Question

Explain the mechanism of transcription in a prokaryotic cell.

Answer 1

Transcription is the process of copying genetic information from sense strand of DNA into RNA, only a segment of DNA is transcribed that only one of the two strands is copied. In prokaryotes, transcription occurs in contact with cytoplasm as their DNA lies in cytoplasm. Transcription requires a DNA dependent enzyme RNA polymerase. Prokaryotes have only one RNA polymerase which synthesises all types of RNAs. The transcription includes following steps :

1. Activation of nucleotides in the nucleoplasm through phosphorylation. These are ATP, GTP, UTP and CTP.
2. A single RNA – polymerase binds to the promoter site/region of DNA template. Chain opening of DNA segment by uncoiling occurs from the site of polymerase binding.
3. Initiation of transcription occures by the complementary pairing of free activated ribonucleotides with the nitrogen bases of DNA template.
4. Activated ribonucleotide triphosphate acts as a substrate and also provides energy for polymerization on a template following the rule of complementarity.
5. As the RNA chain formation initiates, the sigma(cr) factor of RNA polymerase separates. RNA polymerase moves along the coding region of DNA template causing elongation of RNA chain.
6. RNA synthesis stops as soon as the polymerase reaches the terminator region. Rho(p) factor is required for this. Terminator region has a stop signal.
7. Rho factor helps the release of nascent RNA, RNA polymerase falls off. It is the termination of transcription.
8. In prokaryotes, the /n-RNA synthesis does not require any processing to become active and both transcription and translation occur in the same cytosol.
9. Translation can start much before the m-RNA is fully transcribed, i. e., transcription and translation can be coupled.