Genetics - Invent PCR: You May Win a Nobel Prize

Invent PCR: You May Win a Nobel Prize

Teacher: Craig R. Kuchel
Subject: Understanding exactly how the polymerase chain reaction works
Grade Level(s): 11 12
Target Audience: Biology II, AP Biology
Materials Needed: handouts, background materials on PCR, printout of template DNA, primers, polymerase, felt-tipped markers, several rolls of cash register tape
Class Time: 2 fifty-minute class periods
Brief Summary: The polymerase chain reaction has revolutionized medical diagnostics, microbiology and forensic science. While the technique is rather simple and straightforward, few students understand it as they should. The typical illustration included in textbooks and articles about the subject generally dies not lead to a true understanding of how target sequences are selectively amplified. After working through this activity, students should have a clear picture of how each step in PCR works, and the means by which the desired DNA sequences is amplified.
Student Objective(s): Gain an appreciation for the simplicity, but also the complexity and elegance of PCR. Understand all the components of the protocol. Be able to explain exactly how CPR works, and why, after the 3rd cycle, the predominant DNA being formed is the desired target sequence. Comprehend the impact on their personal lives of newly-emerging technologies and the information yielded by those technologies.
Integration (tying it all together): This activity precedes an activity on DNA fingerprinting, and, while RFLP's are used for many DNA fingerprints, its relevance to forensic science is discussed. Ethical problems, uncertainties of using PCR in forensic work, and the potential for using PCR in medical diagnostic work are all explored. The value of basic research is discussed and a comparison of basic and applied research is made. The importance of understanding life at the molecular level is underscored.
Description of Activities: Students read a selection of articles about PCR, the way it works, its incredible impact on microbiology, genetics, and forensics, and its inventor. They then work through a paper and pencil activity in which they imagine that they are inventing PCR. They learn about the various tools and techniques involved in the protocol, and develop some understanding of the chemical components involved and the requirements for thermal cycling. They use information from the New England Biolabs catalog to help them to make some decisions about which polymerase may work best and what are optimal temperatures for denaturing, annealing, and elongation of the DNA strand. Finally, the class period is spent actually performing several PCR cycles using a large paper model and constructing new DNA copies using markers and cash register tape. It quickly becomes obvious that, after the 3rd cycle, the target sequence becomes the predominant species being amplified.
Further Information and References: Anon. 1994. Polymerase Chain Reaction. Carolina Tips Vol. 54 (4). Guyer, R.L., and D.E. Koshland. The Molecule of the Year. Science 246:1541-1546. Marks, J.L. DNA Fingerprinting Takes the Witness Stand. Science 240:1616-1618. Mullis, K.B. 1990. The Unusual Origin on the Polymerase Chain Reaction. Scientific American 262:56-65. May 1992, November 1994. Articles in Popular Science.

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