When students have varying prior knowledge, personalized instruction is desirable. One way to personalize instruction is by using adaptive e-learning to offer training of varying complexity. In this study, we developed a web-based adaptive tutor to teach PCR primer design: the PCR Tutor. We used part of the Taxonomy of Educational Objectives (the three cognitive processes: remember, understand, and apply) to design exercises of varying complexity. Using this method, we demonstrated that we were able to systematically categorize exercises. There was also a good learning effect and a positive student perception when using the PCR Tutor. (Contains 3 tables and 1 figure.)