Milestones in Biology 2 Test Your Knowledge
Which technique did Rosalind Franklin use to examine the structure of DNA?
a. mass spectrometry
b. gel electrophoresis
c. x-ray diffraction
d. model building
e. all of the above
c
Which of the following statements about DNA structure is not true?
a. DNA is a double helix.
b. The phosphate groups are on the outside of the helix.
c. The two strands run in opposite orientations.
d. A pairs with A, T with T, C with C, and G with G.
e. The helix has a constant diameter along its length.
d
Describe the scientific contributions of Watson, Crick, Franklin, and Wilkins to the discovery of the structure of DNA.
Franklin carried out the key X-ray diffraction studies, as did Wilkins, who recognized that Franklin’s data might be helpful to Watson and Crick. Watson and Crick synthesized existing information to build models that were consistent with the observed data. Franklin’s experimental observations were critical in assembling the correct model, the one that accurately represented the structure of DNA.
How did differing amounts of water in the DNA crystals help explain the x-ray diffraction patterns?
The DNA stretched out in the presence of water, suggesting that the water must be interacting with and coating the molecule. As the charged phosphate groups are hydrophilic, and as the water was interacting with DNA on the surface of the DNA, the phosphate groups had to be on the outside of the molecule—a key insight.
Summarize the structure of a DNA double helix.
The DNA double helix is arranged as antiparallel strands. Each strand has its phosphate groups oriented along the external backbone of the molecule, and the nucleotide bases oriented toward the interior of the helix. The bases from one strand pair with bases on the other strand according to specific rules: A pairs with T and G pairs with C. The helix has a constant diameter along its length.