Multiple Choice
Why is DNA replication described as semiconservative?
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8. DNA Replication
Semiconservative Replication
2:41 minutesProblem 1a
Textbook Question
In the discussion, we focused on how DNA is replicated and synthesized. We also discussed recombination at the DNA level. Along the way, we encountered many opportunities to consider how this information was acquired. On the basis of these discussions, what answers would you propose to the following fundamental questions?
What is the experimental basis for concluding that DNA replicates semiconservatively in both bacteria and eukaryotes?
Verified step by step guidance1
Understand the concept of semiconservative replication: In semiconservative replication, each of the two strands of the original DNA molecule serves as a template for the synthesis of a new complementary strand. This results in two DNA molecules, each consisting of one original strand and one newly synthesized strand.
Review the Meselson-Stahl experiment: This landmark experiment conducted in 1958 provided direct evidence for semiconservative replication in bacteria. Meselson and Stahl used isotopes of nitrogen (¹⁵N and ¹⁴N) to label DNA and employed density gradient centrifugation to distinguish between DNA molecules containing heavy (¹⁵N) and light (¹⁴N) nitrogen.
Examine the experimental design: Meselson and Stahl grew E. coli in a medium containing ¹⁵N (heavy nitrogen) until the DNA was fully labeled. They then transferred the bacteria to a medium containing ¹⁴N (light nitrogen) and allowed replication to occur. After each round of replication, they isolated the DNA and analyzed its density using centrifugation.
Interpret the results: After one round of replication, the DNA showed an intermediate density, indicating that each DNA molecule contained one strand labeled with ¹⁵N and one strand labeled with ¹⁴N. After subsequent rounds of replication, the DNA showed both intermediate density and light density, consistent with semiconservative replication.
Extend the findings to eukaryotes: Similar experiments were conducted in eukaryotic systems, such as using labeled thymidine in mammalian cells. These studies confirmed that DNA replication in eukaryotes also follows the semiconservative model, demonstrating the universality of this mechanism across different domains of life.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Semiconservative Replication
Semiconservative replication is the process by which DNA is copied, resulting in two DNA molecules, each containing one original strand and one newly synthesized strand. This model was established through experiments, notably the Meselson-Stahl experiment, which demonstrated that after one round of replication, the DNA consisted of hybrid molecules, confirming that each daughter DNA retains one parental strand.
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Meselson-Stahl Experiment
The Meselson-Stahl experiment used isotopes of nitrogen to label DNA in bacteria, allowing researchers to track the distribution of parental and newly synthesized strands after replication. By analyzing the density of the DNA after centrifugation, they provided strong evidence for the semiconservative model, as the results showed a mixture of heavy and light DNA after one replication cycle.
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DNA Replication in Bacteria and Eukaryotes
DNA replication occurs in both bacteria and eukaryotes but involves different mechanisms and structures. In bacteria, replication starts at a single origin and proceeds bidirectionally, while eukaryotic replication involves multiple origins on each chromosome, with more complex regulation and additional proteins. Despite these differences, both processes ultimately support the semiconservative nature of DNA replication.
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