Prokaryotic Transcription.

• What are the three types of RNA and their role?

  Messenger RNA (mRNA) - encodes for protein.

  Ribosomal RNA (rRNA) - forms part of the ribosome structure.

  Transfer RNA (tRNA) - adapter for protein synthesis.
• How is RNA synthesised in prokaryotes?

  • Synthesised by DNA-dependent RNA polymerase.
  • Requires DNA template, all 4 NTPs (ATP, GTP, UTP, CTP), and Mg²⁺.
  • Core enzyme: 5 subunits + Sigma factor (holoenzyme).
  • Errors occur every 10⁵ NTPs, but RNA is quickly degraded.
• What is the role of the Sigma factor in prokaryotic transcription?

  The Sigma factor guides RNA polymerase to a promotor and increases binding specificity.
• What are prokaryotic promotors?

  Sequences directing the start of transcription, with varying affinities for RNA polymerase, affecting gene transcription levels.
• What are the important regions in prokaryotic promotor sequences?

  -10 and -35 regions for RNA polymerase binding and upstream promotor element (-60 to -40) important for high gene expression.
• Describe the initiation phase of prokaryotic transcription.

  RNA polymerase binds to the promotor, forming a ‘closed’ complex, then an ‘open’ complex, allowing access to DNA. Initiation includes abortive events.
• What happens during prokaryotic elongation?

  RNA polymerase unwinds 17 bp of DNA at a time, creating RNA while topoisomerases relieve torsional stress.
• What are the two types of termination signals in prokaryotic transcription?

  Rho-independant - Forms a hairpin structure and has strings of A's with U's, terminating transcription.

  Rho-dependant - Involves a protein (Rho) that migrates to the 3' end, contributing to mRNA release, mechanism unknown.
• What is the significance of the TATA sequence during transcription initiation?

  The TATA sequence is a region of weak hydrogen bonds that RNA polymerase recognises to "unzip" the DNA and start transcription.
• What happens during the abortive events in transcription?

  Abortive events are short RNA fragments synthesised and released during initiation before RNA polymerase commits to elongation.
• What is the primary role of topoisomerases during transcription?

  Topoisomerases relieve the torsional stress generated by the unwinding of DNA as RNA polymerase moves along the template strand.
• Why is RNA degradation important in prokaryotic cells?

  RNA degradation allows for the quick turnover of mRNA, enabling cells to rapidly respond to changing conditions and regulate gene expression.
• What distinguishes the core RNA polymerase from the holoenzyme in prokaryotes?

  The core RNA polymerase consists of the 5 subunits, while the holoenzyme includes the Sigma factor, which is necessary for specific promoter binding.
• How does the strength of a prokaryotic promotor affect gene expression?

  Stronger promotors have sequences with higher affinity for RNA polymerase, leading to increased transcription levels of the associated genes.
• What is the consequence of RNA polymerase lacking proofreading ability?


  Errors introduced during RNA synthesis can lead to incorrect RNA sequences, but these errors are generally tolerated because RNA is not permanently retained.