What is Gene Expression?

Gene expression is a process by which the instructions present in deoxyribonucleic acid (DNA) are converted into useful molecules such as proteins, and functional messenger ribonucleic (mRNA) molecules in the case of non-protein-coding genes. 

Regulatory Network

The human genome is the entire genetic instruction of a human being. Each gene in the human genome is capable of encoding proteins and protein plays a significant role in dictating several functions of the cell. Thus, the several genes expressed in the nucleus of the cell decide the cellular role in an organism. The flow of information takes place as the double-stranded DNA transcribe into single-stranded mRNA and mRNA translates into protein. This flow of information helps the cell to regulate its functions on its own by regulating the protein’s quantity and type. The balance maintained between the biochemical pathways of synthetic and degradation is reflected by the quantity of specific protein in the cell at any time. The transcription process and translation process are two important processes that play a significant role in determining the type and the quantity of the protein.

Transcription    

It is considered as the initial step in the gene expression where DNA sequences are copied in the format of the ribonucleic acid sequence. This can be explained with an everyday example when a person rewrites information that is obtained via phone call for reviewing later. Rewriting information is a short brief to describe the role of transcription. Our cells should undergo the transcription process in a specialized and narrowed manner. RNA transcripts are essential to construct protein molecules. mRNA is the information mediator for protein construction. The transcription process is mainly facilitated with the help of RNA polymerase. This enzyme utilizes a DNA template that is single-stranded to transcribe the complementary strand of RNA. mRNA strands are synthesized in the direction of 5’ to 3’ in which every new nucleotide is added to the 3’ end.   

Stages of Transcription 

Transcription includes three main stages namely initiation, elongation, and followed by termination stage. They are defined as follows:  

• The initiation process takes place once the enzyme RNA polymerase gets attached to the promoter region of the gene. This helps in separating the strands resulting in a required single strand for the transcription process.     

• Elongation is the process of creating complementary strands of RNA from the template strand of DNA. The RNA molecules get elongated by reading one base at a particular time. The formed RNA strand carries the same information as the non-coding or non-template strand of DNA in the direction of 5’ to 3’. But the Uracil (U) base is replaced for the Thymine (T) base in the RNA strand. 

• Termination is facilitated by the terminators which indicate the completion of the RNA transcript. The hairpin formation is one example of the terminating mechanism in RNA. Once the termination process takes place, the RNA transcripts are released from the enzyme RNA polymerase molecule.    

The pre-mRNA formed must undergo post-transcriptional modification for producing the final form known as mRNA.  

The determination of gene expression patterns, specifically at the transcription level under particular circumstances or in a distinct cell to produce a global picture of the cellular function, is termed gene expression profiling. RNA-seq is one of the sequence-based techniques that helps in providing information on the gene sequences in addition to their levels of expression.

Translation

The mRNA obtained using the transcription process is decoded to produce proteins. Proteins are composed of a specific series of amino acids. The mRNA does not encode protein in a direct manner, rather it encodes polypeptides, which is a linear amino acid chain. In messenger RNA, RNA nucleotides such as A, U, C, and G act as instructions for constructing the polypeptide chain. These nucleotides exist as a group of three which is called a codon. There are about 61 codons for amino acids, in which codons specify one particular amino acid among the twenty amino acids that are commonly found in the protein molecules. AUG is a start codon, but then it also encodes methionine. UAG, UAA, and UGA are the stop codon that informs the cell about the completion of the polypeptide. This conversion of codon into amino acid is altogether referred to as genetic code.      

The translation is facilitated mainly by two significant molecules namely tRNA and ribosomes. The tRNA’s main role is to connect mRNA codons with their specific encoded amino acids. Ribosomes are composed of protein and RNA in which the construction of polypeptides takes place.  

Stages of Translation

Similar to transcription, translation can also be categorized into three stages for the polypeptide construction. They are:   

• Initiation is the stage where the ribosome gets assembled around the mRNA that has to be read along with the initial tRNA that carries the start codon.       
• The elongation process helps in elongating the chain of amino acids. tRNA move across the A, P, and E sites to carry out the elongation process.       
• Termination is carried out with the help of the stop codon. This helps in releasing the amino acid chain from the tRNA molecule and further moving away from the ribosomes. 

The translated protein should undergo Post Translational Modification (PTM) to reach its target cell and perform its function.    

Gene Regulation    

Gene regulation helps the cells to decide which gene has to be expressed in a particular cell. Although all the genes are composed of DNA, each type of cell tends to have certain active gene sets. These modifications in the expression pattern of genes provide varying protein in distinct cell types and specialized for unique functions. This can be briefly explained with an example: In our body, each organ is specialized for different functions. For example, the liver helps in removing substances that are toxic to our body via the blood, but it is not in the case of brain neurons where its role is entirely different from that of the liver cells. Thus, in neurons, those active genes that are responsible for removing toxic substances are turned off. Similarly, liver cells turn off the active genes that are responsible for sending signals using neurotransmitters. In this way, all the cell types express their gene varyingly according to their role. Certain regulatory proteins such as enhancers or repressors are involved in the regulation of gene expression in both prokaryotes and eukaryotes. But the gene regulation is heavily complex in the case of eukaryotes when compared to prokaryotes. Thus, gene expression analysis and regulation of expression levels are important in playing a significant role in our body by providing different cell types with their regulation mechanisms.   

Context and Applications

This topic is significant in the professional exams for both undergraduate and graduate courses, especially for   

Bachelor of Science Genetics.        

Bachelor of Science B.Sc. Biochemistry and Molecular biology        

Bachelor of Science Ecology and Evolutionary biology        

Master of science Biological science         Masters in Biotechnology