ABSTRACT
Specific genes are used only when and where they are needed. For example, we have made much of the classic example of the lac operon, which governs the enzymes responsible for lactose digestion. Similar control is exercised over genes in other bacteria, archaea, and eukaryotes. One of the most intriguing reasons why the chemistry of the cell cannot be viewed as a bag of reactants and products is the fact that this chemistry is under the strict control of the genetic machinery of the cell. In particular, if left to its own devices, some particular chemical pathway in the cell might just travel a path to eventual equilibrium. Often, genetic networks serve as the basis of the developmental decisions that send a cell or collections of cells down some developmental path. Quantitative measurement of gene expression can be made at many stages between the decision to start transcription and the emergence of a functional protein product.
