[en] A large experimental and modeling activity is currently taking place, aimed at better understanding the biological effects of ionizing radiation at the molecular scales. Considerable amounts of experimental data have been accumulated over the past decades in order to measure quantities such as macroscopic cellular survival curves and DNA strand damages after irradiation. In parallel, computer codes have been proposed to use a stochastic approach based on Monte Carlo technique to model physical interaction in the irradiated medium. The Geant4 tool-kit uses the object-oriented technology (C++) to describing particle-matter interactions, such as bio-medical physics and space physics, from sub-micrometer cells up to planetary scales. Geant4-DNA project is included in the Geant4 tool-kit and benefits from the easy accessibility of the Geant4 code for the development of a computing platform allowing estimation effects of ionizing radiations. In my thesis, firstly, I have contributed in the project the validation of various models with the experimental data collections extracted from the recent literature. A good agreement between total and differential cross section values corresponding to each available Geant4-DNA model and experimental data is validated by Kolmogorov-Smirnov testing. Secondly, I have improved elastic scattering process and working on the calculation of the DDCS for proton elastic scattering in water in the Geant4-DNA. In addition, I have combined Geant4 electromagnetic processes with the Geant4-DNA. This combination brought additional Geant4 simulation capabilities in complement of the possibility to combine Geant4-DNA models with other Geant4 electromagnetic models at different sizes and energy scales in a single simulation application. Finally, we have presented the usage of Geant4-DNA physics processes in Nano-meter-size targets fully implemented in a single Geant4 application. The frequencies of the deposited energy and number of direct DNA single strand break and double strand break in the simplified nucleus model are compared with other codes results and with a collection of experimental data on direct DNA dimensions on plasmid DNA. Furthermore I have implemented in Geant4-DNA theoretical cross sections of physics processes based on a Classical Trajectory Monte Carlo (CTMC) approach for modeling the detailed transport of protons and neutral hydrogen atoms in liquid water and in DNA nucleobases. (author)