Transcription in protein synthesis: -

TRANSCRIPTION UNIT

1) The synthesis of MRNA chain over DNA is called as transcription.
2) Coded information for protein synthesis is obtained from
DNA called as cryptogram.
3) DNA dependent RNA polymerase enzyme plays the key sole in transcription. It is able to carry out all three steps involved which are initiation, elongation and termination.
4)  RNA polymerase associates transiently with initiation factor and termination factor for initiation and termination of the transcription process.
5) Eukaryotes have three different RMA polymerases while in Prokaryotes single RNA polymerases.
6) The segment of DNA which participates in transcription is the transcription unit. It is primarily consisting of three regions in the DNA.

a) Promoter
b) structural gene
c) terminator
Promoter - Sequence of DMA needed for RNA polymerase to bind to the template and accomplish the initiation reaction defines promoter.
The sequence of DNA required so that no further bases are added to the RNA chains and complex of RNAT and DNA comes apart is called as terminator. Only one strand is copied in transcription.

DNA Template. 

On specific signals, segments of DNA corresponding to one or more cistrons become derepressed and ready to transcribe. Each segment has a promoter region and a terminator region. A promoter region has RNA polymerase recognition site and RNA polymerase binding site. Chain opening occurs in a specific region occupied by 6bp i.e. ., TATAAG (Pribnow box) in most prokaryotes.

There is convention in defining two strands of the DNA in the structural gene of transcription unit. Since the two strands have opposite polarity and the DNA dependent RNA polymerase also catalyze the polymerization in only one direction that is 5’ to 3’ the strand that has the polarity 3’ to 5’ acts as a template and referred as template strand. The other strand which has polarity 5’ to 3’ which does not code for anything is called as coding strand.

The promoter and terminator flank the structural gene in a transcription unit. The promoter is said to be located towards 5' end (upstream) of the structural gene (the reference is made with respect to the polarity of coding strand). It is a DNA sequence that provides binding site for RNA polymerase, and it is the presence of a promoter in a transcription unit that also defines the template and coding strands. The terminator is located towards 3'-end (downstream) of the coding strand.