Genetic Networks

Abstract

Genetic networks control the execution of the genetic program stored in an organism's deoxyribonucleic acid (DNA) by orchestrating gene expression. Any biological function, in physiology or development, is dependent on the combined action of many genes, requiring their precise control. Expression of individual genes is determined by associated regions of regulatory DNA. These are bound by sequence‐specific regulatory proteins, called transcription factors, leading to activation or repression of transcription. The interactions among regulatory genes display the features of a network where the linkages are determined by the binding sites in the regulatory region of downstream genes. The architecture of genetic networks is intrinsically hierarchical, and discrete subcircuits that accomplish particular tasks can be identified. Certain linkage patterns are recurrent in subcircuits with similar biological function, although the regulatory genes involved differ. This suggests that the topology of the network, and not the identity of its constituents, determines the function. Key Concepts Genomic sequence determines the phenotype by specifying the amino acid sequence of proteins and their timing of expression. Regulation of gene expression occurs primarily at the level of transcription. Regulatory genes are sequence‐specific transcription factors that activate or repress transcription and signalling molecules that affect transcriptional activity in receiving cells. Modular arrays of transcription factor target sites within the cis‐regulatory domains of genes determine the transcription of individual genes. A genetic network is a set of regulatory genes that are functionally linked through sequence‐specific interactions. Each regulatory interaction is contingent on the state of other interactions in the network. Recurrent linkage patterns indicate that the network topology and not identity of individual regulators determines the biological function. In development, the regulatory processes at each stage determine what will happen next, leading to progressive partitioning of the embryo, and ensuring that development is unidirectional. Changes in network architecture are the underlying cause of phenotypic changes in evolution.