This thesis covers the development of a novel genetically encoded light responsive system, or so called optogenetic system, to induce protein-protein interactions, based on the ultraviolet-B sensitive protein UVR8 and its interacting partner COP1 from Arabidopsis thaliana. With such a system, cellular functions can be perturbed instantaneously, while the use of light also enables control over the location, time, and intensity of the perturbation, if suitable optical techniques are available. By making a key step or reaction in a pathway or process light-controlled, optogenetics can be applied to the study of many biological processes, including in the study of DNA damage repair or cell cycle regulation, two processes crucial for maintaining genomic integrity. A specific step is the initial generation of a DNA double strand break. Light controlled dCas9 guided recruitment of endonucleases allows specifying the time in the cell cycle, and the exact location in the genome where a DNA double strand break is produced. This would facilitate the study of repair pathway selection in different cellular contexts. As dCas9 is foreign to human DNA, the consequences of having dCas9 bound to DNA are examined.