DNA is our information store, so how does it get copied from one cell to another?
The special pairing of Adenine (A) with thymine (T), and cytosine (C) with guanine (G) makes it easy to copy a strand of DNA.
Imagine this sequence of DNA bases which we read from left to right:
| start | A | T | C | A | G | C | T | A | G | C | T | G | C | A | T | G | T | T | C | G | stop |
It will have a partner strand where A pairs with T and C with G:
| start | A | T | C | A | G | C | T | A | G | C | T | G | C | A | T | G | T | T | C | G | stop |
| stop | T | A | G | T | C | G | A | T | C | G | A | C | G | T | A | C | A | A | G | C | start |
We read the top strand from left to right and the bottom strand from right to left.
Now, if we pull the two strands apart, we have one strand which looks like:
| start | A | T | C | A | G | C | T | A | G | C | T | G | C | A | T | G | T | T | C | G | stop |
and another which looks like:
| stop | T | A | G | T | C | G | A | T | C | G | A | C | G | T | A | C | A | A | G | C | start |
Either of these strands can act as a template to build the partner strand since we know that A must always pair with T and C with G.
So, when a cell divides to make two new cells, the two strands of DNA separate and get copied using this template idea. We then have two copies of the double helix which can go to the two offspring cells.