Disciplinary silos and large amounts of specialized information in chemistry and biology courses undermines how students can make sense of these disciplines. This manuscript reports on how mechanistic reasoning across undergraduate courses may guide students towards more enduring and meaningful science learning. This team of chemistry, biology, and education researchers engaged in conversation about core mechanisms important for student learning in each disciplinary area that would connect to core mechanisms in the other disciplinary areas. The team also engaged in design work around written mechanistic explanations assessment items in each area. Those discussions prompted awareness of disciplinary and pedagogical similarities and differences about mechanisms. Our findings report on the mechanistic reasoning we focused on in each disciplinary area and how those were embodied in the assessment prompts. We discuss implications for teaching students who are traversing different subject matter information and ways of knowing.