1. Chargaff’s rule states that the amount of in DNA is always roughly equal to the amount of thymine, and that the amount of guanine in DNA is always roughly equal to the amount of cytosine.
2. The outside of the DNA double helix is made of alternating groups of deoxyribose sugars and .
3. The DNA double helix is , meaning that the two strands run in opposite directions.
4. What's x-ray crystallography?
5. Explain how Rosalind Franklin's x-ray crystallography work helped Watson and Crick in their quest to determine the physical structure of DNA.
6. What did Watson, Crick, and Franklin discover about the structure of DNA?
7. How did Watson, Crick, and Franklin's work reiterate Chargaff's findings?

1. adenine

2. phosphates

3. antiparallel

4. X-ray crystallography is a technique used to learn more about the three-dimensional structure of biological molecules. In x-ray crystallography, a molecule (for example, DNA) is crystallized. Then, x-ray beams are directed onto the molecular crystal. The pattern of diffraction, or bending, of the x-rays provides information about the physical properties of the molecule. Rosalind Franklin used x-ray crystallography to investigate the three-dimensional structure of DNA.

5. Watson and Crick were interested in learning more about the physical structure of DNA. They had come up with several hypotheses about how DNA could be structured, but they needed more concrete evidence of the molecule's structure. Rosalind Franklin's Photo 51 provided important information to Watson and Crick as they worked on a model of the three-dimensional structure of DNA.

6. Photo 51 confirmed that DNA is a double helix. Watson and Crick's model of DNA included that it is antiparallel, and that each base pair is held together by hydrogen bonds. The DNA chain is comprised of an outer backbone alternating between the sugar deoxyribose and phosphate groups. The base pairs, which are held together by hydrogen bonds, are located on the inner portion of the double helix.

7. Chargaff found that the number of adenine nucleotides was always roughly the same as the number of thymines, and the number of cytosines was about the same as the number of guanines. This provided the insight that in the DNA double helix, A selectively bonds with T, while G bonds with C. This explains why Chargaff consistently found that the number of As and Ts, and the number of Cs and Gs, were the same in his studies.

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