Learning Intention: Students will understand the semi-conservative model of DNA replication and the experiment used to prove it.

Introduction: Begin the lesson by asking students if they know how DNA replicates. Ask them if they have heard of the semi-conservative model of DNA replication. Explain that this model is the most widely accepted theory of DNA replication and that it was first proposed by Watson and Crick in 1953.

Body: 1. Explain the semi-conservative model of DNA replication: - DNA replication is the process by which DNA makes a copy of itself during cell division. - The semi-conservative model of DNA replication suggests that each strand of the original DNA molecule serves as a template for the synthesis of a new complementary strand. - This means that after replication, each new DNA molecule consists of one original strand and one newly synthesized strand.

1. Describe the experiment used to prove the semi-conservative model of DNA replication: - In 1958, Meselson and Stahl conducted an experiment to test the semi-conservative model of DNA replication. - They grew E. coli bacteria in a medium containing a heavy isotope of nitrogen (15N) for several generations until all the DNA in the bacteria contained 15N. - They then transferred the bacteria to a medium containing a lighter isotope of nitrogen (14N) and allowed them to replicate for one generation. - They then extracted the DNA from the bacteria and separated it using a technique called density gradient centrifugation. - The DNA was placed in a tube with a gradient of cesium chloride, which separates molecules based on their density. - The DNA containing 15N was denser and settled at the bottom of the tube, while the DNA containing 14N settled higher up. - The results showed that the DNA from the bacteria grown in the 14N medium was a mixture of heavy and light DNA, indicating that the DNA had replicated in a semi-conservative manner.

2. Discuss the significance of the experiment: - The Meselson-Stahl experiment provided strong evidence for the semi-conservative model of DNA replication. - It showed that DNA replication is a semi-conservative process, with each new DNA molecule consisting of one original strand and one newly synthesized strand. - This has important implications for genetics and evolution, as it ensures that genetic information is faithfully passed from one generation to the next.

Conclusion: Summarize the key points of the lesson, emphasizing the importance of the semi-conservative model of DNA replication and the experiment used to prove it. Encourage students to ask questions and clarify any misunderstandings they may have.