In the field of biotechnology and medicine it is of interest to model and simulate metabolic processes. The usual methods to model metabolic pathways are chemical descriptions and differential equations. Moreover, the graph theoretical aspect is discussed and the development of expert systems is in process. In this paper we present the formalization of metabolic processes. Our formalization is based on the theory of formal languages. This formalization is called genetic grammar and represents an expansion of the Semi-Thue-System.