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Before the After one switch round of to 14N replication

After two After three rounds of rounds of replication replication

Before the After one switch round of to 14N replication

After two After three rounds of rounds of replication replication

(a) semiconservative;

DNA labeled with 15N will be denser than DNA labeled with 14N; therefore 15N-labeled DNA will sink lower in the density-gradient tube. Before the switch to medium containing 14N, all DNA in the bacteria will contain 15N and will produce a single band in the lower end of the tube.

(a) With semiconservative replication, the two strands separate, and each serves as a template on which a new strand is synthesized. After one round of replication, the original template strand of each molecule will contain 15N and the new strand of each molecule will contain 14N; so a single band will appear in the density gradient halfway between the positions expected of DNA with 15N and of DNA with 14N. In the next round of replication, the two strands again separate and serve as templates for new strands. Each of the new strands contains only 14N, thus some DNA molecules will contain one strand with the original 15N and one strand with new 14N, whereas the other molecules will contain two strands with 14N. This labeling will produce two bands, one at the intermediate position and one at a higher position in the tube. Additional rounds of replication should produce increasing amounts of DNA that contains only 14N; so the higher band will get darker.

(c) In dispersive replication, both nucleotide strands break down into fragments that serve as templates for the synthesis of new DNA. The fragments then reassemble into DNA molecules. After one round of replication, all DNA should contain approximately half 15N and half 14N, producing a single band that is halfway between the positions expected of DNA labeled with 15N and of DNA labeled with 14N. With further rounds of replication, the proportion of 14N in each molecule increases; so a single hybrid band remains, but its position in the density gradient will move upward. The band is also expected to get darker as the total amount of DNA increases.

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