RNA is synthesized from a single transcription unit, a segment of DNA whose transcription gives rise to a continuous RNA molecule. The process begins with the binding of RNA polymerase to a DNA promoter sequence, which triggers local unwinding of the DNA double helix.
DNA promoter sequence
(a specific sequence of several dozen base pairs that determines where RNA synthesis starts and which DNA strand is to serve as the template strand)
Using only one of the two DNA strands as a template, RNA polymerase then initiates the synthesis of an RNA chain
After initiation has taken place, the RNA polymerase molecule moves along the DNA template, unwinding the double helix and elongating the RNA chain as it goes. During this process, the enzyme catalyzes the polymerization of nucleotides in an order determined by their base pairing with the DNA template strand.
Eventually, the enzyme transcribes a special sequence called a termination signal, which terminates RNA synthesis and causes the completed RNA molecule to be released and RNA polymerase to dissociate form the DNA template.
Transcription is mediated by
DNA-dependent RNA polymerase (or just RNA polymerase):• Uses only one strand of the DNA as a template• Synthesizes in 5’ to 3’ direction• Does not require a preformed primer to initiate synthesis• Initiates transcription de novo at specific sites at the beginning of a gene
stage one of transcription (binding)
Binding of RNA polymerase to the promoter
Promoter has three functions
o Directs RNA Polymerase to the templateo Dictates which of the two DNA strands is to be used as the templateo Positions RNA Polymerase over the initiation site
Directs RNA polymerase to specifically bind to promoter
Steps of RNA transcription
Binding to TATA box of TFIID. TBP binds to tata box --TBP associating factors (TAF)--then the binding of TFIIB