Lecture on Transcription in Biology

Overview

• Transcription is enzymatically similar to DNA replication.
• Involves synthesizing a new strand of nucleic acid complementary to the DNA strand.
• Key difference:
  • DNA polymerase adds deoxyribonucleotides.
  • RNA polymerase adds ribonucleotides on the DNA template.

RNA Polymerase in Bacteria

• Composed of five subunits: beta prime, beta, alpha 1, alpha 2, and omega.

Subunits

• Beta Prime:
  • Largest subunit.
  • Contains the active site for RNA synthesis.
• Beta:
  • Second largest.
  • Also involved in RNA synthesis.
• Alpha 1 & Alpha 2:
  • Each has two domains: N-terminal and C-terminal.
  • N-terminal domain: Assists in enzyme assembly.
  • C-terminal domain: Interacts with promoters and binds to DNA sequences.
• Omega:
  • Smallest subunit.
  • Facilitates assembly and stabilizes the enzyme.

Sigma Factor

• RNA polymerase requires a sigma factor for initiating transcription.
• Sigma Factor:
  • Protein aiding specific binding of RNA polymerase to the promoter.
  • Forms a holoenzyme with RNA polymerase, comprising six subunits.

Steps in Transcription

1. Initiation

   • Divided into three sub-steps: closed complex, open complex, and abortive initiation.
   • Closed Complex:
     • RNA polymerase binds to promoter.
     • DNA remains double-stranded.
   • Open Complex:
     • DNA strands unwind at transcription start site.
     • Allows RNA polymerase to start adding RNTPs.
     • No primer needed (unlike DNA polymerase).
   • Abortive Initiation:
     • Formation of short RNA stretches released.
     • Occurs due to sigma factor blocking RNA exit channel.

2. Elongation

   • Begins once sigma factor is released.

3. Termination

   • Occurs after the gene is transcribed.
   • Types in prokaryotes:
     • Rho-independent Termination:
       • Intrinsic terminators.
       • Consist of inverted repeats and AT-rich sequences forming AU base pairs (weakest bonds).
       • RNA release ends transcription.
     • Rho-dependent Termination:
       • Requires Rho factor (ATP-dependent hexameric helicase).
       • Binds to cytosine-rich Rho utilization sites (Rho sites) on RNA.
       • Transcription stops 100 nucleotides from Rho site.
       • Rho protein unwinds RNA-DNA hybrid to terminate transcription.