Overview
This lecture explains how the structure of DNA suggests a replication mechanism, describes the main models of DNA replication, and details the Meselson and Stahl experiment that determined the actual process.
DNA Structure and Replication Mechanisms
- DNA's double helix structure allows each strand to serve as a template for copying.
- Specific base pairing enables prediction of one strand from the other.
- Three models for DNA replication were proposed: conservative, semi-conservative, and dispersive.
Models of DNA Replication
- In the conservative model, parental DNA remains together, and new DNA strands pair together.
- The semi-conservative model suggests each new DNA molecule contains one old strand and one new strand.
- The dispersive model proposes new DNA contains interspersed segments of old and newly synthesized DNA.
The Meselson and Stahl Experiment
- Meselson and Stahl grew E. coli in nitrogen-15 (15N) media so the DNA was heavy due to 15N incorporation.
- Bacteria were switched to nitrogen-14 (14N) media and allowed to replicate for several generations.
- DNA samples were centrifuged to separate based on density, revealing patterns for each replication model.
- After one generation in 14N, DNA formed an intermediate band between 15N and 14N, compatible with semi-conservative or dispersive models.
- After two generations, two bands appeared (one intermediate, one light), confirming the semi-conservative model.
- Semi-conservative replication was supported, and the other models were ruled out.
The Process and Outcome of Replication
- During replication, each DNA strand serves as a template for a new, complementary strand.
- Two daughter DNA molecules, each with one old and one new strand, are formed and separated into daughter cells.
Key Terms & Definitions
- Semi-conservative replication — each new DNA molecule has one parental (old) strand and one newly synthesized strand.
- Conservative replication — parental DNA strands stay together, and new strands pair together.
- Dispersive replication — parental and new DNA segments are interspersed in both strands.
- Gradient (centrifugation) — solution used to separate molecules based on density when spun at high speeds.
- Buoyant density — the density at which a molecule floats during centrifugation.
Action Items / Next Steps
- Watch the linked video on DNA replication for further understanding.
- Review the figures illustrating the three models and the Meselson-Stahl experiment.