Previous research has shown that undergraduate science students learn from peer discussions of in-class clicker questions. However, the features that characterize such discussions are largely unknown, as are the instructional factors that may lead students into productive discussions. To explore these questions, we recorded and transcribed 83 discussions among groups of students discussing 34 different clicker questions in an upper-level developmental biology class. Discussion transcripts were analyzed for features such as making claims, questioning, and explaining reasoning. In addition, transcripts were categorized by the quality of reasoning students used and for performance features, such as percent correct on initial vote, percent correct on revote, and normalized learning change. We found that the majority of student discussions included exchanges of reasoning that used evidence and that many such exchanges resulted in students achieving the correct answer. Students also had discussions in which ideas were exchanged, but the correct answer not achieved. Importantly, instructor prompts that asked students to use reasoning resulted in significantly more discussions containing reasoning connected to evidence than without such prompts. Overall, these results suggest that these upper-level biology students readily employ reasoning in their discussions and are positively influenced by instructor cues.