What is DNA Replication?
DNA replication is how a cell makes a copy of its DNA before it divides. It happens during the S-phase of interphase. The reason it's important is that every new cell needs a complete copy of the DNA so it can work right. The model below shows the 6 main steps. Click Play to watch it go through, or use the buttons to step through it yourself. There's also an Error Mode button that shows what happens if a mistake gets made.
Interactive Model
Step 1: Starting DNA
We start with a normal DNA double helix. The two strands are held together by hydrogen bonds between the bases. A pairs with T, and G pairs with C. The strands are also antiparallel, which just means they run in opposite directions (one is 5' to 3' and the other is 3' to 5').
Vocabulary
| Term | Definition |
|---|---|
| Helicase | The enzyme that unzips the DNA double helix. |
| Primase | Makes a short RNA primer so DNA polymerase has somewhere to start. |
| DNA Polymerase III | Adds new nucleotides to build the new strand (5' to 3' only). |
| DNA Ligase | Glues the Okazaki fragments together on the lagging strand. |
| Replication Fork | The Y-shape where the DNA is being split apart. |
| Leading Strand | The new strand that gets built smoothly in one piece. |
| Lagging Strand | The new strand that gets built in short chunks (Okazaki fragments). |
| Semiconservative | Each new DNA has one old strand and one new strand. |
What If Something Goes Wrong?
DNA polymerase is really fast (about 1,000 nucleotides every second), but it's not perfect. Sometimes it puts the wrong base in. Most of the time the cell catches the mistake and fixes it, but if it doesn't, the mistake becomes a mutation that gets passed on to the next cell. You can see this in the model by clicking Error Mode and going to step 4 or later. A wrong base shows up in red.
Mutations can be a big problem. If the mutation happens in a gene that controls when a cell divides, the cell might start dividing way too much, which is how cancer can start. Other mutations can cause genetic diseases like sickle cell anemia. If helicase fails and the DNA can't even be unwound, replication stops completely and the cell usually just dies instead of dividing.
Common Misconception
"DNA replication and cell division are the same thing."
They're not. DNA replication just copies the DNA. Cell division (mitosis) is when the cell actually splits into two. Replication has to happen first, but the cell doesn't divide right after - it goes through more steps in interphase before mitosis starts.