Transcription Unit and Gene

Every transcription unit on a DNA molecule is defined by three functionally distinct regions arranged in a fixed order: a promoter, the structural gene itself, and a terminator. The promoter sits toward the 5' end of the unit and is the specific DNA sequence that RNA polymerase physically recognises and binds to before transcription can begin; because a promoter is present at only one end of the unit, its position is what fixes which of the two DNA strands will act as the template strand and which will act as the coding strand for that particular gene. The terminator sits at the opposite end, toward the 3' end of the coding strand, and consists of a DNA sequence that signals RNA polymerase to stop transcribing and release the finished transcript; in eukaryotes, terminator-associated sequences that form a hairpin loop in the resulting RNA are recognised directly, while a distinct subclass of terminators additionally requires a dedicated recognition protein called rho (denoted by the Greek letter r) to function. Because the two strands of DNA within the structural gene run in opposite chemical directions (they are antiparallel), and RNA polymerase can only synthesise RNA in one direction, only the strand running 3' to 5' can be read as template, and this is accordingly named the template strand; the other strand, running 5' to 3', is displaced during transcription and is called the coding (or sense) strand, because its sequence matches the resulting RNA transcript exactly, apart from carrying thymine wherever the RNA carries uracil. Eukaryotic structural genes are described as monocistronic, meaning each individual mRNA molecule carries the coding information for, and is later translated into, only a single protein. Prokaryotic genes, by contrast, are frequently polycistronic: clusters of functionally related genes, called an operon, sit next to one another on the chromosome and are transcribed together from a single promoter into one shared mRNA molecule that goes on to direct synthesis of several different proteins. Before transcription can start, RNA polymerase must first bind to the promoter, and in bacteria the RNA polymerase enzyme itself is built fro …