Scientists figure out the order of bases in a piece of DNA from
an X-ray film like this. This is called sequencing. Each dark band on the X-ray ladder
matches a colored base. Each row of bands corresponds to one of the four
bases. You can read the sequence from left to right, in order.
Right now, scientists are trying to sequence every base in a human cell. They're trying to create a complete map of the human genome. With this information they hope to cure disease.