http://mindbites.com/lesson/1457-biology-watson-and-crick-the-clues Professor Wolfe explains the history behind the discovery of the strucutre of DNA. Molecules are too small to observe, and this means it was impossible to determine the molecular structure of DNA. In 1950, Franklin and Wilkins used X-ray defraction to create images of DNA, exposing a possible helical structure. They also determined 3 repetitive ratios, 0.34, 3.4, and 2.0, but did not know what these represented. This information was used by Watson and Crick to develop the first accurate approximation of the structure of DNA. They began with the belief that the structure was a double-helix and used molecular models, to piece together the structure. Watson and Crick discovered that the 0.34 nm number was the distance between each nucleotide, and the 3.4 nm number was 10 nucleotides. The 2.0 nm number they determined to be the diameter of the helix. They also discovered that purines and pyrimidines created a hydrogen bond across the double-helix. Using these dimensions and information, they created the model of DNA that has become the foundation for molecular genetics. Taught by Professor George Wolfe, this lesson was selected from a broader, comprehensive course, Biology. This course and others are available from Thinkwell, Inc. The full course can be found at http://www.thinkwell.com/student/product/biology. The full course covers evolution, ecology, inorganic and organic chemistry, cell biology, respiration, molecular genetics, photosynthesis, biotechnology, cell reproduction, Mendelian genetics and mutation, population genetics and mutation, animal systems and homeostasis, evolution of life on earth, and plant systems and homeostasis. George Wolfe brings 30+ years of teaching and curriculum writing experience to Thinkwell Biology. His teaching career started in Zaire, Africa where he taught Biology, Chemistry, Political Economics, and Physical Education in the Peace Corps. Since then, he's taught in the Western NY region, spending the last 20 years in the Rochester City School District where he is the Director of the Loudoun Academy of Science. Besides his teaching career, Mr. Wolfe has also been an Emmy-winning television host, fielding live questions for the PBS/WXXI production of Homework Hotline as well as writing and performing in "Football Physics" segments for the Buffalo Bills and the Discover Channel. His contributions to education have been extensive, serving on multiple advisory boards including the Cornell Institute of Physics Teachers, the Cornell Institute of Biology Teachers and the Harvard-Smithsonian Center for Astrophysics SportSmarts curriculum project. He has authored several publications including "The Nasonia Project", a lab series built around the genetics and behaviors of a parasitic wasp. He has received numerous awards throughout his teaching career including the NSTA Presidential Excellence Award, The National Association of Biology Teachers Outstanding Biology Teacher Award for New York State, The Shell Award for Outstanding Science Educator, and was recently inducted in the National Teaching Hall of Fame.
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