DNA Replication 2 - unvoiced.pptx

DNA Replication 2

Models of DNA replication

dispersive model

Evidence for semi-conservative replication of DNA

• Meselson and Stahl experiment

Meselson and Stahl experiment

• Explain why they used isotopes of nitrogen in their experiments

• Nitrogen labels the nitrogenous bases in the DNA.
• By using the heavy nitrogen first it labels the parental DNA so it can be traced through generations.
• By moving then to light nitrogen any new daughter strands will be traced and can be distinguished from the parental strands.

Questions

• Often questions on this evidence will involve interpretation of material, you are not expected to learn details of the specific experiment that they did – rather use your analysis skills to answer questions

Questions

Questions are then asked based on your interpretation – rather like a comprehension.

Extension material – Alternative models

• In conservative replication the two parent strands stay together after replication and the two daughter strands stay together after replication.

Predict what band patterns would be seen if this model was true after in the first, second and third samples in the experiment above.

Extension material – Alternative models

• Conservative replication

• Tube 1 – 1 heavy band
• Tube 2 – 2 bands – 1 heavy and 1 Light – equal sized bands
• Tube 3 – 2 bands – 1 heavy and 1 light – ¼ heavy and ¾ light

Extension material – Alternative models

• Dispersive replication

• Tube 1 – 1 heavy band
• Tube 2 – 1 intermediate band
• Tube 3 – 1 intermediate band