Prokaryotic gene regulation (basics concept of Operon) for NEET Biology and NET life science students

 

Prokaryotic gene regulation is very simple in comparison to eukaryotic gene regulation. The whole idea for prokaryotic gene regulation is that we want to control the expression of certain genes. Let's say, we have a dsDNA. From this DNA, mRNA is produced. This mRNA contains all the genes present in different regions which are present in DNA. The mRNA will eventually be transcribed into different proteins. Protein is the ultimate product of genes that we needed.

What is gene regulation?

Gene regulation is the process of regulating the expression of certain genes present in DNA, that will make the protein. For example, let's say, protein A is only required when bacteria need to break down lactose. So that is the condition. If lactose is present and bacteria want to break down lactose, then only it will express the gene for protein A. On the other side, when there is no lactose present in the medium, bacteria will not produce the gene A product. Therefore, the expression of gene A will be blocked or off here. So the whole idea behind prokaryotic gene regulation is a simple process called ON/OFF switch. So simply 2 things can happen, either on or off the gene. To understand prokaryotic gene regulation, we need to know some basic features of prokaryotic gene structure. A specific region called Promoter is present and RNA polymerase binds here. All the structural genes let's say A, B, and C are present downstream of the promoter. After the promoter, another specific region called the Operator is present. The operator usually overlaps 3’ end of Promoter and sometimes 5’ end of the structural genes. So, multiple genes are clustered one after another and all those genes are expressed by the control of promoter and operator. The controller/regulatory element will interact with the operator and will regulate the binding of RNA polymerase to the Promoter. The regulatory element will tell whether the gene sequence should be transcribed into mRNA or not. So, in this case, if the promoter turns off, the whole system of gene expression will be off and if it turns on, all the gene expression will start again. So, multiple genes are getting ON or OFF simultaneously due to the regulation of one promoter and operator. This is called OPERON/Operon concept.

There are two types of prokaryotic genes present.

1.     Structural genes- the genes whose products are needed for metabolism/biosynthesis or required to play a structural role in cells called structural genes.

2.     Regulatory genes- the genes whose products act as controller or regulator of structural genes. Regulatory genes have their own promoter and are transcribed into mRNA. Its products either RNA or proteins bind with the Operator and affect the transcription of structural genes. It is not part of the operon structure.

 

There are 2 types of transcriptional control-

1.     Negative control-when regulatory protein is a repressor and it inhibits gene expression.

2.     Positive control-regulatory protein is an activator and it stimulates transcription.

Negative control is more important in bacteria, whereas eularyotes are more likely to use positive control mechanisms.

            Operon can also be either inducible or repressible.

1.     Inducible operon- in this operon, transcription is normally OFF (not taking place) and something must happen to induce transcription or turn it ON.

2.     Repressible operon- transcription is ON or taking place and something must happen to repress transcription or turn it OFF.

 

 

So, operons may be

Negative inducible

Negative repressible

Positive inducible

And positive repressible