Lac operon in Prokaryotes

What's an operon?

A group of genes that are controlled by the same regulatory mechanism and are expressed at the same time

Why are operons more common in prokaryotes than eukaryotes?

Prokaryotes have smaller and simpler structures due to their genome. They are efficient in saving vital resources, as if a certain gene product is not needed, then all of the genes involved in their production can be switched off.

What are structural genes?

Genes that code for proteins not involved in DNA regulation. They make enzymes, proteins.

In lac operon, what are the 3 structural genes?

lacZ, lacY, lacA

What will lacZ, lacY, lacA make?

β-galactosidase

Lactose permease

Lactose transacetylase

What do β-galactosidase, Lactose permease, Lactose transacetylase do?

These are enzymes which metabolise lactose and are transcribed onto a long single molecule of mRNA.

Regulatory genes

Genes that code for proteins that are involved in DNA regulation. They code for the repressor protein/transcription factor. The regulatory gene controls/affects the expression of structural genes

Name the regulatory gene in lac operon

lacI

What will the lac I gene do?

It will code for the repressor protein. This protein will prevent the transcription of the structural genes in the absence of lactose.

Operator

DNA sequence where the repressor protein binds to

Promoter

DNA sequence where the RNA polymerase binds to

Lac Operon Process

1. When there is glucose, LacI is expressed to make the repressor protein, which binds to the operator

2. Due to its shape/size, it blocks the RNA polymerase binding site on the promoter, hence transcription of the structural genes cannot occur

3. Lactose binds to the repressor protein, causing a conformational change (i.e. change of protein shape) and hence the repressor is released from the operator

4. The release of the repressor from the operator unblocks the binding site on the promoter, hence RNA polymerase can now bind to it to transcribe the 3 structural genes.

5. cAMP receptor protein binds with cAMP and upregulates the activity of RNA polymerase to make transcription more efficient

6. The upregulated RNA polymerase then goes on to transcribe the structural genes to make Beta-galactosidase, lactose permease, and lactose transacetylase.