Chapter 12: Gene Transcription and RNA Modification

Chapter 12: Gene Transcription and RNA Modification

Learning Objectives

  • Central Dogma

    • Understand the flow of genetic information: DNA → RNA → Protein

  • Parts of a Gene

    • Identify and differentiate components: promoter, enhancers, silencers, termination sequences.
    • Prokaryotic vs Eukaryotic gene structure.

  • Strand Functions

    • Define template (3' to 5') vs coding (5' to 3') strands.

  • Prokaryotic Transcription

    • Detail initiation, elongation, termination processes, and associated enzymes.

  • Eukaryotic Transcription

    • Examine initiation, elongation, and termination stages with enzyme roles.

  • Terminology

    • Utilize specific terms: cis, trans, transcriptional start site, 5'UTR, 3'UTR, consensus sequence, holoenzyme, sigma factor, mediator, etc.

  • Splicing Comparison

    • Analyze group I, II, spliceosome processes, and benefits of alternative splicing.

  • RNA Processing in Eukaryotes

    • Outline processing of eukaryotic structural gene transcripts before cytoplasmic translation.
Transcription Overview
  • Molecular Structure of Genes
    • Genes are segments of DNA coding for functional products (RNA/polypeptides).
    • Transcription: Copying DNA sequence into RNA without altering original DNA structure.
Important Concepts of Transcription
  • Gene Regulatory Elements
    • DNA sequences determine gene start/end and RNA synthesis levels.
  • Gene Expression
    • Process of converting genetic information to functional products affecting traits.
  • RNA Transcripts' Roles
    • Serve as mRNA for protein synthesis or functional RNAs without translation (e.g., ribosomal RNA, tRNA).
Transcription in Prokaryotes
  1. Initiation
    • Promoters signal RNA polymerase binding.
    • Open complex formation at the TATAAT box (Pribnow box), characterized by weak A-T bonds.
  2. Elongation
    • RNA polymerase synthesizes RNA in the 5' to 3' direction using the template strand (3' to 5').
    • DNA rewinds behind the open complex.
  3. Termination
    • Two mechanisms: rho-dependent (requires rho protein) and rho-independent (stem-loop structure + uracil-rich sequence).
Eukaryotic Transcription
  • More complex than in prokaryotes:
    • Three RNA polymerases (I, II, III) for different RNA types (rRNA, mRNA, tRNA).
    • Core promoters with TATA box and regulatory elements.
  • Basal transcription machinery consists of RNA polymerase II, general transcription factors (GTFs), and mediators.
Eukaryotic RNA Polymerases and Their Functions
  • RNA Pol I: Transcribes all rRNA (except 5S).
  • RNA Pol II: Transcribes protein-coding genes and some snRNA.
  • RNA Pol III: Transcribes tRNA, 5S rRNA, and microRNA genes.
Eukaryotic Promoters
  • Core promoter typically contains TATA box (-25 region) and transcription start site (+1).
  • Regulatory elements (enhancers and silencers) affect transcription efficiency and rate.
RNA Processing in Eukaryotes
  • Capping
    • 7-methylguanosine cap added to 5' end during transcription for stability, recognition.
  • Polyadenylation
    • Addition of a poly-A tail at the 3' end for stability and transport out of the nucleus.
  • Splicing
    • Introns removed, exons joined by spliceosomes or through self-splicing mechanisms in group I and II.
  • Alternative Splicing
    • Different mRNA variants from the same gene, increasing proteomic diversity and allowing fewer genes in the genome.
Conclusion
  • Understand transcription processes and RNA modifications, focusing on distinctions between prokaryotes and eukaryotes, and their implications in gene expression and functional diversity.
Key Terms
  • Cis-acting elements: Regulatory DNA sequences affecting the same gene.
  • Trans-acting elements: Regulatory proteins binding to cis-elements.
  • Polycistronic mRNA: Found in prokaryotes, encodes multiple proteins.
  • Monocistronic mRNA: Found in eukaryotes, typically encodes a single protein.
  • Ribozyme: RNA with catalytic activity, assists in splicing processes.