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The key points covered in this video include:
1. The lac Operon
2. Structure of the lac Operon
3. Repressing Transcription of the lac Operon
4. Inducing Transcription of the lac Operon
The lac Operon
In prokaryotic cells, genes are often found in groups called operons. An operon is a group of genes that are under the same regulatory mechanism and are all transcribed in one single unit. Operons are useful as the genes that code for proteins involved in a specific metabolic pathway can all be switched on or off. For example, many bacteria like E. coli use glucose as their main respiratory substrate - glucose is very easy to metabolise. However, glucose may not always be available so these bacteria can also use other respiratory substrates such as lactose. Metabolism of lactose requires different proteins compared to metabolism of glucose. These proteins are only produced when glucose is absent and lactose is present in order to conserve resources. The genes that code for proteins involved in lactose metabolism are therefore located together in an operon - the lac operon. The expression of all the proteins can therefore be easily repressed when they are not needed.
Structure of the lac Operon
The lac operon is a group of genes that are involved in the metabolism of lactose. The lac operon is a section of DNA that has an operator region, a promoter region and three structural genes: lacZ, lacY and lacA. Another section of DNA located near the lac operon contains the regulatory gene, lacl. The gene lacZ codes for the enzyme B-galactosidase which catalyses the hydrolysis of the disaccharide lactose into glucose and galactose. The gene lacY codes for the membrane carrier protein lactose permease which helps transport lactose into the bacterial cell. The promoter region is the section of DNA where RNA polymerase binds in order to begin transcription. The regulatory gene lacl codes for a protein that prevents transcription of the structure genes - it is a repressor protein. The operator region is the section of DNA that the repressor protein binds to in order to prevent transcription.
Repressing Transcription of the lac Operon
In the absence of lactose, transcription of the lac genes is always repressed - the genes are switched off. This occurs because the regulatory gene lacl is always switched on, resulting in the production of the repressor protein. The repressor protein binds to the operator region which is next to the promotor region. This prevents RNA polymerase from being able to bind and therefore prevents transcription of the structural genes.
Inducing Transcription of the lac Operon
The presence of lactose induces transcription of the structural genes, allowing production of B-galactosidase and lactose permease. Lactose acts as an inducer by binding to the repressor protein, causing it to change shape. This prevents the repressor protein from being able to bind to the operator region. Therefore, RNA polymerase is able to bind to the promotor and transcription of the structural lac genes can occur. In summary, the lac operon is only expressed when the inducer molecule, lactose, is present - which conserves resources.
Summary
Prokaryotes do not express all genes all the time - they regulate their gene expression by regulating transcription
This allows prokaryotes to respond to changes in the environment and conserve resources
Prokaryotes contain groups of genes called operons that are transcribed together - one example is the lac operon
The genes in the lac operon are repressed when lactose is not present due to a repressor protein that stops RNA polymerase from binding to the promoter
When lactose is present, it binds to the repressor protein which prevents it from functioning
This allows RNA polymerase to bind to the promotor and transcription of the lac structural genes can occur
Lactose therefore acts as an inducer for the transcription of the genes needed for its metabolism - this conserves resources