Prokaryotic Transcription Process

Overview

This lecture explains the process of transcription in prokaryotes, detailing how messenger RNA (mRNA) is synthesized from DNA and highlighting key steps, enzymes, and differences from eukaryotic transcription.

Basics of Transcription

• Transcription creates messenger RNA (mRNA) from a DNA template.
• The DNA double helix unwinds to form a transcription bubble.
• Transcription uses the template strand; the non-template (coding) strand matches the mRNA sequence except T is replaced with U.
• mRNA is synthesized in the 5' to 3' direction, using the 3' to 5' DNA template.

Regions and Directions

• The first mRNA nucleotide is the +1 or initiation site.
• Nucleotides before +1 are upstream; after +1 are downstream.
• In prokaryotes, transcription and translation both occur in the cytoplasm and can happen simultaneously.

Steps of Transcription

• Initiation: RNA polymerase binds to the promoter region upstream of a gene.
• Within promoters are conserved "consensus sequences" (e.g., -10 and -35 regions).
• The sigma factor subunit of RNA polymerase recognizes consensus sequences.
• The complete RNA polymerase with sigma factor is called the holoenzyme.
• Elongation: RNA polymerase moves along the template strand, synthesizing mRNA and releasing the sigma factor.
• Synthesis occurs rapidly (about 40 nucleotides/second in prokaryotes).
• Termination: Transcription ends by either rho-dependent (requires rho protein) or rho-independent (hairpin loop structure and weak A-U bonds) mechanisms.

Prokaryotic vs. Eukaryotic Transcription

• Prokaryotes: Transcription and translation occur simultaneously in the cytoplasm.
• Eukaryotes: Transcription occurs in the nucleus, and mRNA must be transported to the cytoplasm for translation.

Visual Description

• Multiple RNA polymerases can transcribe the same gene, creating several mRNAs at once.
• Ribosomes attach to mRNA as soon as it is available, forming polyribosomes.

Key Terms & Definitions

• Template Strand — DNA strand used to synthesize mRNA.
• Non-template (Coding) Strand — DNA strand whose sequence matches mRNA (T replaced by U).
• Promoter — DNA region upstream of a gene; RNA polymerase binding site.
• Consensus Sequence — Conserved DNA motifs in promoters (e.g., -10 and -35 regions).
• RNA Polymerase — Enzyme complex that synthesizes RNA from DNA.
• Sigma Factor — Subunit of RNA polymerase that recognizes promoter consensus sequences.
• Holoenzyme — Complete RNA polymerase with sigma factor.
• Rho-dependent Termination — Transcription ends using the rho protein.
• Rho-independent Termination — Transcription ends by forming a hairpin loop and weak A-U base pairs.
• Polyribosome — Multiple ribosomes translating a single mRNA molecule at once.

Action Items / Next Steps

• Watch the assigned YouTube video linked on Canvas to review transcription steps (initiation, elongation, termination).
• Optional: Review related material in the OpenStax textbook as referenced.