To understand genetics, students need to be able to explain and draw connections between a large number of concepts. The purpose of the study reported herein was to explore the way upper secondary science students reason about concepts in molecular genetics in order to understand protein synthesis. Data were collected by group interviews. Concept maps were constructed using the interview transcripts, and analysed. The most central concept was DNA, which served as a link between the concepts of genes and proteins. Students spontaneously introduced concepts from "classical genetics" to explain "molecular genetics." The concept maps generated from the different group interviews were similar in that various concepts consistently appeared within specific sub-groups of interconnected concepts, ie clusters. Five main clusters were identified. The students were better able to relate between concepts within a cluster than between concepts in different clusters. The clusters can be seen as representations of the students' knowledge structures, and could be used as starting points in teaching genetics. We recommend that courses in genetics should begin by focusing on students' existing connections between concepts from different clusters and then point out concepts that feature in two or more clusters such as "DNA," "gene," and "protein." (Contains 5 figures.)