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Eukaryotic Transcription Initiation

• Eukaryotic transcription initiation involves multiple transcription factors.

  • Key factors include:

  • TFIIA

  • TFIIH: Functions as a helicase, aiding in DNA unwinding upstream of the TATA box.

  • Phosphorylation of RNA polymerase (RNA pol):

  • Facilitated by TFIIH acting as a kinase, enabling RNA polymerase to "escape" the promoter region.

  • TFIID:

  • Comprised of TATA binding protein (TBP) that binds to the TATA box.

  • TFIIB:

  • Positions RNA polymerase correctly at the transcri ption start site.

• Overall, RNA polymerase is recruited to the promoter to begin transcription.

Class Activity: Understanding Transcription Organization

• transcript components:

  • 5' UTR: Untranslated region on the 5' end of the mRNA.

  • CDS (Coding Sequence): Contains the coding information that will be translated into proteins.

  • 3' UTR: Untranslated region on the 3' end of the mRNA.

  • Promoter: Region of DNA that initiates transcription.

• Incorrect aspects of a model for a mature eukaryotic transcript:

  1. The promoter region is NOT transcribed; it serves only as a regulatory sequence.

  2. The model mistakenly labels "Introns" instead of "Exons."

  3. Exons must include the 5' UTR in the definition.

Eukaryotic Transcript Structure

• The Coding Sequence (CDS) is represented in orange and includes the information that gets translated into proteins.

• Introns:

  • Can be found within both the 5' and 3' UTRs (untranslated regions) and are removed during primary transcript processing.

• Exons:

  • Defined as regions that are retained in the mature transcript, containing both UTRs and CDS.

Gene Expression Process in Eukaryotes

• Diagram Interpretation:

  1. Exon 1 and Exon 2:

  • Positioned separated by introns and includes 5' UTR and 3' UTR, enhanced by splice sites.

  1. Enhancers/Silencers: Elements that modulate transcriptional activity.

  2. Promoter and Terminator: Essential sequences for gene regulation.

  3. Transcription activities include:

  • Splicing of introns

  • Capping of 5' end

  • Polyadenylation at 3' end.

Bacterial Transcription

• Operons:

  • Defined as transcriptional units that express multiple coding sequences (CDS) or genes from a single promoter.

  • Reason for multiple genes from a single promoter:

  • Provides a coordinated regulation of genes with related functions or pathways.

RNA Processing

• Begins before the transcript is fully synthesized, demonstrating a model that RNA polymerase II is responsible for RNA synthesis.

• Key features of RNA processing:

  • Involves polyadenylation, splicing, and capping to create mature mRNA.

Distinctions Between Eukaryotic and Bacterial Genes

• Introns: Typically absent in bacterial genes, contrasting with eukaryotic genes.

• UTRs and operons: In bacterial diagrams, UTRs are generally not emphasized; bacteria often organize genes as operons for efficient transcription.

Alternative Splicing

• Definition: A process that allows for multiple distinct mRNA transcripts to be produced from a single gene, impacting protein diversity.

• Exon arrangements in splicing:

  • Exon 1 includes the 5' UTR while Exon 4 includes the 3' UTR, integral to the mature transcript's final makeup.

• Question on mature transcripts from pre-mRNA:

  • How many different mature transcripts can be derived from the pre-mRNA?

Example of Alternative Splicing and Gene Diversity

• Dscam gene: Produces an astonishing variety of over 38,016 different mRNAs.

  • Structure includes: 115 exons, with 20 exons invariant across all mature mRNAs.

  • Specifically: 4 exons feature multiple alternative options (calculating combinations: 12 × 48 × 33 × 2 = 38,016 presentations) requiring that only one variant of each alternative exon is included in the final transcript.

References

• All diagrams and figures referenced are sourced from the "Essential Cell Biology" textbook, as well as supporting images from Wojtowicz et al., 2004 Cell.