Gene expression is the process by which information in DNA is used to produce functional molecules, mainly proteins and certain RNA molecules. It involves transcription of DNA into RNA, processing of that RNA, and translation of messenger RNA into proteins. Cells control gene expression at multiple stages, allowing different cell types to arise from the same genome and enabling dynamic responses to the environment.

Regulation begins with chromatin structure. DNA is wrapped around histone proteins to form nucleosomes, and chemical modifications such as methylation or acetylation alter how tightly DNA is packed. This affects whether transcription factors and RNA polymerase can access genes. Epigenetic marks are heritable yet reversible, providing a flexible regulatory layer.

Transcription factors recognize specific DNA sequences in promoters and enhancers, where they recruit or block the transcription machinery. Enhancers can act over long distances through DNA looping. Noncoding RNAs, including microRNAs and long noncoding RNAs, modulate gene expression by affecting mRNA stability, translation, or chromatin state.

After transcription, RNA processing steps such as capping, splicing, and polyadenylation further shape gene output. Alternative splicing allows one gene to produce multiple protein isoforms. Export of RNA from the nucleus, control of translation initiation, and selective degradation of mRNA fine tune protein levels.

Gene expression noise and cell to cell variability are important in processes like development and responses to stress. Misregulation underlies many diseases, including cancer, developmental disorders, and neurological conditions. Advances in high throughput sequencing and single cell technologies now allow detailed mapping of gene expression patterns and regulatory networks, deepening understanding of cellular identity and function.