Review the key concepts, formulae, and examples before starting your quiz.
🔑Concepts
Regulation of gene expression refers to a very broad term that may occur at various levels. In eukaryotes, it can be exerted at: (i) Transcriptional level (formation of primary transcript), (ii) Processing level (regulation of splicing), (iii) Transport of mRNA from nucleus to the cytoplasm, (iv) Translational level.
The Lac Operon (Lactose Operon) is a classic model for gene regulation in , proposed by Francois Jacob and Jacques Monod. It consists of a polycistronic structural gene regulated by a common promoter and regulatory genes.
The Lac Operon contains three structural genes: , , and . The gene codes for -galactosidase (-gal), which hydrolyzes lactose into galactose and glucose. The gene codes for permease, which increases the permeability of the cell to -galactosides. The gene encodes a transacetylase.
The gene (inhibitor) codes for the repressor of the lac operon. The repressor is synthesized constitutively. In the absence of an inducer, the repressor binds to the operator region () and prevents RNA polymerase from transcribing the operon.
Lactose (or allolactose) acts as an inducer. It binds to the repressor, forming an inactive complex that cannot bind to the operator. This allows RNA polymerase access to the promoter, switching 'ON' the transcription of structural genes.
Regulation of the lac operon by a repressor is termed as negative regulation. The operon can also be under positive regulation, involving proteins like the Catabolite Activator Protein (CAP).
📐Formulae
💡Examples
Problem 1:
In an culture, if the gene of the Lac operon is mutated such that its product cannot bind to the operator, what will be the state of the operon expression?
Solution:
The Lac operon will be constitutively expressed (always 'ON').
Explanation:
The gene produces the repressor protein. Normally, the repressor binds to the operator to block transcription. If the mutation prevents the repressor from binding to the operator, RNA polymerase will have constant access to the structural genes , , and , leading to continuous synthesis of the enzymes regardless of whether lactose is present or absent.
Problem 2:
Why is a very low level of expression of the lac operon required in the cell even in the absence of lactose?
Solution:
To allow the initial entry of lactose into the cell.
Explanation:
Lactose enters the cell through the action of the enzyme permease (coded by the gene). If the operon were completely 'OFF' with zero expression, no permease would be present, and lactose could never enter the cell to act as an inducer. Thus, a basal level of transcription is necessary.