A gene is a unit of heredity consisting of a sequence of nucleotides in DNA that encodes a functional product, such as a protein or RNA.
Explanation
In molecular biology, a gene is defined as a sequence of DNA that is transcribed into RNA and, in most cases, translated into a polypeptide. Genes include a coding region and regulatory sequences that control when and where transcription occurs. In bacteria and archaea, genes are often arranged contiguously in operons under the control of a single promoter. Eukaryotic genes are more complex: coding regions (exons) are interrupted by non‑coding introns, and transcription is influenced by promoters, enhancers, silencers and epigenetic modifications. Not all genes encode proteins; some produce functional RNAs such as ribosomal RNA, transfer RNA and various small and long non‑coding RNAs that regulate gene expression. Genes vary in length from a few hundred base pairs to over a million bases. Mutations within genes can alter their products or expression, contributing to variation, adaptation or disease. With modern sequencing technologies, the concept of a gene has expanded beyond simple one gene–one enzyme models to encompass overlapping reading frames, alternative splicing and complex regulatory architectures. Environmental signals and developmental programs modulate gene expression, integrating genetic information with physiology.
Examples and important facts
The lacZ gene in Escherichia coli encodes β‑galactosidase and is part of the lactose operon that is induced when lactose is present. Mutations in the human CFTR gene, which encodes a chloride channel, cause the inherited disorder cystic fibrosis. The 16S rRNA gene encodes the RNA component of the bacterial small ribosomal subunit and is widely used for phylogenetic studies. The XIST gene produces a long non‑coding RNA responsible for X‑chromosome inactivation in female mammals. Hemoglobin genes exist as clusters that undergo developmental switching; defects produce thalassemia and sickle‑cell disease. Gene families arise through duplication, and pseudogenes are nonfunctional remnants of once‑active genes.
Genes are the fundamental units of heredity, directing the synthesis of functional molecules. Insights into gene structure, expression and evolution underpin modern genetics, medicine and biotechnology.