Legend:

The base sequences, which code for proteins, are shown to overlap on a very small chromosome.

This circle represents the chromosome of the [phi]X174 virus which encodes for nine separate proteins. Scientists knew how small a DNA base sequence could be and still code for a protein. However, they were puzzled by this little virus whose DNA is too small to code for all the different proteins it produces.

In 1977, Fredrick Sanger and colleagues mapped out the complete DNA structure of this peculiar virus and discovered that the codings for two of the proteins are embedded within two of the other genes. They discovered that the same stretch of DNA was encoding for more than one protein!

In the illustration, the letters indicate the proteins coded for by the genes in the colored regions. Notice how in some regions coding for one protein also includes information for other proteins. For example, the region coding for protein A also codes for protein B and part of protein K. (The short black areas indicate parts of the chromosome that do not code for proteins).