DNA Replication — Semi-Conservative Copying

Before a cell can divide, it must make an accurate copy of every chromosome — every base of its DNA. The process by which this is achieved is called DNA replication. This lesson covers the OCR A-Level Biology A specification point 2.1.3 (e) — the semi-conservative nature of DNA replication, the roles of DNA helicase and DNA polymerase, and the classic Meselson–Stahl experiment that proved semi-conservative replication was correct.

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1. The Three Proposed Models of Replication

When the structure of DNA was first solved, three competing hypotheses existed for how it is replicated:

1. Conservative replication — the original double helix stays together, and an entirely new double helix is made as a copy.
2. Semi-conservative replication — the two parental strands separate, and each acts as a template for a new strand. Each daughter molecule contains one old strand and one new strand.
3. Dispersive replication — the new DNA contains a mixture of old and new fragments along both strands.

graph TD
  A[Parental DNA] --> B[Conservative: one all-old, one all-new]
  A --> C[Semi-conservative: each daughter = 1 old + 1 new strand]
  A --> D[Dispersive: patches of old and new on both strands]

Only the semi-conservative model turned out to be correct — as demonstrated by Meselson and Stahl in 1958.

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2. The Meselson–Stahl Experiment

Matthew Meselson and Franklin Stahl devised a beautifully elegant experiment using isotopes of nitrogen:

Procedure

1. Bacteria (Escherichia coli) were grown for many generations in a medium containing the heavy isotope of nitrogen, ¹⁵N. This caused all their DNA bases (which contain nitrogen) to be "heavy".
2. The bacteria were then transferred to a medium containing only the normal light isotope, ¹⁴N, and allowed to divide.
3. Samples of DNA were extracted after one and two generations, and the DNA was spun in a caesium chloride density gradient. Denser DNA settles lower in the tube.

Results

Sample	Observed band(s)	Interpretation
Before transfer (all ¹⁵N)	A single heavy band	All DNA contains only ¹⁵N
After 1 generation	A single intermediate band	Each molecule contains one ¹⁵N strand and one ¹⁴N strand — refutes conservative model
After 2 generations	Two bands — one intermediate, one light	Half the molecules are hybrid, half are entirely light — refutes dispersive model

The results matched the predictions of the semi-conservative model exactly, and ruled out both the conservative and dispersive hypotheses.

Exam Tip: You should be able to predict the banding patterns expected under each model and explain why the observed results support semi-conservative replication.

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3. Semi-Conservative Replication in Detail

The modern understanding of replication includes several named enzymes and a clear sequence of events.

3.1 The Four Key Steps

1. DNA unwinds and unzips. The enzyme DNA helicase moves along the double helix and breaks the hydrogen bonds between complementary bases, separating the two strands to form a replication fork.
2. Each original strand acts as a template. The exposed bases of each parental strand are available for base pairing with free nucleotides.
3. Free DNA nucleotides align and pair with their complementary bases on the template strand (A–T, C–G).
4. DNA polymerase catalyses condensation reactions to join adjacent nucleotides via phosphodiester bonds, forming the new strand. This continues until two complete double helices have been produced.

graph TD
  A[Double helix] --> B[DNA helicase breaks H-bonds]
  B --> C[Strands separate at replication fork]
  C --> D[Free nucleotides pair with exposed bases<br/>A–T, C–G]
  D --> E[DNA polymerase joins nucleotides<br/>by phosphodiester bonds]
  E --> F[Two identical daughter double helices<br/>each: 1 old strand + 1 new strand]

3.2 Where Does the Energy Come From?