While it is generally agreed that all students should read to learn a subject, content area teachers in Argentine high schools hardly guide students on how to do it. In Biology instruction, there is even less precisions about how lessons can be organized in order to integrate reading and writing to learn tasks. Taking a design-based research approach, we analyze how two Biology teachers conduct multivoiced classroom discussions, interweaving reading to write with oral discourse, so that students learn about Protein Synthesis. To this end, we developed (together with two Biology teachers), observed and registered a teaching sequence that intertwines reading, writing and image interpretation in the penultimate year of two high schools. To analyze it, we considered Brousseau‘s theory of didactical situations and its further developments, which offer conceptual tools to distinguish and characterize teacher’s action. Specifically, four functions of teachers’ mediation are taken from the theory: activity definition, return of responsibility to the students about classwork, regulation of student activity and institutionalization of knowledge constructed. As a result, we found that the institutionalization of teacher’s disciplinary knowledge before reading obstructs the opportunity for students to experience the need for searching for information in texts when they are required to write. On the other hand, when this institutionalization is postponed, and professors regulate and institutionalize how to use disciplinary texts, and with what purposes, students learn to use them and gain autonomy in their comprehension and writing. Thus, in order to read, write, and learn in science, the institutionalization of disciplinary knowledge should be accompanied and preceded by the regulation and institutionalization of reading and writing practices. This analysis contributes to literacy instruction research showing the heuristic power of the theory of didactical situations, born in Mathematical teaching research, to new fields of inquiry.