B.Sc I year BIOTECHNOLOGY Major I

Prokaryotic Transcription:

Prokaryotic transcription, copying DNA to mRNA, has three stages: Initiation (RNA polymerase binds promoter with sigma factor), Elongation (polymerase moves along DNA, synthesizing RNA 5' to 3'), and Termination (hairpin loops or Rho factor stop synthesis) . It occurs in the cytoplasm, allowing simultaneous translation, and involves RNA polymerase, sigma factor, and specific promoter/terminator sequences like -10/-35 boxes. 

1. Initiation

• RNA Polymerase Holoenzyme: The core enzyme plus the sigma (σ) factor forms a holoenzyme.
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• Promoter Binding: The sigma factor recognizes and binds to specific DNA sequences (promoter, like -10 and -35 boxes) near the gene's start.
• Unwinding: The DNA double helix unwinds at the promoter, creating a transcription bubble, exposing the template strand.
• Sigma Factor Release: Once a short RNA segment is made, the sigma factor detaches, leaving the core enzyme to continue. 

2. Elongation

• RNA Synthesis: The core RNA polymerase moves along the DNA template strand in the 3' to 5' direction.
• mRNA Growth: It adds complementary ribonucleotides (A, U, C, G) to the growing mRNA strand in the 5' to 3' direction, forming phosphodiester bonds.
• Rate: This process occurs rapidly, about 40 nucleotides per second. 

3. Termination

• Terminator Sequence: The polymerase encounters a specific DNA sequence that signals the end.
• Two Mechanisms:
  • Rho-Independent (Intrinsic): The RNA transcript forms a stable hairpin (stem-loop) structure, causing the polymerase to stall and the RNA to detach.
  • Rho-Dependent: A Rho (ρ) protein binds to the mRNA and travels toward the polymerase, causing dissociation.
• Release: The newly formed mRNA transcript and the RNA polymerase are released from the DNA template.