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Meselson and Stahl grew bacteria in the presence of a heavy isotope of nitrogen ($^{14}N$). The DNA was then transferred to a growth medium containing nucleotides with a lighter isotope of nitrogen ($^{12}N$) which then proceeded to replicate.

The DNA was centrifuged to separate it by weight forming bands. After one round of DNA replication, there was one band in the middle of the centrifuge tube. After two rounds of DNA replication in the lighter nitrogen, two separate bands of DNA could be seen. This demonstrated that replication is semi-conservative.

Which of the following BEST explains why these results support this idea?


After two rounds of replication, only two of the four daughter DNA molecules still contained the heavy nitrogen.


After both the first and second round of replication, the ratio of heavy nitrogen versus light nitrogen varied randomly.


After two rounds of replication, one molecule of DNA contained the only heavy nitrogen in its structure and the other three contained light nitrogen.


After each round of replication, only hybrid $^{14}N$-$^{15}N$ DNA molecules formed, creating two bands after two rounds.

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