A more complete understanding of the relationship of cell physiology to genomic structure is desirable. Because of the intrinsic complexity of biological organisms, only the simplest cells will allow complete definition of all components and their interactions. The theoretical and experimental construction of a minimal cell has been suggested as a tool to develop such an understanding. Our ultimate goal is to convert a ''coarse-grain'' lumped parameter computer model of Escherichia coli into a genetically and chemically detailed model of a ''minimal cell.'' The base E. coli model has been converted into a generalized model of a heterotrophic bacterium. This coarse-grain minimal cell model is functionally complete, with growth rate, composition, division, and changes in cell morphology as natural outputs from dynamic simulations where only the initial composition of the cell and of the medium are specified. A coarse-grain model uses pseudochemical species (or modules) that are aggregates of distinct chemical species that share similar chemistry and metabolic dynamics. This model provides a framework in which these modules can be ''delumped'' into chemical and genetic descriptions while maintaining connectivity to all other functional elements. Here we demonstrate that a detailed description of nucleotide precursors transport and metabolism is successfully integrated into the whole-cell model. This nucleotide submodel requires fewer (12) genes than other theoretical predictions in minimal cells. The demonstration of modularity suggests the possibility of developing modules in parallel and recombining them into a fully functional chemically and genetically detailed model of a prokaryote cell. T he basic design rules relating the regulation of cellular function to genomic structure is of broad interest. Bioinformatics emerged as an approach to convert static linear sequence genomic data into an understanding of the dynamic nonlinear function of living organisms. Initial efforts have focused on identifying the proteins encoded in the genome and, subsequently, identifying protein function and regulatory elements in the genome. These efforts are able to address specific questions but cannot translate genomic data broadly into an understanding of cell function. We propose a reverse approach. We ask how we would design a cell to achieve expected functions and, from that design, how we would write the genomic instructions. This approach follows the typical engineering design approach where desired performance dictates functional design, which is then translated into blueprints. To accomplish this goal, we are constructing a chemically and genetically minimal cell computer model. By modeling the essential regulatory structure and functions to maintain a living cell, we expect to better understand the relationship of genomic instructions to cell function and regulation.A "minimal cell" is a hypothetical cell possessing the minimum functions required for sustained growth and reproduction in a maximally supportive culture env...