M03_REEC5174_09_IE_16-20 - Notes to Instructors Chapter 16...

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Notes to Instructors Chapter 16 The Molecular Basis of Inheritance What is the focus of these activities? Almost all introductory biology students know that DNA is the hereditary material in living cells. Many of them have a difficult time visualizing its overall structure, however, and how that structure and the characteristics of associated enzymes determine its mode of replication. What are the particular activities designed to do? Activity 16.1 Is the hereditary material DNA or protein? This activity is designed to help students organize and review the experiments and thought processes that lead first to an understanding that DNA is the hereditary material and later to the structure of the DNA double helix. Activity 16.2 How does DNA replicate? This activity is designed to give students a better understanding of both the overall process of DNA replication and the experimental evidence used to support the semiconservative model of replication. What misconceptions or difficulties can these activities reveal? Activity 16.1 This activity asks students to review and integrate the evidence from a series of experiments that together demonstrated that DNA (not protein) is the hereditary material. The vast majority of introductory biology students already know that DNA is the hereditary material (and have known this “all their lives”). As a result, they often “can’t see the point in rehashing old experiments.” It is often necessary to be very explicit about what you want them to learn from this review. You may need to state that you want students to understand the logic behind the experiments in addition to the evidence that the experiments provide. In addition, many students don’t understand the experiments of Meselson and Stahl. We offer a couple of reasons: 1. Many don’t understand why growing bacteria in a medium that contains only 15 N means that the DNA the bacteria produce will ultimately contain 15 N in its nucleotides. It is useful to let them know that, like plants, many bacteria are capable of manufacturing macromolecules (like nucleotides) from inorganic precursors. 2. Students may not understand why DNA that contains 15 N will layer out in a centrifuge tube lower than DNA that contains 14 N. They generally don’t understand that the centrifuge tube contains a density gradient established prior to the addition of DNA and that centrifugation separates compounds based on their density. ( Note: A density gradient can be set up by centrifuging a concentrated solution of CsCl in an ultracentrifuge for up to 24 hours.) Activity 16.2 Students tend to encounter a number of difficulties, misconceptions, and missing conceptions as they model DNA replication. Here are several possible problems: 1. Many students don’t understand what 5 and 3 mean relative to the structure of DNA. To explain this, it is useful to draw a deoxyribose molecule and show how the carbons are numbered clockwise in the ring (from the oxygen). The phosphate group is attached to the number 5 carbon, the base is attached to the
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This note was uploaded on 02/08/2012 for the course BIOLOGY 101 taught by Professor Hayes during the Spring '09 term at Harvard.

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M03_REEC5174_09_IE_16-20 - Notes to Instructors Chapter 16...

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