Transcription & Regulation Bio1090

Overview of Transcription and Gene Regulation

  • Focus on transcription (RNA synthesis) and gene regulation mechanisms

  • Genes in DNA; significance of RNA products

  • 22,000 genes in the human genome; not all genes are expressed simultaneously

Central Dogma of Molecular Biology

  • Central Dogma: DNA → RNA → Protein

  • Transcription: Process of synthesizing RNA from DNA template

  • Translation (to be covered later): Converting RNA into protein

DNA and RNA Structure

  • DNA is double-stranded with anti-parallel strands (5' to 3' direction)

  • RNA is single-stranded

  • During transcription, RNA is synthesized from DNA template in the 5' to 3' direction

  • Transcription involves the template strand and non-template strand of DNA

    • Template strand: Read by RNA polymerase

    • Non-template strand: Holds structure but is not transcribed

Transcription Unit Structure

  • Composition of transcription unit:

    • Promoter: Initiation point for RNA polymerase attachment

    • RNA coding sequence: Part of gene leading to RNA

    • Terminator: Signals end of transcription

  • Example in bacteria: DNA strand unwinds, RNA synthesized along DNA

  • Multiple RNA polymerases can transcribe from one gene simultaneously

Transcription Process

  • Transcription initiation: RNA polymerase binds to promoter; starts at transcription start site

  • Transcription elongation: RNA polymerase moves down, synthesizing RNA molecule

  • Transcription termination: Reaches terminator sequence; RNA synthesis stops

Key Terms

  • Upstream: Referring to position before transcription start site

  • Downstream: Referring to position after transcription start site

  • Transcription start site: Point where RNA synthesis begins

  • Negative numbers: Indicate nucleotides upstream of the start site

Complementarity in Transcription

  • RNA synthesized is complementary to template DNA strand

  • Differences:

    • DNA has Thymine (T); RNA has Uracil (U)

    • RNA resembles the non-template strand, but T is replaced by U

RNA Polymerase Function

  • RNA polymerase unwinds DNA strands to access the template strand

  • Synthesized RNA is produced in a 5' to 3' direction; phosphodiester bonds form between nucleotides

  • DNA rewinds behind the polymerase after transcription bubble moves forward

Regulation of Gene Expression

  • Genes need regulation to control when transcription occurs

  • Promoter region: Key for regulating transcription—allows polymerase binding

  • Operator sequences: Control transcription by allowing or blocking polymerase access

Prokaryotic Gene Regulation

  • Operon: Cluster of genes transcribed together—produces multiple proteins from one mRNA

  • Example of negative inducible operon: Lac operon in bacteria (E. coli)

Lac Operon Details

  • LacI: Gene produces repressor that binds to operator, inhibiting transcription in absence of lactose

  • Allolactose: Inducer that binds LacI, preventing it from blocking transcription when lactose is present

  • Upon lactose entering, transcription rates can increase significantly as it activates operon genes for lactose metabolism

Negative Inducible Operon Dynamics

  • Repressor normally inhibits transcription; removal of the repressor allows transcription to proceed

  • Inducer (e.g., allolactose) inactivates the repressor, enabling RNA polymerase to bind the promoter and start transcription

  • Example: When lactose is available, transcription of LacZ, LacY, and LacA structural genes occurs; these proteins enable lactose metabolism

  • When lactose is depleted, the absence of allolactose allows the repressor to attach to the operator, halting transcription

Conclusion

  • Understanding transcription and regulation crucial for biology; lays groundwork for studying protein synthesis in future lectures.