Gene expression is the process by which genes direct the synthesis of functional products, such as proteins, tRNA, rRNA, etc. This process utilizes the information present in the DNA to synthesise proteins or other molecules.

Gene expression occurs in two main steps: transcription and translation. Firstly, the information present in DNA is used to synthesise the complementary RNA molecule. Then, a polypeptide chain is synthesised using the RNA template, allowing a gene to be expressed in the form of a functional product.

Gene expression is carefully controlled at various stages, which determines both the type and amount of proteins produced. Regulation of gene expression can take place at the transcription or translation level.

Mechanism of Gene Expression

Genes are short stretches of DNA that code for a functional product. They are the basic functional unit of inheritance, passed down from parents to offspring. The information within the DNA is known as the genotype, while the expression of genes in terms of functional products or observable traits is referred to as the phenotype. Gene expression leads to the formation of proteins, as well as the synthesis of non-coding RNAs such as tRNA, rRNA, and more. All three RNAs are needed for the synthesis of proteins.

Genes are the foundation of all cellular functions. They are responsible for deciding the fate and function of a cell, as there are thousands of genes present in an organism. When expressed in a cell, a gene can be used to perform one or many functions, allowing the cell to respond to various internal and external changes. Additionally, genes control the synthesis of proteins, and proteins control the structure, metabolic functions, and development of an organism.

The two steps involved in the synthesis of a protein are Transcription and Translation.

Transcription

The process of RNA synthesis using DNA templates is catalysed by an enzyme called RNA polymerase. This enzyme is DNA-dependent. In eukaryotes, there are multiple RNA polymerases that synthesise different types of RNA molecules. RNA polymerase II is responsible for the synthesis of pre-mRNA or hnRNA which acts as a template for protein synthesis.

The synthesis of RNA occurs in the 5’ to 3’ direction, with the 3’ to 5’ strand of the DNA acting as a template. The RNA produced contains the same sequence as the 5’ to 3’ (coding) strand of the DNA, except that thymine is replaced by uracil.

The process of transcription is a three-step process:

1. Initiation - The RNA polymerase binds to the promoter located at the 5’ end of the structural gene (coding strand) to begin transcription.

2. Elongation - Nucleoside triphosphates serve as a substrate and each base is incorporated in accordance with the complementary base sequence of the template strand of the DNA.

3. Termination - When the RNA polymerase reaches the terminator region located downstream of the structural gene, the newly formed RNA and RNA polymerase are released and transcription is complete.

RNA polymerase I transcribes rRNAs in eukaryotic cells, while RNA polymerase II transcribes pre-mRNA transcript and RNA polymerase III transcribes tRNA, 5srRNA and snRNA.

All three RNAs, i.e. mRNA, rRNA and tRNA are required for protein synthesis, but mRNA acts as a template for translation. The hnRNA or pre-mRNA is further processed to form a mature and fully functional mRNA that is ready for protein synthesis. The mRNA undergoes splicing, capping and tailing in the nucleus. The fully processed mature mRNA is transported out of the nucleus for protein synthesis.

Translación

Translation is the process of converting the information contained in mRNA into a polypeptide chain of amino acids. This is done by using mRNA as a template, with three base sequences or codons in the mRNA coding for one amino acid. The sequence of amino acids in the polypeptide chain is determined by the sequence of bases in the mRNA transcript.

A cistron is a gene or a DNA segment that codes for a protein. In eukaryotes, genes are usually monocistronic, meaning that a single structural gene codes for one mRNA and one protein. In prokaryotes, structural genes are generally polycistronic, meaning that multiple mRNAs and proteins are synthesised using the same promoter and operator.

The process of translation starts with the addition of an amino acid to the specific tRNA (charging of tRNA) that acts as an adapter molecule. tRNA brings amino acids to ribosomes for protein synthesis. Translation is also carried out in three steps, i.e. initiation, elongation and termination.

1. Initiation - The process of translation begins with the binding of ribosomal subunits and the recognition of the start codon (AUG) by the initiator tRNA (Met-tRNA).

