Transcription is the biological process by which prokaryotic and eukaryotic cells generate single stranded RNA through the enzymatic action of a specific isoform of RNA polymerase. It is the intermediary step in the production of proteins, and the final step for non-protein coding RNAs. Protein-coding genes are transcribed into mRNA which are then translated by the ribosome to create a nascent polypetide chain.
In virally-infected cells, both RNA and DNA can be transcribed, and the resulting double-stranded RNA is an alarm signal for infection, as it is captured by the RNA interference pathway, imported into the endplasmic reticulum and then displayed on the cell surface in a specific subtype of MHC class II molecules. These short interfering RNAs (siRNAs) cause the production of interferons, and the activation of the immune system.
Non-protein coding RNAs are transcribed from DNA for the production of ribosomal RNA, transfer RNA, and small nuclear RNAs which serve various other functions in the cell. The biochemical process for transcription is similar to DNA replication in that it requires the action of multiple enzymes to unravel double-stranded genomic DNA, which allows an enzyme (polymerase) to processively catalyze the formation of a complementary strand based on proper nucleotide base pairing.