To the reader <p> Since I worked on two separate projects towards my doctorate thesis, the arrangement of my thesis is rather unusual. The reader will find that my thesis is divided into four parts. Part 1 is dedicated to a very general introduction about the basic knowledge needed to guide you, the reader, through the rest of the thesis. Within this part, different sections focus on different fundamental aspects of Biophysics related to my work. In Part 2, I discuss my studies of the distribution and dynamics of the nuclear protein Ran in live interphase HeLa cells. This part contains a background section specific to this project, the materials and methods used for this study, experimental results, a discussion of our findings, and it ends with conclusions. Part 3 is dedicated to the study of the dynamical mechanisms responsible for the establishment of the Bed protein concentration gradient along the anterior-posterior axis in live Drosophila melanogaster embryos. Again, a specific background section is included in this part, followed by the materials and methods used to perform this research, results, discussions and finally I will summarize my results to conclude this work. The last part, part 4, is rather short and contains the summary of the overall results of my work on both nuclear proteins with some emphasis on the similarities and differences in their dynamical behavior.</p> / Thesis / Doctor of Philosophy (PhD)