11 Jun Cuny BIOLOGY 211 – An operon is composed of 3 basic DNA
Question
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An operon is composed of 3 basic DNA components:
Promoter – a nucleotide sequence that enables the transcription of a gene. It is recognized by RNA polymerase, which then initiates transcription. In RNA synthesis, promoters indicate which genes should be used for messenger RNA creation – and, therefore control which proteins the cell produces.
Operator – a segment of DNA that a repressor binds to. In the lac operon it is classically defined as a segment between the promoter and the genes of the operon.In the case of a repressor, the repressor protein physically obstructs the RNA polymerase from transcribing the genes.
Structural genes – these are genes that are co-regulated by the operon.
Regulation of operons.
Control of an operon enables organisms to regulate the expression of various genes depending on the conditions in their environment.
Regulation of operon can be either negative or positive by induction or repression.
Negative inducible operons.
A regulatory repressor protein is normally bound to the operator. This protein prevents the transcription of the genes on the operon. If an inducer molecule is present, it binds to the repressor and changes its conformation leading to its inability to bind to the operator, thus allowing for expression of the operon. The lac operon is an example of a negatively controlled inducible operon.
Negative repressible operons.
Transcription of the operon normally takes place.
Repressor proteins are produced by a regulator gene but the proteins can’t bind to the operator in their normal conformation.
But, molecules known as corepressors are bound by the repressor protein This causes a conformational change to the active site.
The activated repressor protein binds to the operator,preventing transcription. The trp operon is an example of a negative repressible operon.
Positive inducible operons.
Activator proteins can’t normally bind to the pertinent DNA. When an activator protein binds an inducer, the inducer undergoes a onformational change so that it can bind to the DNA and activate transcription.
Positive repressible operons.
The activator proteins are normally bound to the pertinent DNA segment. However, when the activator binds the inhibitor, the inhibitor is prevented from binding the DNA. This,therefore, stops activation and transcription of the system.
Example : The lac operon.
It is an example of a negatively controlled inducible operon. It consists of three adjacent structural genes, a promoter, a terminator, and an operator.
This operon is regulated by several factors , the availability of glucose and lactose inclusive. It can be activated by allolactose
Lactose binds to the repressor protein , preventing it from repressing gene transcription. This allows RNA polymerase to bind with the promoter, and express the lactase-synthesizing genes.
The lactase will eventually digest all of the lactose, until there is no lactose remaining to bind to the repressor. The repressor will then bind to the operator, stopping the manufacture of lactase.
This way, the organism is able to utilize lactose in their environment.
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