Describe the structure, function and effects of gene mutations on the lac operon.

Answer 1

An operon is a group of genes in bacteria that are transcribed together as a group. They function to perform related functions for the bacterium. A commonly used example is the lac operon, which is an example of an inducible operon. That means that it is ‘off’ or not functioning and needs to be turned on. It is turned on in the presence of the sugar lactose, which the operon uses as an energy source. There are five parts to this operon. The promoter is the site of attachment of RNA polymerase, the enzyme used to transcribe the lac genes. The operator is the site of attachment of a repressor protein which is present in the absence of lactose, and therefore serves to prevent the transcription of the lac genes when lactose is not present. There are three genes in the operon: the lacZ gene, which codes for the enzyme β-galactosidase; the lacY gene, which codes for the enzyme permease; the lacA gene, codes for the enzyme acetyltransferase. β-galactosidase functions to break down lactose into glucose and galactose. Permease codes for a membrane protein allowing the movement of lactose into the cell to be metabolized. Acetyltransferase is an enzyme functioning in the removal of an acetyl group from acetyl CoA to other molecules.

               As previously mentioned, the lac operon is normally not functioning, as a repressor protein is bound to the operator, preventing the transcription of the lac operon genes by RNA polymerase. It physically blocks RNA poly from transcribes the genes. This protein is made by another gene, the lacI gene which is located upstream from the lac operon, and while its function is important to the functioning of the lac operon, it is not considered part of the lac operon. In the absence of lactose, the repressor protein binds to the operator. However, when lactose is present, the lactose binds allosterically to the repressor and changes the shape of the repressor such that it cannot bind to the operator. Consequently, RNA polymerase is free to transcribe the lac operon genes.

               There are consequences if there are mutations to any of the components of the operon or repressor. If there is a mutation in the lacI gene, a repressor protein will be made that is unable to bind to the operator, resulting in the lac operon always being ‘on’, regardless of whether there is lactose present or not. A mutation in the promoter will result in RNA polymerase being unable to attach and therefor transcription will not occur, regardless of whether there is lactose present or not. A mutation in the operator will result in the repressor being unable to bind here, so the lac operon will always be turned on. Finally, a mutation in the lacZ gene will result in transcription of a β-galactosidase enzyme which is not functional, consequently lactose will not be broken down.