Hi there,
can someone explain about the lac operon?
I'm quite confused about it.
Hey
An operon is a group of linked structural genes, that are transcribed together, with a common promoter (binding site of RNA polymerase) and operator (binding site of repressor protein).
The lac operon is a specific type of operon found in E. coli bacteria located in our gut. The lac operon contains three structural genes that encode the information for the synthesis of lactase (an enzyme that breaks down the sugar lactose).
However, the thing is that E. coli bacteria prefer glucose over lactose so when glucose is around (or when there is no lactose in the first place), synthesising lactase would be a complete waste of energy - this is where the regulatory gene comes in.
The regulatory gene is located right next to the lac operon and codes for the synthesis of a repressor protein (a transcription factor) . Remember how I mentioned the operator region earlier? The thing is that the operator is located right in between the promoter region (where RNA polymerase binds) and the 3 structural genes coding for the synthesis of lactase. So when the regulatory gene is transcribed, and all the normal translation stuff happens, a repressor protein is synthesised which binds to the operator region, pretty much blocking RNA polymerase from being able to transcribe the 3 genes.
In the above explanation, it is assumed that lactose is absent, however when E. coli obtain lactose (and no glucose) something bizarre happens. The lactose in the cytosol actually binds to the repressor protein that has been synthesised, rendering it inactive. This results in the RNA polymerase being able to move past the promoter region and freely transcribe the structural genes, which in turn leads to translation and finally the synthesis of lactase which breaks down the lactose.
Have you picked up the cycle yet? As soon as the lactose runs out (i.e. all of it has been broken down by the lactase molecules), the repressor protein straight away starts to block the RNA polymerase from transcribing the genes until more lactose is obtained.
This mechanism prevents bacteria from wasting energy producing excess amounts of lactase molecules that can't do anything when there is not lactose.
Hope this helps