Preview
IB DP Biology — D: Continuity and Change | SL | D1.1
📋 What You Will Learn
- Identify the key enzymes involved in DNA replication and their specific roles at each stage
- Explain the semi-conservative mechanism of replication, including the roles of helicase, DNA polymerase, and DNA ligase
- Analyse the Meselson–Stahl experiment and explain how its results provide evidence for semi-conservative replication
- Apply understanding of base complementarity and antiparallel strand orientation to predict replication outcomes
- Evaluate the significance of semi-conservative replication for genetic continuity across cell generations
🔍 Guiding Question How does the structure of DNA allow it to be copied with extraordinary fidelity, and what does the mechanism of replication reveal about the relationship between molecular structure and biological function?
🌍 Why This Matters Every time a cell divides — whether a skin cell replacing itself or a cancer cell multiplying unchecked — the entire genome must be copied with near-perfect accuracy. Errors in this process are the molecular origin of mutations, and understanding replication is the foundation for understanding cancer, genetic disease, and the biotechnology tools used to fight them.
The Problem the Cell Had to Solve
Here is the fundamental challenge of heredity: every time a cell divides, it must pass an identical copy of its genetic information to each daughter cell. Not a rough approximation — an exact copy of roughly three billion base pairs in human cells. The molecule that carries this information, DNA, is a double helix held together by hydrogen bonds between complementary bases. That structure, it turns out, is not just a storage format. It is the solution to the copying problem itself.
Before Watson and Crick published the double helix structure in 1953, three models of replication were proposed: conservative (the original stays intact and a completely new copy is made), semi-conservative (each strand serves as a template for a new strand), and dispersive (old and new DNA are interspersed throughout both strands). The structure of DNA immediately suggested the semi-conservative model — but it took an elegant experiment to prove it.
📖 Key Definition — DNA Replication DNA replication is the process by which a DNA molecule is copied to produce two identical double-stranded DNA molecules, each consisting of one original (parental) strand and one newly synthesised strand.
📖 Key Definition — Semi-Conservative Replication Semi-conservative replication is the mechanism by which each strand of the parental DNA double helix serves as a template for the synthesis of a new complementary strand, resulting in two daughter molecules each retaining one parental strand.
The Meselson–Stahl Experiment: Proof of Semi-Conservative Replication
In 1958, Matthew Meselson and Franklin Stahl designed one of the most elegant experiments in the history of molecular