Transcription is the formation of RNA from DNA.
It occurs in the nucleus during the G1 & G2 stage of the cell cycle.
Of the two strands of DNA only one strand called the template strand takes part in transcription.
A transcription unit in DNA consists of three regions:
- A promoter
- The structural gene
- A terminator.
Structural gene is the part of DNA from which RNA is formed.
- It is polycistronic in prokaryotes and is monocistronic in eukaryotes.
Transcription occurs with the help of DNA dependent RNA polymerase.
Polymerization occurs in 5′ → 3′ direction.
- The 3′ → 5′ strand of DNA acts as the template strand/ anti-sense strand.
- The 5′ → 3′ strand is the coding strand/ sense strand, it has the base sequence similar to the template strand. Only Uracil replaces by Thymine.
All reference points while defining the transcription unit are made with the coding strand.
Lying towards 5′ end (upstream) of the structural gene is the promoter. It is the binding site for RNA polymerase for transcription to start.
- Pribnow box is found in the promoter of prokaryotes.
- Hogness or TATA box is found in the promoter of eukaryotes.
Lying towards 3′ end (downstream) of the structural gene is the terminator, where transcription ends.