DNA Replication Lab

Every time a cell divides, it must copy its entire genome — around three billion base pairs in a human cell — with astonishing accuracy. This simulation walks through how that happens.

The base-pairing rule

DNA is a double helix of two strands held together by pairs of bases that fit like puzzle pieces:

• A pairs with T (adenine–thymine), joined by two hydrogen bonds
• G pairs with C (guanine–cytosine), joined by three hydrogen bonds

This is the key to copying: each strand already contains the information needed to rebuild its partner. Split them apart and each one serves as a template.

Step by step

1. Unwinding. Helicase opens the helix at a replication fork, breaking the hydrogen bonds.
2. Priming. Primase lays down a short RNA primer — DNA polymerase cannot start from nothing, only extend.
3. Elongation. DNA polymerase adds nucleotides, reading the template 3′→5′ and building the new strand 5′→3′.
4. Proofreading. The polymerase checks each addition and backs up to fix mistakes.

Why one strand is made in pieces

DNA polymerase only works in one direction (5′→3′). On the leading strand that direction points the same way the fork is moving, so copying is continuous. On the lagging strand it points the wrong way, so the enzyme works backwards in short stretches — Okazaki fragments — each needing its own primer. Later, another enzyme removes the primers and DNA ligase welds the fragments together.

Try this

• Step slowly through the fork and watch fragments appear only on one side.
• Watch the G–C rich regions: three hydrogen bonds make them harder to separate than A–T rich regions.
• Note that each finished double helix contains one old strand and one new one — semiconservative replication.

Why it matters

Understanding replication is the basis of PCR (which amplifies DNA for testing), DNA sequencing, and a whole class of antibiotics and chemotherapy drugs that work by jamming the enzymes you have just watched in action.