ABSTRACT
Cellular integrity, physiological homeostasis, identical replication of the genome, and cell division are regulated in a complex network of interactions of different kinds of molecules. These encompass not only proteins, but also lipids and derivatives thereof, nucleic acids, and low molecular weight components. The function and interaction of the various cellular components are regulated essentially in two different ways:
The physiological action might be constitutive, as it is, for example, for the energy-supplying biochemical pathway. Most of these housekeeping genes and proteins are expressed constitutively during the cell cycle. This is also valid for many luxury proteins expressed in differentiated cells.
64 The physiological function of genes or proteins might be necessary only during a distinct period of the cellular life cycle. Most of the molecules necessary for the replication of the genome (i.e., DNA synthesis proteins, or regulatory molecules like cyclins) are expressed, or at least regulated, in a cell cycle-dependent fashion.
