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This activity can be used in conjunction with the short film The Double Helix. It introduces students to the classic experiment by Matthew Meselson and Franklin Stahl, which revealed that DNA replication follows the semiconservative model.

All introductory biology textbooks, and many sophomore-level genetics textbooks as well, describe several classic experiments in molecular biology. This interrupted case study takes students through two of these classic experiments, namely, those by Griffith and Avery, McCarty and MacLeod that showed DNA to be the genetic material in Streptococcus pneumoniae, and the experiment by Meselson and Stahl that demonstrated DNA replication to be semiconservative. Engaging students with the experiments in a more exploratory manner can reinforce the nature of scientific discovery and the logic behind these findings. The case, which has been formatted as two separate exercises that can be used independently, was developed for use in introductory biology classes for biology majors. The material is accessible enough to also be useful for non-majors college biology or high school AP biology students.

This case study provides an overview of the seminal experimental work that led to the discovery of DNA structure and the confirmation of the semi-conservative model of DNA replication. By guiding students through a chronological series of historic experiments and discussing some of the collaborations and controversies involved in the original research, students learn about the history and nature of science in addition to several important biological concepts. A number of recommended videos, including one created by the author, enable instructors to use the "flipped-classroom" mode of instruction according to which students read primary literature and watch videos on their own before group discussions and activities. The case study was developed for use in an introductory undergraduate biology course, and would also be appropriate for use in a high school biology course. Some prior knowledge or instruction may be required, depending on the level and learning objectives of the course.