Transcription and mRNA Processing

  • Lecture Overview

    • Topic of Transcription:

    • Understanding genes as units of information.

    • Mechanism of transcription and its regulation.

    • Discussion on mRNA processing integrated with transcription.

  • Central Dogma of Molecular Biology

    • Flow of information: DNA → RNA → Proteins.

    • DNA Replication: copying of DNA.

    • Transcription: interpreting DNA to produce RNA.

    • Translation: RNA translates to protein (developing a phenotype).

    • Coined by Francis Crick.

    • Notable exceptions:

      • Reverse Transcription: RNA → DNA (e.g., in retroviruses like HIV).

      • Non-coding RNAs (e.g., rRNA, tRNA, and microRNAs).

      • Prions: protein-only inheritance, not traditionally aligned with the dogma.

  • Definition of a Gene

    • Gene as a sequence on a chromosome (unit of information).

    • Structure breakdown:

    • Promoter: initiates transcription, upstream of gene body.

    • Coding Sequence: region that codes for proteins, includes exons (coding) and introns (non-coding).

    • Termination Sequences: signals the end of transcription.

  • Transcription Process

    1. Initiation:

      • RNA polymerase binds to the promoter, unwinds DNA.

    2. Elongation:

      • RNA polymerase synthesizes RNA strand (5' to 3' direction), copes from the template strand.

    3. Termination:

      • Signals RNA polymerase to stop transcription and release the pre-mRNA.

        Transcription Overview

  • RNA Polymerase

    • Multi-subunit enzyme responsible for transcription.

    • Different types:

    • Eukaryotic: RNA polymerase I (rRNA), II (mRNA), III (tRNA).

    • Prokaryotic: One type, which performs all functions.

    • Importance: Targeted by various biological drugs due to its essential role.

  • Promoter Characteristics

    • Functions as a landing site for RNA polymerase.

    • Must be upstream of the transcription start site (TSS).

    • Orientation-dependent: sequence and directionality matter.

  • Transcription Factors

    • General Transcription Factors: required by all genes to initiate transcription.

    • Regulatory Transcription Factors: specific to certain genes or cell types, triggered by cellular signals.

  • Enhancers

    • Regulatory sequences promoting transcription, can be located far from the gene.

    • Involved in forming complexes and looping DNA to connect to promoters for transcription initiation.

  • mRNA Processing

    • Occurs simultaneously with transcription.

    • Key modifications:

    • 5' Capping: adds methylguanosine for stability and signaling.

    • Polyadenylation: adds poly-A tail for stabilization and export.

    • Splicing: Removal of introns and joining of exons.

    • Alternative Splicing: Allows production of multiple proteins from one gene by including/excluding specific exons.

  • Eukaryotic vs Prokaryotic Transcription

    • Eukaryotic transcription occurs in the nucleus; prokaryotic in cytoplasm.

    • Eukaryotes: extensive mRNA processing; prokaryotes: no processing.

    • mRNA is translated simultaneously with transcription in prokaryotes.

  • Final Notes

    • Transcription is a multilayered and complex process involving initiation, elongation, and termination.

    • Understanding the specific mechanisms of regulation is essential for detailed molecular biology.

    • Further discussions will continue on eukaryotic gene regulation in upcoming lectures.