2. Elongation - The polypeptide chain continues to grow as specific amino acid-tRNAs bind to mRNA codons sequentially, and peptide bonds are used to add amino acids to the expanding polypeptide chain.

3. Termination - Translation comes to an end when one of the stop codons (UAA, UAG or UGA) is reached, and the polypeptide chain is released.

Regulation of Gene Expression

Regulation of gene expression is essential for controlling the amount, location, type and timing of functional product synthesis. This is necessary for the proper functioning and maintenance of cellular machinery, and allows a cell to produce the correct gene product in the correct amount and at the correct time. Thus, it enables cells to adjust to changing environments. Genes are regulated based on physiological, metabolic and environmental conditions.

The control of gene expression is essential for cellular differentiation, which is the process of how cells with the same genome mature to perform different functions. Through the switching on and off of certain genes in one cell and not in another, this key mechanism is responsible for development, homeostasis, morphogenesis, and evolution. The development of an embryo to an adult is the result of the coordinated regulation of the expression of multiple gene sets.

Regulation of gene expression can occur at various levels, including transcription, translation, and post-translational modifications.

At the time of formation of RNA, known as transcription.

At the time of protein synthesis, i.e. translation.

In eukaryotes, gene expression is also regulated during mRNA processing and export.

Transcription initiation is an efficient and primary control point, with regulatory sequences present upstream or downstream to the promoter region of a transcription unit. Accessibility of the promoter site, to which RNA polymerase binds and initiates transcription, is regulated by accessory proteins. These regulatory proteins bind to the operator region adjacent to the promoter region and can act as either activators or repressors. If a repressor protein binds to the promoter, it inhibits the binding of RNA polymerase to the promoter site and thus hinders transcription.

Example of Gene Expression and Regulation

The lac operon is a classic example of a transcriptionally regulated system in prokaryotes, wherein a number of mRNAs are transcribed from structural genes with a common promoter and operator regions. These genes usually work together or have related functions.

The lac operon consists of three structural genes: z, y, and a, that are regulated by a regulatory gene i.

The Z gene codes for β-galactosidase.

The gene ‘y’ codes for permease.

A gene codes for transacetylase.

I gene - codes for an inhibitor or repressor protein of the lac operon.

Beta-galactosidase catalyzes the hydrolysis of lactose into glucose and galactose. Permease increases the cell permeability to β-galactosides and transacetylase transfers an acetyl group from acetyl-CoA to β-galactosides. The repressor protein binds to the operator region and inhibits the transcription of the lac operon.

When lactose is present, the lac operon is switched on due to the requirement of β-galactosidase for the hydrolysis of lactose, acting as an inducer of the operon. Conversely, when lactose is not present, bacteria do not need β-galactosidase, thus the lac operon is switched off.

Positive Regulation

Normally, the lac operon is switched off due to the continuous formation of repressor protein from the i gene. The repressor binds to the operator region and makes the promoter region inaccessible to RNA polymerase, thereby inhibiting the transcription of z, y and a gene. When an inducer, such as lactose or allolactose are present, then they act as inducers of the operon. The inducer binds to the repressor and inactivates it, thereby enabling the RNA polymerase to access the promoter site for transcription of the genes. Here, the substrate of the enzyme regulates the synthesis of the enzyme. This regulation of a gene by the repressor is known as negative regulation.

Positive Regulation

The lac operon is positively regulated when glucose catabolites are absent. This causes the levels of cAMP to increase, which binds to catabolite activator protein (CAP). The cAMP-CAP complex then binds to the region upstream to the lac operon operator, thus allowing for the binding of RNA polymerase and increased expression of the lac operon.

The presence of glucose catabolites causes a decrease in cAMP levels when glucose levels are high. This decreases the binding efficiency of RNA polymerase, leading to a decrease in the gene expression of lac operon even when lactose is present.

The regulation and operation of the lac operon allows bacteria to use glucose or lactose as a source of energy.