Transcription is the cellular process in which RNA polymerase reads a DNA template and produces a complementary RNA strand, synthesizing the first step in gene expression.
Explanation
During transcription, genetic information encoded in DNA is copied into RNA to provide a working blueprint for protein synthesis and other functions. In prokaryotes a single RNA polymerase associates with a sigma factor to recognize promoter sequences upstream of genes. After binding, the enzyme unwinds a short segment of the double helix and begins polymerization by adding ribonucleotides to the growing RNA chain in the 5′ to 3′ direction. As the polymerase moves along the template, the DNA helix reforms behind it. Termination occurs when the polymerase encounters specific terminator sequences or a rho factor. Eukaryotes have three major nuclear RNA polymerases: polymerase I transcribes rRNA precursors, polymerase II transcribes protein‑coding mRNAs and many non‑coding RNAs, and polymerase III synthesizes tRNAs and 5S rRNA. Initiation by polymerase II requires general transcription factors and promoter elements such as the TATA box; transcription can be modulated by enhancers and repressors located far from the gene. Nascent transcripts from polymerase II undergo co‑transcriptional capping, splicing and polyadenylation before export to the cytoplasm. Regulation of transcription ensures appropriate levels of gene products in response to environmental cues and developmental signals, and errors can lead to disease. Antibiotics like rifampin selectively inhibit bacterial RNA polymerase, and many viruses encode or hijack polymerases for their replication.
Examples of transcription in biology
Transcription of the human beta‑actin or GAPDH genes occurs continuously in most cells to provide mRNAs for essential cytoskeletal and metabolic proteins. In bacteria, the lac operon is transcribed as a single polycistronic mRNA when lactose induces the operon, while termination factors stop transcription when glucose is abundant. The yeast 35S rRNA precursor is transcribed by RNA polymerase I and later processed into 18S, 5.8S and 25S rRNAs. Reverse transcription in retroviruses copies an RNA genome into DNA, illustrating that transcription can proceed in reverse. Inhibitors like alpha‑amanitin selectively block eukaryotic polymerase II, whereas actinomycin D intercalates DNA and inhibits both prokaryotic and eukaryotic transcription.
Transcription translates the static information of DNA into a dynamic RNA intermediate. By orchestrating initiation, elongation, termination and RNA processing, cells control gene expression and respond to changing conditions.
Related Terms: DNA, RNA polymerase, Gene expression, Promoter, mRNA