Overlapping Protein Codes
The base sequences, which code for proteins, are shown to overlap on a very
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