Gene expression is the process by which information encoded in a gene is used to produce a functional product, either RNA or a protein. It connects the information stored in DNA with the activities and properties of a cell. Expression is regulated in amount, location, and timing: a gene may be active in one cell type but inactive in another, or its output may change in response to environmental conditions. Not all expressed genes encode proteins; many produce RNAs that perform structural, catalytic, or regulatory functions. (genome.gov)
Molecular pathway
For protein-coding genes, expression involves two principal stages: transcription, which produces an RNA copy of a DNA sequence, and translation, which uses an RNA template to synthesize a protein. During transcription, RNA polymerase assembles an RNA strand complementary to the DNA template. Repeated transcription of the same gene can generate many RNA molecules without consuming or permanently altering the template. Different genes can be transcribed at different rates. (ncbi.nlm.nih.gov)
In eukaryotes, newly synthesized protein-coding transcripts generally undergo processing before serving as mature messenger RNA (mRNA). This commonly includes addition of a 5′ cap, removal of intervening sequences through RNA splicing, and formation of a poly(A) tail. Alternative splicing can generate distinct mature transcripts from the same gene, sometimes yielding different protein variants. For nuclear genes, processed mRNA is exported from the cell nucleus to the cytoplasm. (ncbi.nlm.nih.gov)
Translation takes place on a ribosome, which reads mRNA in successive three-nucleotide units called codons. Transfer RNAs deliver amino acids corresponding to these codons according to the genetic code. The ribosome joins the amino acids into a polypeptide until a stop codon is reached. Producing a functional protein may additionally require protein folding, chemical modification, transport, or assembly with other subunits. Consequently, mRNA abundance alone does not determine the quantity or activity of the final protein product. (ncbi.nlm.nih.gov)
Non-protein-coding genes do not require translation to produce their functional products. Examples include ribosomal RNAs, transfer RNAs, and regulatory RNAs. Their expression nevertheless involves transcription and may involve substantial RNA processing or assembly into molecular complexes. (genome.gov)
Regulation at multiple levels
Gene expression can be controlled at several stages, including transcription, RNA processing, RNA transport and degradation, translation, and protein turnover. Regulation at one stage can reinforce or counteract regulation at another. The abundance of a product therefore reflects both its production and its removal, rather than simply whether its gene is “on” or “off.” (ncbi.nlm.nih.gov)
Transcriptional regulation depends partly on interactions between regulatory proteins and specific DNA sequences. A promoter helps establish where transcription begins. Activators and repressors, including transcription factors, influence transcription by binding regulatory sequences or interacting with the transcription machinery. Some regulatory elements act close to a gene, whereas others influence transcription from more distant positions. Combinations of regulatory inputs allow genes to respond differently across cell types and conditions. (ncbi.nlm.nih.gov)
In eukaryotic cells, DNA accessibility also depends on chromatin, the complex of DNA and associated proteins. DNA methylation and modifications of histone proteins can influence how DNA interacts with regulatory machinery. These mechanisms are important in epigenetics because they can alter gene use without changing the underlying DNA sequence. Some regulatory states are maintained through cell division, helping preserve cellular specialization. Their effects depend on the genomic and cellular context. (genome.gov)
Post-transcriptional regulation changes the fate of RNA after it is made. RNA-binding proteins and regulatory RNAs can influence transcript stability or translation. These controls allow cells to adjust protein production without necessarily changing transcription. Protein degradation provides an additional means of changing the abundance of an expressed product. (pmc.ncbi.nlm.nih.gov)
Differences among organisms and cells
In bacteria, transcription and translation can occur concurrently because no nuclear envelope separates the processes. Bacterial genes with related functions are often organized into an operon, allowing several genes to be transcribed together and regulated coordinately. The lactose operon illustrates how regulatory proteins and DNA control sequences govern expression in response to nutrient conditions. (ncbi.nlm.nih.gov)
Most nucleated cell types in a multicellular organism contain essentially the same genome, yet produce different combinations of RNA and proteins. These expression differences underpin cellular differentiation and specialization. A cell’s expression pattern can include relatively persistent features associated with its identity and temporary changes caused by signals or environmental conditions. Cellular differentiation generally does not require wholesale alteration of the genome’s nucleotide sequence. (mcb.berkeley.edu)
Measurement and interpretation
Expression can be assessed through RNA abundance, protein abundance, product activity, or an associated phenotype. These measurements address different aspects of expression and are not interchangeable. Detecting a transcript establishes that RNA is present, but does not by itself establish how much functional protein is being produced. (genome.gov)
Microarrays estimate the abundance of selected transcripts through complementary binding to probes. RNA sequencing provides broader expression profiling and supports analysis of transcript variants. Bulk measurements characterize a mixture of cells, whereas single-cell approaches examine expression in individual cells. Interpretation requires attention to experimental design, sample quality, normalization, and biological variation; differences in measured expression must be distinguished from differences introduced by sample preparation or data processing. (genome.gov)