Introns and Exons

 
Below is a diagram of a typical gene. The notation 5' and 3' indicates the direction of the DNA strand. Within it are stretches which code for amino acids, called exons and stretches of unused code called introns.
 

 
The section between the grey squares is the transcription unit which is copied to the pre-messenger RNA.

The introns are removed, and the exons are joined to together to form the final functional messenger RNA.

We have referred to the introns as "redundant code." However, although they are not used to produce the amino acid sequence, this may be a misleading description. In fact their function is largely unknown. Although they are a feature of all eurkaryotic organisms, and may differ considerably in the same gene in different species, often they are remarkably similar. That is to say, they have been conserved through evolution. One suggestion is they have a role in the regulation of gene expression.