Question

Explain the process of transcription in prokaryotes.

Answer 1

Transcription is the process of the formation of the m-RNA strand on a DNA strand in the nucleus. It takes place in three steps—initiation, elongation and termination.
(a) Initiation:
i. In prokaryotes, the structural genes are polycistronic and continuous.
ii. A single DNA-dependent RNA polymerase catalyses transcription of all the three types of RNA (mRNA, tRNA and rRNA).
iii. RNA polymerase binds to the promoter region and starts the process of transcription. The RNA polymerase enzyme contains a detachable subunit called the sigma (σ) factor. It helps the enzyme to bind firmly to DNA.
iv. The RNA core polymerase (minus sigma factor) moves down DNA at a faster pace and this continues to synthesise a new RNA chain.
(b) Elongation:
i. Ribonucleoside triphosphates help in the polymerisation on a DNA template, following the rule of complementarity.
ii. The enzyme facilitates the opening of the DNA helix and continues elongation.
(c) Termination: The process of elongation during transcription continues until the enzyme, RNA core polymerase reaches the terminator sequence in the sense DNA strand (3′–AAAAAAT–5′). At this point, another protein particle, the rho (ρ) factor, forms a complex with RNA-polymerase. This causes the enzyme to go off the DNA track, and thus, new m-RNA is released.