Inquiry in Action: Interpreting Scientific Papers
by Ruth Buskirk and Christopher M. Gillen
Key to Questions
Article 1
Inquiry Figure 16.11: Does DNA Replication Follow the Conservative, Semiconservative,
or Dispersive Model?
Key to Questions
1. Meselson and Stahl, California Institute of Technology
2. Proceedings of the National Academy of Science, 1958
3. National Foundation for Infantile Paralysis and the National Institutes of Health
4.
density-gradient centrifugation: separation of a mixture of macromolecules, placed in a
concentrated solution of cesium chloride, according to their density in a centrifuge
generation time: the time between the birth of one organism (here, a bacterium) and the birth of
its offspring
isotope: one of several atomic forms of an element, each containing a different number of
neutrons and thus having a different mass
5. Heavy nitrogen isotopes, N15
6. DNA composed of N15 has greater density. Higher-density material is to the right in the
photograph.
7. (a) About one-half; (b) by 4.3 hours, the band of DNA begins to narrow; and by 19.2 hours,
the band was fairly narrow; (c) middle.
8. The line with two peaks is a graph of the darkness of the band images in Figure 2a, showing
the position of the same two bands along the same horizontal scale of density.
9. They added the growing bacteria culture to a growth medium containing a tenfold excess of
N14H4Cl.
10. The density of DNA varies directly with the fraction consisting of N15. Therefore, the relative
position of the band between the band of fully labeled DNA and the band of fully unlabeled
DNA indicates how much of the DNA was labeled with N15.
11. (a) 0.81 hours for experiment 1 and 0.85 hours for experiment 2; (b) about 3.5 hours for 4
generations; (c) switch bacteria from growth media containing only N15 molecules to growth
media containing only N14 molecules.
12. (a) After 1.0 generation, the peak DNA density was lower than the density of initial DNA.
(b) After two generations, there were two peaks, and one of those was at the same density as the
peak after one generation. (c) None
13. One generation time after the addition of N14, these half-labeled or “hybrid” molecules alone
are observed. Thus, all daughter molecules are half-labeled.
14.
(a)
(b)
(c)
15. If the two images in each pair were different, there would be two peaks in sketch a.
16. The two chains separate, then each chain serves as the template for the synthesis of its
complement.
17. (a) Bases (purines and pyrimidines); (b) You could label the phosphate with a radioisotope of
phosphorus, P32, or label the deoxyribose sugar with a radioisotope of carbon, C14.
18. In “conservative” replication, all the atoms of the original DNA molecule would be together
in one of the daughter molecules and the other daughter molecule would be made from all new
(nonlabeled) atoms.
19. As long as the pairs of nitrogen bases are present in the DNA fragments, we can still see the
proportions of heavy and light nitrogen that support the Watson-Crick model of replication.
20. You could label a different part of the DNA molecule, as suggested in question 16, and see if
the density results are the same. You could repeat these tests with another species of bacteria or
with cells from a eukaryotic organism to see if the results can be generalized to all cells.