Overview of Gene Expression and RNA Processing

• DNA and RNA Stability
  • DNA is a stable repository of genetic information, referred to as a library of all gene info that is present.
  • RNA is considered unstable and acts as a messenger of the library, indicating its ephemeral nature.

Gene Expression

• Gene Expression Overview
  • Gene expression is the process by which genes are transcribed into RNA and subsequently translated into proteins.
  • Characteristics of RNA:
  • More readily degradable than DNA due to its chemical structure, making it less stable.

RNA Structure

• RNA's Secondary Structure

  • Typically single-stranded but can fold into intricate secondary structures with functional significance.
  • Examples of secondary structures include:
  • Hairpins
  • Interior Loops
  • Bulge Loops

• Features of RNA Structures:

  • Secondary structures are formed post-transcription and play roles in RNA stability and function.

Gene Structure and Nomenclature

• Basic Gene Structure
  • Comprised of:
  • Promoter: DNA sequence where RNA polymerase binds.
  • Coding Region: The sequence transcribed into RNA.
  • Terminator: Signals the end of transcription.
  • Orientation:
  • Typical DNA layout includes 5' promoter region, a coding region, and a 3' termination region.

Bacterial Transcription

• Bacterial RNA Polymerase

  • E. coli has a single RNA polymerase responsible for synthesizing all forms of RNA (mRNA, tRNA, rRNA).
  • Rifampicin: An antibiotic that inhibits RNA synthesis by preventing the formation of the first phosphodiester bond, effectively impeding mRNA, tRNA, and rRNA synthesis.

• Promoters in Bacteria:

  • Promoters consist of consensus sequences located upstream of the coding sequence.
  • Common bacterial promoter sequences include:
  • -10 consensus sequence (Pribnow Box): TATAAT
  • -35 consensus sequence: TTGACA

Transcription Initiation

• Process Overview
  • RNA polymerase initiation involves a two-step process:
  1. Holoenzyme loosely attaches to the promoter sequence, forming a closed promoter complex.
  2. DNA unwinds to form the open promoter complex and begins transcription.
• Holoenzyme binds to the consensus sequences and unwinds DNA around the transcription start site.

Transcription Elongation and Termination

• The holoenzyme synthesizes RNA from the transcription start site (+1) while the sigma subunit initially remains bound.
• Termination Mechanisms:
  • Transcription is terminated by specific sequences that signal RNA polymerase to stop.
  • Intrinsic Termination: Involves a repeat sequence that forms a hairpin structure.
  • Rho-Dependent Termination: Requires a helper protein (rho) to terminate transcription.

Eukaryotic Transcription

• More complex than bacterial transcription, involving multiple RNA polymerases:

  • RNA Polymerase I: Transcribes ribosomal RNA (rRNA).
  • RNA Polymerase II: Transcribes messenger RNA (mRNA) and requires processing, such as intron removal.
  • RNA Polymerase III: Transcribes tRNA and certain rRNAs.

• Transcriptional Regulation in Eukaryotes:

  • More complex than in prokaryotes; involves various regulatory elements and proteins (enhancers and silencers).

• Enhancer Sequences:

  • Enhance transcription levels and can be located at varying distances from the genes they regulate.
  • They bind proteins that communicate with promoter-bound proteins to regulate gene expression.

Chromatin Structure and Regulation

• Chromatin-Based Regulation:
  • Eukaryotes exhibit chromatin modifications that affect transcriptional accessibility.
  • Epigenetic Processes: Describe reversible changes in gene expression without altering the primary DNA sequence.

Conclusion

• Understanding gene expression and the regulatory mechanisms involved is crucial for comprehending cellular function and organismal biology. This regulation varies significantly between prokaryotes and eukaryotes, reflecting the complexity of eukaryotic cells, including the roles of enhancers, silencers, and chromatin structure in gene transcription regulation.