1. Gene structure in prokaryotes * In prokaryotic cells such as bacteria, genes are usually found grouped together in operons. * The operon is a cluster of related genes, often coding for enzymes in a metabolic pathway, which are under the control of a single promotor regulatory region.

2. *One example is the Lac operon, which coded for the enzymes responsible for lactose catabolism. *Within the operon there are three genes that code for proteins (structural proteins) and an upstream control region including the promotor and a regulatory site called The operator

3. *Laying outside the operon is the repressor gene, which codes for a protein (Lac repressor) that binds to the operator site and is responsible for the suppression of the operon by blocking the binding of RNA polymerase.

4. * The system of grouped genes in prokaryotic cells means that the transcribed mRNA may contain information for more than one protein. Such a molecule is known as a polycistronic mRNA. The synthesis of these mRNA is regulated in a cordance with the needs of the cell at any one time thus enables the cell to adapt quickly to changing environmental conditions. Fig 6

6. Gene structure in eukaryotes *Gene structure and function in eukaryoties is more complex than in prokaryotes. *Eukaryotic genes contain extra pieces of DNA that did appear in the mRNA. *These sequences are known as intervening sequences or introns, with the sequences that will make up the mRNA being called exons

7. *Within the nucleus the introns are spliced out of the primary transcript and a cap at the - 5 terminus and a tail of adenine residues at the - 3 terminus, were added. *The end product of RNA processing is a fully functional mRNA that is ready for export to the cytoplasm for translation ( Fig 7)