Despite numerous studies on the tumor suppressor p53, a complete picture of its role in cell arrest and killing in G(1), S and G(2)M phases after drug treatment is lacking. We tackled the analysis of the complexity of cell cycle effects combining the time-course measures with different techniques with the aid of a computer program simulating cell cycle progression. This mixed experimental-simulation approach enabled us to decode the dynamics of the cytostatic and cytotoxic responses to cisplatin and doxorubicin treatments in a p53--proficient colon carcinoma cell line (HCT-116) and in its p53-deficient counterpart. We achieved a separate evaluation of the activity of each cell cycle control and we connected these results with measures of p53 level in G(1), S and G(2)M. We confirmed the action of p53 in all cell cycle phases, but also the presence of strong p53-independent cytostatic and cytotoxic activities exerted by both drugs. In G(1) phase, p53 was responsible for a medium/long term block, distinct from the short-term block, which was p53-independent. The delay in traversing S phase was reduced by the presence of p53. In G(2)M phase, despite a strong p53-independent block, there was a weaker but more persistent p53-dependent block. At cytotoxic concentrations, p53-dependent and p53-independent cell death was observed. The former was poorly phase-specific, occurred earlier and exploited the apoptotic mechanism more than p53-independent death. Computer simulation produced a framework where previous partial and sometimes apparently contradictory observations of the p53-mediated effects could be reconciled and explained.