Transcription in Prokaryotes | B.Sc notes | Wandofknowledge
The process of conversion of the de-oxyribonucleotide sequences of DNA into ribonucleotide sequences of RNA is called Transcription.
- In prokaryotes, all the types of RNA (m-RNA, t-RNA, r-RNA) are synthesized from the DNA by single RNA polymerase.
- Prokaryotic RNA polymerase enzyme is multimeric consists of α2, β, β’, ω.
- RNA polymerase has ability to unwind the double stranded DNA helix before transcription and rewind it after the process of transcription completed.
- RNA polymerase is capable of de-novo synthesis of RNA strand without involving primer.
RNA polymerase have specific protein subunit :
- σ- factor : It helps RNA polymerase in locating the transcription initiation site called promotor region. Every gene needs specific σ-factor for specifically recognization of its promoter.
- RNA polymerase holoenzyme : The RNA polymerase holoenzyme acts as crab claw having two positively charged pincer. After the separation of the two strands of DNA by RNA polymerase, the transcription occurs on the sense strand.
The RNA polymerase catalyse the synthesis of RNA in 5’ to 3’ end by forming phosphodiester bond between the 3’ OH of first ribonucleotide with the 5-nucleotidyl phosphorous of the incoming ribonucleotide triphosphate.
Mechanism of Transcription
- Initiation– It involves binding of σ-factor of RNA polymerase at the promoter site or transcription initiation site (+10, +35 upstream).
Unwinding of the DNA helix by core enzyme RNA polymerase forming transcription bubble.
Synthesis of about 9-10 ribonucleotide sequences followed by removal of σ-factor.
- Chain Elongation– The RNA polymerase catalyse the synthesis of RNA transcript by the process of transcription in 5’ to 3’ direction by forming 3-OH-5P phosphodiester bond between the pre-existing ribonucleotide and incoming new ribonucleotide.
- Chain Termination– Every gene contains termination nucleotide sequence at which transcription process terminates.
Termination of transcription may occur:
- By ρ- protein dependent mechanism- The ρ-protein specifically binds with the termination sequences and cleave the nascent RNA transcript.
- By ρ- protein independent mechanism- In this case the sense DNA contains repeated inverted copies of G≡C nucleotide followed by large number of A nucleotide.
When the RNA polymerase transcribe these sequences, a hairpin like loop and as result nascent RNA transcript get cleaved at this point blocking the process of transcription.
- Translation: Mechanism of Protein Synthesis
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