Escherichia coli bacteria containing only ¹⁵N-labeled DNA were grown in media containing only ¹⁴N nucleotides. What percentage of double helices would be composed of one ¹⁵N strand and one ¹⁴N strand after three generations?

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11ecba2d_0e9c_46d5_a3bf_f708e813395d_MD0008_00 DNA replication is a semiconservative process in which DNA polymerase synthesizes each daughter strand using one of the parental strands as a template. Each new double helix contains one parental strand and one daughter strand. In the experiment described, Escherichia coli bacteria initially have the heavy isotope ¹⁵N in their DNA. Once the bacteria are transferred to ¹⁴N media and allowed to replicate, newly synthesized strands will contain ¹⁴N. In Generation 1, all new strands will be paired with parental strands that contain ¹⁵N. Therefore, 100% of double helices produced in Generation 1 will have one ¹⁵N strand and one ¹⁴N strand (Choice D). In Generation 2, both the ¹⁵N and ¹⁴N strands from Generation 1 are used as templates for the synthesis of new ¹⁴N strands. In the DNA that used a ¹⁴N strand as a template, both parental and daughter strands will contain ¹⁴N nucleotides. In DNA that used a ¹⁵N strand as a template, the parental strand will contain ¹⁵N and the daughter strand will contain ¹⁴N. Because half of the parental strands are ¹⁵N and half are ¹⁴N, 50% of the Generation 2 double helices will contain one ¹⁵N strand and one ¹⁴N strand (Choice C). The other 50% will contain two ¹⁴N strands. In each new generation, the percentage of double helices containing one ¹⁵N strand and one ¹⁴N strand will be half the percentage of the previous generation. Therefore, in Generation 3, 25% of double helices will contain a ¹⁵N strand and a ¹⁴N strand. (Choice A) 12.5% of double helices will contain a ¹⁵N strand and a ¹⁴N strand in Generation 4, not Generation 3. Educational objective: DNA replication is a semiconservative process that results in each double helix containing one parental strand and one newly synthesized daughter strand. DNA polymerase synthesizes each new daughter strand by using a parental strand as a template.

Escherichia Coli Bacteria Containing Only 15N-Labeled DNA Were Grown in Media

Escherichia Coli Bacteria Containing Only ¹⁵N-Labeled DNA Were Grown in Media