Name ___________________________________ Date __________ Block ____________ Discovering DNA’s Structure Introduction: Even though DNA is considered large when it comes to molecules, it is still very tiny in size. The most powerful microscopes that we have are still unable to see what the molecule looks like. If we can’t see it, then how are scientists so sure that it looks the way it’s drawn? The purpose of this activity is to take you through some of the data scientists used to figure out DNA’s structure. Part A: History of DNA – briefly examine some of the scientific discoveries that led to Watson and Crick discovering DNA’s structure. Fill in the missing info in the table below Name Event in DNA History Year Gregor Mendel 1878 Showed that chromosomes carry the cell’s units of inheritance Phoebus Levene 1928 Determined the molecule that Griffith discovered was DNA 1950 Rosalind Franklin 1953 1. What is one thing that surprised you, or that you found interesting from the timeline above? Explain why it surprised you. – “Nothing surprised me, or interested me” is NOT a valid answer. 2. Looking at the timeline as a whole, how many scientists were involved in figuring out DNA’s structure (it wasn’t just Watson and Crick)? 3. Based on the timeline as a whole, how many different experiments were involved in building the knowledge Watson and Crick needed to figure out the structure of DNA? 4. Do you think Watson and Crick could have figured out DNA’s structure if Gregor Mendel (or any other scientist) hadn’t discovered the field of genetics by studying pea plants in his monastery’s gardens? Explain. 5. Based on the timeline, why do you think it is important for scientists to publish/share the findings of their experiments with other scientists? Part B: Ewin Chargaff’s Data – Chargaff discovered two rules that played a major role in the discovery of DNA’s structure. Interpret the data below to come up with Chargaff’s rules: Table 1: Relative Composition (mole %) of Bases in DNA Organism Human Chicken Grasshopper Sea Urchin Wheat Yeast E. coli Adenine (A) 30.9 28.8 29.3 32.8 27.3 31.3 24.7 Thymine (T) 29.4 29.2 29.3 32.1 27.1 32.9 23.6 Guanine (G) 19.9 20.5 20.5 17.7 22.7 18.7 26.0 Cytosine (C) 19.8 21.5 20.7 17.3 22.8 17.1 25.7 6. According to what we learned from the 3D DNA Questions worksheet, how do the bases in DNA pair with each other? 7. Look at Chargaff’s Data for humans. Calculate the ratio of A to T and G to C? Show your work A to T ratio: _______________ What does this ratio tell you? G to C ratio: _________________ What does this ratio tell you? 8. Quickly review the data for the other organisms in the table. Do the ratio findings only occur for human DNA? Explain. 9. If you were Chargaff, what conclusion would you draw based on the data comparison? 10. Chargaff also compared the base pairing ratios of different organisms. Compare the human DNA base composition to the other organisms. Do any of the other organisms have the same percentage distribution as human DNA? Explain. Which organism has the closest base composition to the human’s? ______________ Which organism’s base composition is furthest from the human’s? _______________ 11. If you were Chargaff, what conclusion would you draw based on this data comparison? Part 3: Rosalind Franklin’s Data – Franklin was a rarity in her day because, not many females went into science and those that did were often not respected as much as their male counterparts. Rosalind took meticulous pictures (pictures so to speak) of DNA and was able to determine a couple of important details that helped Watson and Crick. The pictures Rosalind took were X-ray diffractions of DNA. See below: A. This X-ray crystallographic pic of DNA told Franklin that DNA was a double helix. B. It was already know by scientists that DNA was a double helix, but because Franklin’s pics were so clear, she was able to calculate the diameter of DNA: 20 Angstroms C. The diameters of A, T, C and G had been measured as well: T and C – 8 Angstroms G and A – 12 Angstroms 12. Based on the data above, which two bases could Adenine (A) bond with? Why these two bases (explain using the data)? 13. How can we use Chargaff’s data to help us figure out which of these two bases Adenine actually bonds with? Read this: Rosalind Franklin did not herself discover the structure of DNA despite her major contribution. Instead two young scientists looking for fame, James Watson and Francis Crick figure it out by using Chargaff’s data, which had been published, and Rosalind Franklin’s data, which she had yet to publish. Some say they ‘stole’ Franklin’s data because she had yet to publish it, and did not knowingly share it with them. Once a scientist’s data has been published, it is considered public property, but before then it is that scientist’s sole academic property. 14. Could Watson and Crick have definitively concluded that Adenine bonds to Thymine and Guanine to Cytosine, using Chargaff’s data only? (Hint: think of all possible combinations) Explain. Read this: Watson and Crick used other data to help them determine the structure of DNA, but Chargaff’s and Franklin’s data were some of the most important that they needed. 15. Based on this diagram, what are two other pieces of data that Watson and Crick needed to know in order to get DNA’s structure correct?
