Biology | Answers to Review Questions

Author Dr. George B. Johnson, Washington University
Author Dr. Peter H. Raven, Missouri Botanical Gardens & Washington University
Contributor Dr. Susan Singer, Carleton College
Contributor Dr. Jonathan Losos, Washington University

Answers to Review Questions

Chapter 14 (p. 298)

1. The cap and stalk died when the foot was removed. When the cap was cut off, the foot and stalk were not affected, and the cap regenerated. The A. mediterranea foot still grew an A. mediterranea cap, even with an A. crenulata stalk grafted onto it.

2. Hershey and Chase labeled viral nucleic acid and viral protein coat with separate radioactive isotopes. After infected cells were analyzed, they found only viral nucleic acid inside the infected cells, not viral protein coat.

3. The three-dimensional shape of the molecule is a double helical strand. Hydrogen bonding between nitrogenous bases is responsible for holding the two strands of the helix together. Purines can only hydrogen-bond with pyrimidines, which is what accounts for the equal number of purines and pyrimidines in a DNA molecule.

4. Meselson and Stahl showed that DNA replication was semiconservative by labeling new DNA with a heavy isotope so that older and newer strands could be identified. After allowing the DNA to replicate awhile, it was spun on a cesium gradient, showing the newly-replicated material to weigh intermediate between the labeled and unlabeled DNA, suggesting that one chain (the new one) was "heavy," while the older (original) chain was "light."

5. The leading strand is replicated continuously while the lagging strand is replicated in fragments (Okazaki fragments). The reason for the discrepancy in replication strategy for the different strands is that the two strands are anti-parallel.

6. Beadle and Tatum wanted to test the hypothesis that DNA coded for specific enzymes, and tested their hypothesis on mold. They irradiated mold to see if they could create mutants incapable of synthesizing necessary enzymes in certain biosynthetic pathways. They were successful. Some of their mutants could not grow on a minimal medium unless they were supplied with the missing enzyme.