Regulation of Gene Expression
Gene expression results in the formation of a polypeptide that is required to perform a particular function or set of functions. Expression of the gene should be at the right time, at the correct amount. Otherwise, energy deprivation and metabolic chaos would occur in the cell. It can be regulated at several levels. In eukaryotes, the regulation could be exerted at;
- Transcriptional level (Formation of primary transcript)
- Processing level (Regulation of splicing)
- Transport of mRNA from the nucleus to the cytoplasm.
- Translational level
In prokaryotes, control of the rate of transcriptional initiation is the predominant site for control of gene expression. The set of genes regulating a metabolic reaction constitutes an Operon e.g. lac operon, trp operon, etc. When a substrate is added to the growth medium of bacteria, a set of genes is switched on to metabolize it. This is called induction. When a metabolite (product) is added, the genes to produce it are turned off. This is called repression.
Lac operon
It was proposed by Francois Jacob and Jacque Monod. The operon controlling lactose metabolism is called Lac Operon. It consists of;
- Three structural genes: Lac z gene that codes for Beta-galactosidase (It hydrolyzes lactose to glucose and galactose), Lac y gene that codes for Permease enzyme (Increases permeability of the cell to Lactose), and Lac a gene that codes for Transacetylase.
- A regulator gene (i gene/ inhibitor gene): It codes for a repressor protein.
- Inducer: Here lactose is the inducer that keeps the switch on and allows the structural gene to transcribe mRNA to synthesize the enzymes.
Functioning of Lac operon;
- When lactose (inducer) is absent: The regulator gene synthesizes mRNA and produces repressor protein which then binds to the operator region (blocks RNA polymerase movement). This prevents the transcription of mRNA from the structural gene (remains switched off).
- When lactose (inducer) is present: Lactose binds to the repressor protein making it inactive and it fails to bind to the operator region. The operator gene becomes free and induces the RNA polymerase to bind with the promoter gene (lac operon “switched on”) and transcribes structural genes.
Molecular Basis of Inheritance Notes Class 12
CBSE Class 12 Molecular Basis of Inheritance: Inheritance is transmitted by certain molecules that Mendel termed as ‘factors’, but their nature was discovered later with the development of various scientific techniques. The molecules which govern the inheritance are called genes and it is of two types; Deoxyribonucleic acid (DNA) and Ribonucleic acid (RNA). In all organisms except viruses, DNA acts as the genetic material whereas RNA acts as the messenger molecule, however in some viruses RNA functions as the genetic material. It is because DNA is a more stable molecule as compared to RNA and hence, it evolved as the genetic material of almost all the organisms.
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