RNA polymerase

RNA polymerase is the machine that performs transcription — the enzyme that crawls along a gene and types out an RNA copy. Where DNA polymerase copies DNA into DNA, RNA polymerase copies DNA into RNA, building the message that will later be translated into protein.

It clamps onto the DNA, pries the two strands apart in a small moving bubble, reads the template strand, and adds RNA nucleotides that base-pair with it (A–U and G–C). Unlike DNA polymerase, it needs no primer — it can start a new strand from scratch, which is one reason a separate enzyme makes RNA primers for DNA replication.

Crucially, RNA polymerase does not transcribe randomly. It is recruited to specific start signals called promoters, and an array of regulatory proteins help it decide whether to begin. Bacteria use a single RNA polymerase for all genes, while eukaryotes use three, each dedicated to different classes of RNA.