DYNAMICAL BEHAVIOR OF BIOLOGICAL REGULATORY NETWORKS .1. BIOLOGICAL ROLE OF FEEDBACK LOOPS AND PRACTICAL USE OF THE CONCEPT OF THE LOOP-CHARACTERISTIC STATE

In the field of biological regulation, models dictated by experimental work are usually complex networks comprising intertwined feedback loo ps. In this paper the biological roles of individual positive loops (m ultistationarity, differentiation) and negative loops (homeostasis, wi th or without oscillations, buffering of gene dosage effect) are discu ssed. The relationship between feedback loops and steady states is the n clarified, and the problem: ''How can one conveniently disentangle c omplex networks?'' is then considered. Initiated long ago, logical des criptions have been generalized from various viewpoints; these develop ments are briefly discussed. The recent concept of the loop-characteri stic state, defined as the logical state located at the level of the t hresholds involved in the loop, together with its application, are the n presented. Biological applications are also discussed.