Transcription unit (Each transcribed segment of DNA) consists of the promoter, the structural gene and the terminator.
(1) The promoter:
- The promoter is located towards 5′ end of structural gene, i.e. upstream.
- It is a DNA sequence that provides binding site for enzyme RNA polymerase.
- In prokaryotes, sigma factor sub unit of the enzyme recognizes the promoter.
(2) Structural genes:
- Template strand (Antisense strand) : DNA strand having 3’→ 5’ polarity acts as template strand as DNA dependentRNA polymerase catalyses polymerization in 5’ → 3’ direction.
- Sense strand : The other strand of DNA having 5’ → 3’ polarity is complementary to template strand. It is called as sense strand. The sequence of bases in this strand, is same as in RNA (where Thymine is replaced by Uracil). It is the actual coding strand.
(3) The terminator:
- The terminator is located at 3’ end of coding strand, i.e. downstream.
- It defines the end of the transcription process.