Before a cell divides (see The Cell Cycle), it copies all of its DNA. The pairing rule makes this possible: each strand carries the information to rebuild the other.
Key idea
In replication, helicase unzips the double helix by breaking the hydrogen bonds between bases. DNA polymerase then adds matching nucleotides to each old strand, A across from T and G across from C. Each new DNA molecule has one old strand and one new strand: replication is semiconservative.
Why it works
Each old strand is a template. Because A can only pair with T and G only with C, there is just one strand that fits each template. So the two copies come out identical to each other and to the original. DNA polymerase also checks its work and fixes most of its own mistakes.
- Helicase unwinds and unzips the helix.
- Primase lays down a short RNA primer, because DNA polymerase can only add to an existing strand.
- DNA polymerase adds nucleotides to the new strand, always building it in the 5′→3′ direction.
- DNA ligase seals the gaps between pieces of new DNA.
Worked example: Building on a template
An old strand reads 3′-TACGGA-5′ after the helix unzips. What new strand is built on it?
- Pair each base: T→A, A→T, C→G, G→C, G→C, A→T.
- The new strand runs the opposite way.
Answer: 5′-ATGCCT-3′
Worked example: The Meselson-Stahl experiment
In 1958 Matthew Meselson and Franklin Stahl grew bacteria on heavy nitrogen, then moved them to light nitrogen. After one round of copying, all the DNA was of middle weight. Why?
- Heavy DNA had two heavy strands.
- In semiconservative copying, each new molecule keeps one heavy strand and adds one light strand.
- One heavy and one light strand weigh in between.
Answer: Every molecule was a hybrid: half old (heavy), half new (light).
Worked example: Counting after several rounds
One heavy DNA molecule is copied 3 times in light nitrogen. How many of the molecules still contain a heavy strand?
- After 3 rounds there are 2 × 2 × 2 = 8 molecules.
- The two original heavy strands are never broken up or lost; each sits in its own molecule.
Answer: 2 of the 8 (the other 6 are all light).
Common mistake
A common picture is that the cell builds a brand-new double helix beside the old one, which stays whole (the "conservative" idea). Meselson and Stahl ruled that out: after one round there was no fully heavy DNA left. Each copy keeps one parent strand as its template.