We present a brief review of molecular biological basis and mathematical modelling of circadian rhythms in Drosophila. We discuss pertinent aspects of a new model that incorporates the transcriptional feedback loops revealed so far in the network of the circadian clock (PER/TIM and VRI/PDP1 loops). Conventional Hill functions are not used to describe the regulation of genes, instead the explicit reactions of binding and unbinding processes of transcription factors to promoters are probabilistically modelled. The model is described by a set of ordinary differential equations, and the parameters are estimated from the in vitro experimental data of the clocks' components. The model is robust over a wide range of parameter variations. Through the sensitivity analysis of the model, roles of VRI and PDP1 feedback loops are investigated, and it is proposed that they increase the robustness of the clock.

Studying the evolutive and adaptative machanisms of prokayotes is a complicated task. As these machanisms cannot be easily studied "in vivo", it is necessary to consider other methods. We have therefore developed the RAevol model, a model designed to study the evolution of bacteria and their adaptationto the environment. Our model simulates the evolution of a population of artificial bacteria in a changing environment, providing us with an insight into the strategies that digital organisms develop to adapt to new conditions. In this paper we describe the principle and architecture of the model, focusing on the mechanisms of the regulatory networks of artificial organisms. Experiments were conducted on populations of artificial bacteria under conditions of stress. We study the ways in which organisms adapt to environmental changes and examine the strategies they adopt. An analysis of these adaptation strategies is presented and a brief overview was proposed concerning the patterns and topological characteristics of the evolved regulatory networks.

No other concepts have shaken so deeply the bases of engineering like those of positive and negative feedback. They have played a most prominent role in engineering since the beginning of the previous century. The birth certificate of positive feedback can be traced back to a pair of patents by Edwin H. Armstrong in 1914 and 1922, whereas that of negative feedback is already lost in time. We present in this paper a short review on the feedback's origins in the fields of engineering and biology. Besides, we compare the main feedback's ideas in control theory and system biology in order to get a better understanding of the lactose and tryptophan operons' regulatory systems (control systems). It is obvious that we need to know the genome of an organism in order to understand how it works, but that is only half of the puzzle, for we also need to know how the underlying genetic regulatory systems work in order to get a complete picture of the cell's dynamics.

Hill functions follow from the equilibrium state of the reaction in which n ligands simultaneously bind a single receptor. This result if often employed to interpret the Hill coefficient as the number of ligand binding sites in all kinds of reaction schemes. Here, we study the equilibrium states of the reactions in which n ligand bind a receptor sequentially, both non-cooperatively and in a cooperative fashion. The main outcomes of such analysis are that: n is not a good estimate, but only an upper bound, for the Hill coefficient; while the Hill coefficient depends quite strongly on the cooperativity level among ligands. We finally use these results to discuss the feasibility and constrains of using Hill functions to model the regulatory functions in mathematical models of gene regulatory networks.