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- Gene expression is the process by which the genetic code – the nucleotide sequence – of a gene is used in the synthesis of a functional gene product.
- It refers to a complex series of processes in which the information encoded in a gene is used to produce a functional product such as a protein that dictates cell function.
- It involves several different steps through which DNA is converted to an RNA which in turn is converted into a protein or in some cases RNA, for example, genes encoding the necessary information for transfer RNAs and ribosomal RNAs (tRNAs and rRNAs).
- The two main stages include:
- Transcription:the production of messenger RNA (mRNA) by the enzyme RNA polymerase, and the processing of the resulting mRNA molecule.
- Translation: the use of mRNA to direct protein synthesis, and the subsequent post-translational processing of the protein molecule.
- Thus, gene expression is the phenotypic manifestation of a gene or genes by the processes of genetic transcription and genetic translation.
Sequence of Events in Gene Expression
When genes are expressed, the genetic information (base sequence) on DNA is first copied to a molecule of mRNA (transcription). The mRNA molecules then leave the cell nucleus and enter the cytoplasm (in eukaryotes), where they participate in protein synthesis by specifying the particular amino acids that make up individual proteins (translation).
Key phases in gene expression
Gene expression consists of steps that finally produce a functional bio-molecule.
Key steps involved in gene expression include the following:
- Transcription – conversion of DNA to RNA
- This is the first step in gene expression in which DNA molecules are transcribed into their corresponding RNA copy.
- This process is aided by an enzyme called DNA-dependent RNA polymerase.
- Post-transcriptional modifications
In this process, the primary RNA obtained after transcription is modified to produce a mature messenger RNA or mRNA.
The processes involved are:
- Splicing which is the cleavage of introns (non-coding sequences) and ligation of exons (coding sequences) with the help of several components that recognize specific sequences in the RNA.
- Capping which involves addition of a cap molecule to the 5’ end.
- Tailing which is the addition of poly A tail to the 3’ end.
- RNA transport (In Eukaryotes)
- Most of the mature mRNAs produced after modifications are transported from the nucleus to the cytoplasm where the next step in gene expression takes place.
- This is achieved by moving the mRNAs through tiny pores in the nucleus to reach the cytosol.
- Translation or protein synthesis
- The sequence in the mRNA is translated into a protein with the help of several components such as ribosomes, tRNAs or transfer RNAs, and enzymes called aminoacyl tRNA synthetases.
- Translation of mRNA involves 3 important steps – initiation, elongation, and termination, leading to the formation of polypeptide chains.
- Protein folding and modifications
- In this final step, the polypeptide chains or random coils formed during translation fold into a 3D structure giving rise to a functional protein.
- Failure to fold leads to protein inactivity and misfolded proteins have abnormal functionalities compared to correctly folded ones.
- Also, proteins can be modified by various methods such as phosphorylation, glycosylation, ADP ribosylation, hydroxylation, and addition of other groups.
Regulation of Gene Expression
- Gene regulation is a label for the cellular processes that control the rate and manner of gene expression.
- A complex set of interactions between genes, RNA molecules, proteins (including transcription factors) and other components of the expression system determine when and where specific genes are activated and the amount of protein or RNA product produced.
Mechanisms of gene regulation include:
- Regulating the rate of transcription. This is the most economical method of regulation.
- Regulating the processing of RNA molecules, including alternative splicing to produce more than one protein product from a single gene.
- Regulating the stability of mRNA molecules.
- Regulating the rate of translation.
- Transcription factors are proteins that play a role in regulating the transcription of genes by binding to specific regulatory nucleotide sequences.