Part 1: Why DNA must be copied

Before mitosis or meiosis, the DNA in the nucleus is replicated. This is essential because each daughter cell needs a complete set of genes so it can make the proteins it requires. If DNA were not copied first, genetic information would be lost when cells divide.

The process is described as semi-conservative replication. In each new DNA molecule, one strand comes from the original DNA and one strand is newly made. This means the base sequence can be copied accurately from an existing template, helping preserve genetic continuity between generations of cells.

Part 2: How semi-conservative replication happens

DNA replication begins when DNA helicase acts on the double helix. DNA helicase unwinds the helix and breaks the hydrogen bonds between complementary bases. This separates the two polynucleotide strands and exposes the bases on each strand.

Each original strand now acts as a template. Free DNA nucleotides are attracted to the exposed bases on the template strands. They pair by complementary base pairing, so adenine pairs with thymine and cytosine pairs with guanine.

DNA polymerase then catalyses the condensation reaction that joins adjacent nucleotides together. This forms the sugar-phosphate backbone of the new strand. As the nucleotides are joined, phosphodiester bonds are formed and each template strand gains a complementary strand.

At the end of the process, two DNA molecules are produced. They have the same base sequence as each other and each contains one original strand and one newly synthesised strand.

By copying the base sequence accurately before cell division, semi-conservative replication helps ensure genetic continuity between generations of cells.

Part 3: Why it is called semi-conservative

The word conservative means "kept" or "retained". Replication is semi-conservative because half of the original DNA molecule is retained in each new molecule. The original double helix does not stay together as one complete molecule. Instead, the two original strands separate and each becomes part of a different daughter DNA molecule.

This matters because the original strands provide the templates for the new strands. Complementary base pairing allows the correct sequence to be rebuilt on each template, so the copied DNA carries the same information as the original DNA.

After one round of replication, both DNA molecules are hybrids: each has one old strand and one new strand. That pattern is the key evidence scientists looked for when testing how DNA replicates.

Part 4: Evidence from scientists

Watson and Crick's model of DNA suggested a copying mechanism because specific base pairing meant each strand could act as a template for a new complementary strand. Scientists then needed evidence to test whether this model was correct.

Meselson and Stahl grew bacteria in a medium containing the heavy isotope nitrogen-15 so the DNA became heavy. They then moved the bacteria to a nitrogen-14 medium and allowed the DNA to replicate. After each generation they extracted the DNA and separated it by density using centrifugation.

After one generation, the DNA formed a single band at an intermediate density. This supported semi-conservative replication because every DNA molecule contained one original heavy strand and one new light strand. It did not match the conservative model, which would have produced one heavy band and one light band after one generation.

That first-generation result ruled out conservative replication, because conservative replication would have left one DNA molecule completely heavy and made one completely light DNA molecule.

After two generations, there was an intermediate band and a light band. This pattern strengthened the case for semi-conservative replication because some DNA molecules still contained one original heavy strand, while others were made entirely from light nitrogen after a second round of copying. The strength of the experiment was that the competing ideas gave different predictions, so the banding pattern provided clear evidence to support the Watson-Crick model and the copying mechanism suggested by Watson and Crick.

Part 5: Feynman check

Try to explain the whole idea as if you were teaching it to a student who knows that DNA carries genetic information but has not yet learned how it is copied.

Use everyday language first, then add the scientific terms. A strong explanation should include these ideas:

• the double helix is unzipped when helicase breaks hydrogen bonds
• each original strand is used as a template
• free nucleotides pair with exposed bases by complementary base pairing
• DNA polymerase joins adjacent nucleotides by condensation reactions, forming the sugar-phosphate backbone
• each new DNA molecule contains one old strand and one new strand, so replication is semi-conservative
• this accurate copying preserves genetic continuity between generations of cells

If your explanation is really secure, you should also be able to explain the Meselson-Stahl evidence in simple language:

"Scientists first made the original DNA heavy by growing bacteria in nitrogen-15. After moving the bacteria into nitrogen-14, any new DNA strands were light. After one generation, every DNA molecule was half heavy and half light, so they all formed one intermediate band. That ruled out conservative replication. After more generations, some DNA became completely light while some stayed intermediate, which is exactly what semi-conservative replication predicts."

Common mistakes to avoid:

• saying helicase breaks phosphodiester bonds rather than hydrogen bonds
• saying whole DNA molecules stay together as complete originals
• forgetting that DNA polymerase joins nucleotides together instead of choosing the base pairs
• claiming the first Meselson-Stahl result proved both models equally well