DNA Transcription Study Notes

Definition: DNA transcription is the process of converting double-stranded DNA into RNA in both eukaryotic and prokaryotic cells.

Transcription Factors and Enzymes: Two important types of proteins that facilitate transcription. - RNA Polymerases: Essential for synthesizing RNA from the DNA template.

Prokaryotic Cells (e.g., Bacteria) - Promoter Region: A specific nucleotide sequence on the DNA where transcription factors and RNA polymerases bind initiating RNA synthesis. - RNA Polymerase Holoenzyme: Consists of a core enzyme and a sigma subunit. - Core Enzyme Components: - 2 Alpha Units - 2 Beta Units (Beta and Beta Prime) - 1 Omega Unit - Function: The core enzyme reads the DNA template and synthesizes RNA. - Sigma Subunit: Binds to the promoter region allowing core enzyme binding and transcription initiation. - RNA Types Made: Prokaryotes can synthesize all types of RNA (mRNA, rRNA, tRNA) from one RNA polymerase.
Eukaryotic Cells - Multiple Promoter Regions: Each type of RNA polymerase responds to specific promoter regions. - RNA Polymerase I: Synthesizes rRNA. - RNA Polymerase II: Synthesizes mRNA; needs general transcription factors. - RNA Polymerase III: Synthesizes tRNA and some small nuclear RNAs. - Difference in complexity: Requires general transcription factors to bind before RNA polymerase can initiate transcription. - Function of General Transcription Factors: Help RNA polymerases to bind at the promoter regions.

Prokaryotes vs. Eukaryotes - Eukaryotic initiation requires both RNA Polymerase II and general transcription factors, notably transcription factor IID (TFIID) that binds to the TATA box promoter. - In prokaryotes, RNA Polymerase binds directly through the sigma subunit.

Direction: RNA polymerases read the DNA template from 3' to 5' and synthesize RNA from 5' to 3'.
Activity of RNA Polymerase: Opens the DNA, stabilizes it, and synthesizes RNA using the template strand (5’ to 3’).

Prokaryotes: Two mechanisms of termination. - Rho-Dependent Termination: The Rho protein disassociates RNA polymerase from the DNA. - Rho-Independent Termination: Formation of a hairpin loop in the RNA transcript signals RNA polymerase to stop transcription.
Eukaryotes: Recognizes a polyadenylation signal (AAUAAA) leading to the termination by cleaving the RNA transcript away from the polymerase.

From hnRNA to mRNA: The initial product is heterogeneous nuclear RNA (hnRNA) undergoing significant modifications to become mature mRNA.

5' Capping
- Process: Cleavage of a phosphate group via RNA triphosphatase, addition of guanylyl transferase to create a guanosine monophosphate (GMP), and methylation of GMP. - Function: Helps in translation initiation and prevents degradation by nucleases.
3' Polyadenylation
- Process: Addition of a poly A tail by poly A polymerase to the 3' end, following recognition of the polyadenylation signal (AAUAAA). - Function: Aids in translation initiation, prevents degradation, and assists in nuclear export.
Splicing
- Splicing Process: Removal of non-coding introns and joining of coding exons through small nuclear ribonucleoproteins (snRNPs). - Significance: Allows for the generation of mature mRNA that codes for proteins.

Definition: A method where one gene can produce multiple mRNAs, leading to different proteins depending on which exons are included or excluded during splicing.
Example: Different antibodies produced by plasma cells derived from the same genetic sequence.

Concept: The process of modifying nucleotide sequences in RNA molecules post-transcription to change their potential output. Example case is in different tissues leading to distinctly sized proteins like apo B-100 versus apo B-48 due to uracil incorporation instead of cytosine.

Key Summary: DNA transcription involves numerous steps and involves specific enzymes and transcription factors that differ between prokaryotic and eukaryotic cells, culminating in RNA synthesis and subsequent processing for functional roles in protein synthesis.