The purpose of this research was twofold. The first was to investigate the development of preservice teachers' knowledge base for science teaching on the rate of reaction topic within the 14-week content representation (CoRe)-based mentoring-enriched practicum course. In this course, CoRe design was used as a form of lesson planning and intensive educative mentoring was provided by teaching assistants. The second purpose was to identify which aspects of the practicum course contributed to preservice teachers' development. Three preservice chemistry teachers voluntarily participated in the study. Qualitative data sources included CoRe, semistructured interviews, and reflection papers. Both inductive and deductive analyses were applied by triangulating findings from multiple data sources. Results revealed that at the end of the practicum, participants were able to successfully link content with teaching to promote student learning. Their pedagogical content knowledge (PCK) developed with its all components coherently integrated with each other. They also started to view the PCK construct as professional pedagogical knowledge. Educative mentoring with the use of an explicit and shared PCK language supported by CoRes, teaching experiences, and the observation of peers were reported as the most valuable aspects in stimulating teacher development. Implications for teachers' professional growth and research are discussed.