Gene expression is the process by which the information encoded in a gene is used to produce a functional product such as an RNA molecule or a protein. It encompasses the transcription of DNA into RNA and, for protein-coding genes, the translation of messenger RNA into a polypeptide. This process functions as an on-off switch and volume control for gene activity and is tightly regulated.
Mechanisms and regulation
Gene expression begins with transcription, during which a DNA template is copied into a messenger RNA (mRNA) molecule. This mRNA then travels to ribosomes where translation occurs: transfer RNAs read the mRNA codons and add the corresponding amino acids to the growing polypeptide chain, producing a protein. Many RNA copies can be produced from a single gene, and each RNA molecule can direct the synthesis of many protein molecules. Genes can also encode non-coding RNAs such as ribosomal RNAs, transfer RNAs and regulatory RNAs that perform structural or regulatory functions. Cells carefully control when, where and how much of each gene product is made. Regulatory proteins, chromatin modifications and small RNAs can turn genes on or off or adjust transcription rates, providing an on/off switch and volume control. In bacteria, transcription and translation are coupled, whereas in eukaryotes transcription occurs in the nucleus and translation in the cytoplasm. Regulation can occur at multiple levels including transcription initiation, RNA processing, mRNA stability, translation efficiency and protein turnover. Such regulation allows microbes to respond to environmental stress, alter virulence factor production and conserve energy.
Selected examples and applications
A classic example of bacterial gene expression control is the lac operon of Escherichia coli, which is activated in the presence of lactose and repressed when glucose is abundant. Pathogenic bacteria modulate expression of toxins, adhesion factors and secretion systems in response to host signals, enabling invasion and immune evasion. Heat shock proteins are induced when cells experience elevated temperatures to help refold denatured proteins. In laboratories, gene expression can be measured using techniques such as quantitative PCR, microarrays and RNA-sequencing, which reveal changes in transcript levels during infection or under different environmental conditions.
Gene expression underlies how cells use genetic information to make molecules and adapt to their environment. Measuring and controlling gene expression is fundamental to microbiology, biotechnology and medicine.
Related Terms: Transcription, Translation, RNA polymerase, Ribosome, Gene regulation