Process of Gene Expression and Regulation

Gene expression is a basic life process that connects the information contained in a gene to the final functional gene product such as protein or non-coding RNA (ncRNA). It is necessary for cellular structure and function to remain normal and for developmental processes including differentiation and morphogenesis. Transcription, mRNA splicing, translation, and post-translational protein modification are all part of the protein expression process. The capacity to control gene expression allows cells to supply a functional protein whenever it is required for normal survival or function. This system is engaged in several physiological and pathological processes, including cellular adaptation to new environments, maintaining homeostasis, and repairing the damage.

Gene expression 

For animal biotechnology and other biological sciences, the capacity to undertake quantitative gene expression analysis has become increasingly significant. This type of study usually relates to approaches that allow one to determine the level of expression of a target gene in a specific cell, tissue, or entire organism. In multicellular organisms, cells of the same origin are united together to form tissues, which then form an organ to perform a shared physiological function. Animal organs have different levels of cellular heterogeneity than other in vitro systems using animal cells, such as tissue cultures, which makes quantitative investigation of gene expression and regulation difficult. During the transcription and translation processes, epigenomic chemicals, which are chemical compounds and proteins that can bind to DNA and influence gene expression, are involved in regulation at several sites.

Gene regulation

The process of turning genes on and off is called gene regulation. Gene regulation can occur at any point throughout the transcription-translation process, but it occurs most frequently during transcription. Regulating RNA processing, mRNA stability and translation rate are all examples of gene control mechanisms. Gene regulation is important for viruses, prokaryotes, and eukaryotes because it boosts an organism’s variety and adaptability by allowing the cell to express protein only when it is required.

Barbara McClintock demonstrated an interaction between two genetic loci, activator (Ac) and dissociator (Ds), in the colour formation of maize seeds as early as 1951. However, the lac operon discovered by François Jacob and Jacques Monod in 1961 is usually considered as the first discovery of a gene control system, in which E. coli expresses some lactose metabolic enzymes only when lactose is present but not when glucose is present.

In multicellular creatures, gene regulation drives cellular differentiation and morphogenesis, resulting in the development of many cell types with varying gene expression profiles from the same genomic sequence.

• It is possible only if the organism has a mechanism of regulating gene activity by allowing some to function and others to restrain their activity through switching on and switching off systems. This means the genes are turned ‘on’ or ‘off’ as per requirement. 
• A set of genes is ‘switched on’ when enzymes are required to metabolise a new substrate. The enzymes produced by these genes metabolise the substrate.
• The molecules of a metabolite that come to switch on of the genes are termed inducers and the phenomenon is called induction.
• Similarly, certain genes which are in their ‘switch on’ state, continue to synthesise a metabolite till the latter is produced in an amount more than required, or else, it is supplied to the cell from outside. In other words, certain genes continue to express themselves till the end product inhibits or represses their expression. Inhibition by an end product is known as ‘feedback repression’.

The transcription of a gene into mRNA and subsequent translation into protein is called gene expression. The majority of gene expression is regulated at the transcriptional level, owing to protein binding to specific DNA locations. Francois Jacob, Jacques Monod, and Andre Lwoff won the Nobel Prize in Medicine in 1965 for their work supporting the hypothesis that DNA transcription regulates enzyme levels in cells that happens due to transcriptional regulation, which can be either induced or suppressed. These researchers claimed that an ‘operon’, which contains many related genes on the chromosome that include an operator, a promoter, a regulator gene, and structural genes, controls the enzyme’s production.

Lac operon in E.coli (example of gene regulation)

 An inducible operon system normally remains in switched-off condition and begins to work only when the substance to be metabolised by it is present in the cell. Inducible operon system generally occurs in catabolic pathways, e.g. Lac operon of E. coli.

 Active repressor + inducer = inactive repressor

An inducible operon system consists of four types of genes

(i)   Structural genes – These genes synthesise mRNAs, which in turn synthesise polypeptides or enzymes over the ribosomes. An operon may have one or more structural genes. Each structural gene of an operon is called cistron. The lac operon (lactose operon) of Escherichia coli contains three structural genes (Z, Y, and A). These genes occur adjacent to each other and thus are linked. They transcribe a polycistronic mRNA molecule (a single stretch of mRNA covering all the three genes), that helps in the synthesis of three enzymes-p” galactosidase, lactose permease, and transacetylase.

(ii)  Operator gene – It lies adjacent to the structural genes and directly controls the synthesis of mRNA over the structural genes. It is switched off by the presence of a repressor. An inducer can take away the repressor and switch on the gene that directs the structural genes to transcribe.

(iii) Promoter gene – This gene is the site for the initial binding of RNA polymerase. When the operator gene is turned on, the enzyme RNA polymerase moves over it and reaches the structural genes to perform transcription.

(iv) Regulator gene – It produces a repressor that binds to the operator gene and stops the working of the operator gene.

(v)  Repressor – It is a protein, produced by the regulator gene. It binds to the operator gene so that the transcription of the structural gene stops. Repressor has two allosteric sites (1) operator gene (2) effective molecule (inducer/corepressor)

(vi)  Inducer – It is a chemical (substrate, hormone, or some other metabolite) which after coming in contact with the repressor, forms an inducer repressor complex. This complex cannot bind with the operator gene, which is thus switched on. The free operator gene allows the structural gene to transcribe mRNA to synthesise the enzymes.

Conclusion:

The mechanism which stimulates the expression of certain genes and inhibits that of others is called regulation of gene expression. Transcription, mRNA splicing, translation, and post-translational protein modification are all part of the protein expression process. The capacity to control gene expression allows cells to supply a functional protein whenever it is required for normal survival or function. This system is engaged in several physiological and pathological processes, including cellular adaptation to new environments, maintaining homeostasis, and repairing the damage.