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Classic Experiments in Molecular Biology


Robin Pals-Rylaarsdam
College of Arts and Sciences
Saint Xavier University


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.


  • Accurately describe the experiments performed by each group, observations made, and conclusions drawn.
  • Articulate several possible outcomes/hypotheses for a given experiment, select the outcome that best fits the data collected, and explain why the other hypotheses are rejected in light of the data.
  • Accurately answer questions regarding the evidence for semiconservative replication and the evidence for DNA being the genetic material in E. coli.


DNA; Griffith; Avery; Streptococcus pneumonia; genetic material; DNA replication; Meselson; Stahl; semi-conservative; experimental data; experimental design

Topical Areas

History of science, Scientific method

Educational Level

High school, Undergraduate lower division



Type / Methods

Discussion, Interrupted



Subject Headings

Cell Biology  |   Molecular Biology  |   Biology (General)  |   Genetics / Heredity  |  

Date Posted


Teaching Notes

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Teaching notes are intended to help teachers select and adopt a case. They typically include a summary of the case, teaching objectives, information about the intended audience, details about how the case may be taught, and a list of references and resources.

Supplemental Materials

An alternative version of the case is provided below, differing only on p. 5 where figure and text are more historically accurate (but may distract from the overall conclusion).

In addition, we provide a link below to materials available from HHMI BioInteractive that can be used in conjunction with this case.

  Alternative Version
  Pulse Chase Primer hhmi/

Answer Key

Answer keys for the cases in our collection are password-protected and access to them is limited to paid subscribed instructors. To become a paid subscriber, begin the process by registering.


The following video(s) are recommended for use in association with this case study.

  • The Double Helix hhmi/
    The Double Helix is the story of the scientists and evidence involved in one of the most important scientific quests of the 20th century: the discovery of the structure of DNA. Running time: 16:53 min. Produced by HHMI BioInteractive.
  • The Chemical Structure of DNA hhmi/
    Short animation showing the detailed structure of DNA. Running time: 2:44 min. Produced by HHMI BioInteractive.
  • DNA Replication Schematic hhmi/
    A 50-second animation that explains the mechanism of replication. The double helix unwinds and each strand acts as a template for the construction of the new DNA molecule. Running time 0:50 min.Produced by HHMI BioInteractive.
  • DNA Replication Basic Detail hhmi/
    Short 3-D animation showing how DNA is replicated at the molecular level. It involves an enzyme that unwinds the DNA and other enzymes that copy the two resulting strands. Running time: 1:07 min. Produced by HHMI BioInteractive.second
  • DNA Replication Advanced Detail hhmi/
    Short animation showing both strands of the DNA double helix acting as templates for the new DNA strands. Incoming DNA is unraveled by the enzyme helicase, resulting in the 3' strand and the 5' strand. The 3' strands and the 5' strands are replicated by a DNA polymerase enzyme but in different ways. Running time: 2:20 min. Produced by HHMI BioInteractive.