The concept of genes as DNA coding for proteins remains the cornerstone for the explanation of heredity in school science. In this paper, the authors, both researchers in genetics education, explore classroom teaching of genes as DNA by analysing and interpreting the discourse of a practical lesson which was part of a larger research project in Western Australian schools. In the practical lesson, Year 10 students did a hands-on experiment to extract DNA from onion cells. The paper discusses the ways in which the teacher and her students interacted in their classroom during the experiment. In particular, it examines the dialogic interactions between two students, one of whom was the most outstanding student in the class and later found to display fruitful learning in developing her gene conception. Although the experiment appeared to provide a more tangible learning experience for most students in the class than did a textbook description of DNA or its computer multimedia representations, such experience did not contribute much to their conceptual understanding of genes. The authors finally discuss how students can benefit more from hands-on investigation as one way of representing information for learning about the complexity of the gene concept.