Gene structure in prokaryotesGene 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.

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

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

**The system of grouped genes in The system of grouped genes in prokaryotic cells means that the prokaryotic cells means that the transcribed mRNA may contain transcribed mRNA may contain information for more than one protein. information for more than one protein. Such a molecule is known as Such a molecule is known as

a polycistronic mRNA.a polycistronic mRNA.The synthesis of these mRNA is The synthesis of these mRNA is regulated in a cordance with the needs of regulated in a cordance with the needs of the cell at any one time thus enables the the cell at any one time thus enables the cell to adapt quickly to changing cell to adapt quickly to changing environmental conditions. environmental conditions. Fig 6Fig 6

Gene structure in eukaryotesGene structure in eukaryotes**Gene structure and function in Gene structure and function in eukaryotieseukaryoties is is more complex than in more complex than in prokaryotes.prokaryotes.

*Eukaryotic genes contain *Eukaryotic genes contain extraextra pieces of DNApieces of DNA that did appear in the mRNA.that did appear in the mRNA.

*These sequences are known as *These sequences are known as intervening intervening sequences or intronssequences or introns, , with the sequences with the sequences that will make up the mRNA being called that will make up the mRNA being called exonsexons

*Within the nucleus *Within the nucleus the intronsthe introns are are spliced out of the primary transcript spliced out of the primary transcript and and a capa cap at the at the --5 terminus and a 5 terminus and a tail of adenine residues at the tail of adenine residues at the --3 3 terminus, were added.terminus, were added.

*The end product of RNA processing *The end product of RNA processing is a fully functional mRNA that is is a fully functional mRNA that is ready for export to the cytoplasm for ready for export to the cytoplasm for translationtranslation ( Fig 7)( Fig 7)