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Following James Watson and Francis Crick’s publication describing the double helical structure of DNA in 1953, scientists were trying to determine how DNA replicated. There were three models proposed: the conservative model, the semi-conservative model, and the dispersive model.

In the conservative model, scientists proposed that a double-stranded DNA molecule was replicated but remained intact, resulting in two double-stranded molecules, one of which was the original double-stranded molecule, and the other of which was composed of two newly synthesized strands. In the semi-conservative model, scientists proposed that each of the two resulting double-stranded DNA molecules was composed of one original strand and one newly synthesized strand.

Finally, in the dispersive model, scientists proposed that the original double-stranded molecule combined with chunks of newly synthesized double-stranded DNA, resulting in two double-stranded DNA molecules, each composed of both original double-stranded DNA chunks and newly synthesized double-stranded DNA chunks.

In 1958, Matthew Meselson and Frank Stahl designed an experiment to differentiate the three models. They grew several generations of cells in a culture medium containing ${ NH }_{ 4 }^{ + }$ with a “heavy” isotope of nitrogen, $_{ }^{ 15 }N$, ensuring that all of the nitrogen incorporated into the DNA was $_{ }^{ 15 }N$.

The two molecular biologists then switched the culture medium to ${ NH }_{ 4 }^{ + }$ with a “light” isotope of nitrogen, $_{ }^{ 14 }N$. They allowed one round of replication to occur, lysed some of the cells, and centrifuged their contents in cesium chloride, separating the DNA based upon density.

They additionally allowed the cells to proceed through a second round of replication in the presence of “light” nitrogen, again lysed some of the cells, and centrifuged their contents in cesium chloride. They expected DNA containing only $_{ }^{ 14 }N$-labeled DNA to form a light band high up in the centrifuge tube, while DNA containing only $_{ }^{ 15 }N$-DNA would form a heavy band lower down in the centrifuge tube. Hybrid DNA composed of similar amounts of $_{ }^{ 15 }N$-DNA and $_{ }^{ 14 }N$-DNA would form a band of intermediate density.

Suppose that, after the first round of replication, cellular lysis, and centrifugation of the DNA in cesium chloride, Meselson and Stahl observed the banding pattern shown in the tube labeled A, above. Then, after the second round of replication, cellular lysis, and centrifugation of the DNA in cesium chloride, they observed the banding pattern shown in the tube labeled B above.

Based upon these results, which of the following is TRUE?

Select ALL that apply.

A

The result following the first round of replication is consistent with the conservative model.

B

The result following the first round of replication is consistent with the semi-conservative model.

C

The result following the first round of replication is consistent with the dispersive model.

D

The result following the first round of replication cannot distinguish between the three models.

E

The result following the second round of replication is consistent with the conservative model.

F

The result following the second round of replication is consistent with the semi-conservative model.

G

The result following the second round of replication is consistent with the dispersive model.

H

The result following the second round of replication cannot distinguish between the three models.

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