2. The work of Rosalyn Franklin using x-ray crystallography established the shape as an  helix

4. Watson and Crick gathered information from various sources and built a model. The model established the method by which DNA replication could occur.

5. A Science Odyssey: You Try It: DNA Workshop Activity

6. Method of DNA replication suggested by Watson and Crick DNA helix unwinds DNA helicase breaks the H bonds between complementary base pairs and double helix separates DNA polymerase moves along the DNA pairing complementary nucleotides to the exposed bases DNA ligase joins the sugar-phosphate backbone to form a new strand of DNA

7. Scan in diagram froggy page 104 Use of cards to show process

9. In 1958 Meselsohn and Stahl conducted a series of experiments that gave strong support to the theory of semi-conservative replication.

10. They grew E.coli bacteria for many generations in a medium containing 15 N, a heavy isotope of nitrogen. The bacteria grew, incorporating the 15 N into their DNA This made their DNA heavier than normal The bacteria grown in 15 N were then transferred to a 14 N medium and left for periods of time that corresponded to the generation time of E.coli

11. Bacteria grown in 15 N (heavy) medium Transfer some bacteria to 14 N (light) medium sample 0 mins sample 20 mins sample 40 mins 15 N/ 15 N 15 N/ 14 N 14 N/ 14 N

12. semi-conservative replication

13. Results In the first generation, all DNA was mid-way between that of light and heavy DNA, so it contained equal amounts of each. In the second generation there were two sorts of DNA, one was light and the other contained equal amounts of 15N and 14N (i.e. it was like the DNA in the first generation bacteria). Throughout the investigation the DNA from the control culture produced only light bands, indicating it contained only 14N