A sequence of DNA that contains the information to produce a functional product, such as a protein or a functional RNA, and serves as a unit of inheritance.
Explanation
Genes are the fundamental units of heredity. In molecular terms a gene is a segment of nucleic acid with regulatory and coding regions that direct the synthesis of specific products. In eukaryotes many genes consist of exons, which code for amino acid sequences or functional RNAs, interspersed with introns that are removed during RNA splicing. Upstream and downstream promoter elements, enhancers and silencers regulate transcription initiation and the level and timing of gene expression. In prokaryotes genes are typically organized into operons and lack introns, with expression controlled by promoters and operators. The information encoded by a gene is transcribed into messenger RNA for protein‑coding genes, or into transfer RNA, ribosomal RNA or other non‑coding RNAs that perform structural and regulatory roles. Translation of mRNA occurs on ribosomes, producing polypeptides that fold into functional proteins. Mutations, insertions, deletions or rearrangements within genes can alter gene function, leading to variation, disease or evolution. Different versions of a gene at the same chromosomal location are called alleles. Diploid organisms carry two alleles of most genes, and the interaction of these alleles underlies Mendelian inheritance patterns. The human genome contains roughly 20 000 protein‑coding genes among about three billion base pairs. Advances in genomics have expanded the definition of a gene to include regulatory sequences and overlapping transcriptional units.
Examples and Significance
The cystic fibrosis transmembrane conductance regulator (CFTR) gene on chromosome 7 encodes an ion channel; mutations in this gene cause cystic fibrosis. In bacteria, the lacZ gene encodes β‑galactosidase, an enzyme that hydrolyses lactose; it is part of the lac operon and regulated by lactose availability. The gene encoding haemoglobin β chain (HBB) is mutated in sickle cell anaemia. In blood typing, alleles of the ABO gene determine the presence of A and B antigens on red blood cells. These examples illustrate how genes determine phenotypes, can be regulated and may be altered in disease.
The concept of a gene integrates molecular structure, function and inheritance. Understanding gene organization and expression is central to genetics, biotechnology and medicine.
Related Terms: DNA, Chromosome, Allele, Gene expression, Genome