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Before a cell divides it copies all its DNA. How does it copy it so well?
DNA Replication lab · Last updated: 16 October 2026
Before a cell divides, it copies every letter of its DNA. In this 3D lab, you unzip the double helix and watch new strands being built along each old one. It reveals the simple pairing rule that lets life copy enormous instructions with astonishingly few mistakes.
What you'll discover
- DNA is a double helix whose two strands are held together by base pairs: A with T and C with G.
- Each old strand acts as a template, so every new DNA molecule keeps one old strand and gains one new one.
- Enzymes check their work as they go, which is why copying errors are so rare.
The science in plain words
DNA is the molecule that carries genetic instructions. It is shaped like a twisted ladder, the double helix, and each rung is a pair of chemical bases. There are four bases, A, T, C and G, and they always pair the same way: A with T, and C with G. The order of the bases along a strand spells out the instructions.
To copy DNA, an enzyme called helicase unzips the ladder by breaking the weak bonds between base pairs. Another enzyme, DNA polymerase, then moves along each exposed strand and adds the matching base opposite every letter. Because A only fits with T and C only with G, each old strand acts as a template for an exact new partner. The result is two identical DNA molecules, each made of one old strand and one new one. This is called semi-conservative replication.
The human genome has about 3 billion base pairs, so accuracy matters. DNA polymerase proofreads as it goes and removes wrongly paired bases, and repair systems fix many remaining slips. Even so, an occasional mistake gets through. These rare changes, called mutations, are usually harmless, sometimes harmful, and over long periods are a source of the variation that evolution acts on.
Where it fits
- India, NCERT: Class 10 science, Chapter 7: How do Organisms Reproduce?
- India, NCERT: The enzymes of DNA copying go beyond the NCERT syllabus for classes 6 to 10; the lab builds on Class 10 DNA copying in reproduction.
- United States: Grades 9 to 12
- Next Generation Science Standards: HS-LS1-1, HS-LS3-2
Common questions
Why does DNA need to be copied?
DNA is copied before a cell divides so that each new cell gets a complete set of instructions. Without copying, the daughter cells would be missing genes and could not work properly. Copying also lets parents pass genetic information to their children.
What are the base pairs in DNA?
DNA has four bases: adenine (A), thymine (T), cytosine (C) and guanine (G). A always pairs with T, and C always pairs with G. This strict pairing is what lets one strand act as a template for building its matching partner.
What happens if DNA is copied wrongly?
A copying mistake that is not repaired is called a mutation. Most mutations have no noticeable effect, some cause problems such as disease, and a few turn out to be useful. Over many generations, mutations provide the variation that natural selection works on.
Related labs
- Cell Division: One cell becomes two. How does each new cell end up with a full set of chromosomes?
- Protein Folding: A protein is a long chain. How does it fold itself into exactly the right shape?
- Genetic Inheritance: Two purple-flowered plants can have white-flowered babies. How?
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Try it yourself
The DNA Replication lab is one of the interactive 3D science labs inside Taz. Change one thing, predict what will happen, then run it and watch. Labs open after you sign in.