A simple and cheap laboratory class is proposed to illustrate the lethal effect of UV radiation on bacteria and the operation of different DNA repair mechanisms. The class is divided into two sessions, an initial 3-hour experimental session and a second 2-hour analytical session. The experimental session involves two separate experiments: one dedicated to illustrating the lethal effect of UV radiation and the protective effect of DNA photorepair; the second to explore the operation of DNA repair mechanisms that prioritise survival but introduce mutations. The procedure makes use of a "Pseudomonas aeruginosa" double auxotroph, which serves to detect UV-induced back-mutations to prototrophy. The proposed scheme is carried out by undergraduate students of the Bacterial Physiology and Genetics course, as part of our Biotechnology curriculum. We think that it will be a valuable tool for microbiology students to increase their understanding of basic genetic concepts. (Contains 1 table and 3 figures.)