Gene expression is a cellular process by which the information encoded in a particular gene is used to produce a functional protein or an RNA molecule. It occurs in all known life forms including eukaryotes, prokaryotes as well as viruses. The transcription of a gene into a mRNA molecule and the translation of the mRNA into a polynucleotide chain of a functional protein are known as the central dogma of molecular biology. Gene expression can be regulated in various steps of the process such as transcription, post-transcriptional modifications, translation, and post-translational modifications. The differential expression of genes allows the cell to produce the required amount of proteins for the functioning of the cell.
Key Areas Covered
Key Terms: Eukaryotes, Gene Expression, mRNA, Prokaryotes, Protein, Transcription, Translation
What is Gene Expression
Gene expression is the process by which genetic instructions are used to synthesize gene products. Generally, the information flows from DNA to mRNA to protein. The two main steps of the gene expression are transcription and translation. The central dogma of molecular biology is shown in figure 1.
Transcription refers to the process of copying the information of a gene into a new RNA molecule. It is the first step of gene expression in both eukaryotes and prokaryotes. RNA polymerase is the enzyme involved in the transcription. Three different types of RNA are produced during transcription: messenger RNA (mRNA), transfer RNA (tRNA), and ribosomal RNA (rRNA). The mRNA carries the genetic information from the nucleus to the cytoplasm. The tRNA is an adaptor RNA that serves as the physical link between mRNA and amino acids. The rRNA forms the integral parts of the ribosome. The process of transcription is shown in figure 2.
However, in some viruses, the genetic material is negative sense RNA. Here, the RNA-dependent RNA polymerase transcribes the negative sense RNA into a mRNA.
Post-transcriptional modifications refer to the process of converting the primary RNA transcript into a mature mRNA molecule. They mainly occur in the eukaryotic gene expression. The mRNA molecule produced by the transcription is known as the primary RNA transcript or pre-mRNA. It is processed to produce the mature mRNA molecule during four steps: 5’ capping, polyadenylation, and alternative splicing. The 5’ capping is the addition of a GTP to the 5’ end of the pre-mRNA molecule. Polyadenylation is the addition of a poly-A tail to the 3’ end of the pre-mRNA molecule. Both 5’ cap and the poly-A tail prevent the degradation of the mRNA molecule. Eukaryotic genes consist of introns and exons. Only introns are coded for the amino acid sequence of a gene. Hence, exons are removed during RNA splicing. Alternative splicing is the production of coding sequences of several polypeptide chains by combining different patterns of introns. Post-transcriptional modification in eukaryotic mRNA is shown in figure 3.
Most prokaryotic genes occur in clusters known as operons. The operons consist of several, functionally-related genes regulated by a single promoter. They transcribe to produce a polycistronic mRNA molecule that synthesis several functionally-related proteins.
Translation refers to the process where the genetic code carried by a mRNA molecule is decoded, producing a polypeptide chain of a particular protein. It occurs in the cytoplasm by ribosomes. A system of three amino acids is involved in the determination of each amino acid in the polypeptide chain. The three nucleotides in the mRNA that represent an amino acid is known as a codon. The complete codon system is known as the genetic code. Different tRNA molecules contain anticodons that fix with each codon in the mRNA. Hence, they carry the corresponding amino acid for the synthesis of the polypeptide chain. Translation is shown in figure 4.
Post-translational modifications are the covalent and enzymatic modification of the polypeptide chain of a functional protein. Different polysaccharide, lipid or inorganic groups are added in order to produce a functional protein. These modifications are known as glycosylation, phosphorylation, sulfation, etc. Different cofactors can also be added to regulate the function of the protein. The post-translational modifications of insulin protein are shown in figure 5.
How is the Gene Expression Regulated
The cell regulates the gene expression either to increase or decrease the number of proteins produced inside the cell. In eukaryotes, it can be achieved through the various steps of gene expression such as transcription, post-transcriptional modifications, translation, and post-translational modifications. However, in prokaryotes, the regulation of the gene expression is achieved during the initiation of the gene expression.
The production of functional proteins inside the cell is achieved through the expression of genes in the genome. The two main steps of the gene expression are transcription and translation in all kinds of living organisms including eukaryotes, prokaryotes, and viruses. Transcription is the production of a mRNA molecule based on the nucleotide sequence of the gene. Translation is the production of a polypeptide chain based on the codon sequence of the mRNA molecule. In eukaryotes, the gene expression can be regulated in both transcriptional and translational levels. However, the gene expression in prokaryotes is regulated during the initiation of transcription.
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