Argumentation is a key scientific practice and a central social and cultural process contributing to the generation, evaluation, and application of new scientific knowledge. As scientists develop arguments and construct knowledge claims, they clarify their own position, attempt to persuade others, and critique and evaluate competing claims. This paper describes the redesign of a common biology lab activity to foreground the roles argumentative critique and collaborative talk play in professional science communities for students. Tasked with designing their own protocols for strawberry DNA extraction, students use their knowledge of cell structure and a list of materials provided to create and test their group’s procedure. After obtaining results, a “lab meeting” is held in which each group presents their protocol, rationale, and results. Then, the students compare procedural details in order to collaboratively redesign a new protocol based on the data provided by each group. These discussions contextualize argumentation within an investigative setting and highlight the importance of questioning, skepticism, and critique. In classroom trials, students reported growth in both their knowledge of the process of DNA extraction and in their ability to use evidence to support ideas about designing and conducting an experiment.