusion genes, in which fragments ofthe mRNA coding regions ofthe cloned human H3 histone gene are fused with fragments of other genes not expressed in a cell cycle-dependent manner. After transfection into HeLa S3 cells, le...
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