SCH2226 Human Molecular Genetics - Lecture 7: Gene Expression Transcription

The Central Dogma and RNA Types

• Central Dogma (Francis Crick, 1956): Describes the flow of genetic information: $\text{DNA} \rightarrow \text{RNA} \rightarrow \text{Protein}$.

• Transcription: The synthesis of a single-stranded RNA molecule from a DNA template catalyzed by RNA polymerase.

• Key RNA Molecules:     

• Messenger (mRNA): Carries genetic information for protein synthesis.     

• Transfer (tRNA): Delivers amino acids to the ribosome.     

• Ribosomal (rRNA): Component of the ribosome.     

• Small Nuclear (snRNA): Involved in pre-mRNA processing and splicing.

The Transcription Process Overview

• Three Stages:    

•  1. Initiation: RNA polymerase binds to the promoter and denatures/opens the DNA helix.    

•  2. Elongation: RNA polymerase adds nucleotides in the $5' \rightarrow 3'$ direction.     

• 3. Termination: RNA polymerase recognizes specific termination sequences and dissociates from the DNA.

• Distinction from Replication: Transcription involves only a small portion of the genome, uses a single strand as a template, does not require a primer, and utilizes Uracil ($U$) instead of Thymine ($T$).

Transcription in Prokaryotes ($E. coli$)

• RNA Polymerase Holoenzyme: Composed of the core enzyme and the $\sigma$ (sigma) factor.

• Promoter Regions:     

• $-35$ Region: Consensus sequence $5'\text{-TTGACA-}3'$.     

• $-10$ Region (Pribnow Box): Consensus sequence $5'\text{-TATAAT-}3'$; marks where the DNA untwists (approx. $17\,bp$).

• Sigma Factor ($\sigma$): Critical for promoter recognition. It dissociates from the core enzyme after $8$ to $9$ nucleotides have been polymerized.

Transcription in Eukaryotes

• RNA Polymerases:     

• RNA Polymerase I: Located in the nucleolus; transcribes $28S$, $18S$, and $5.8S$ rRNAs.     

• RNA Polymerase II: Located in the nucleus; transcribes mRNA and some snRNAs.     

• RNA Polymerase III: Located in the nucleus; transcribes tRNA, $5S$ rRNA, and some snRNAs.

• Promoters and Regulation:     

• Core Promoter: Includes the TATA box at $-30$ ($TATAAAA$).     

• Proximal Elements: Located between $-50$ and $-200$.     

• Transcription Factors: Initiation requires the TFIID complex, consisting of the TATA-binding protein (TBP) and TBP-associated factors (TAFs).     

• Enhancers and Silencers: DNA sequences that regulate transcription rates; can be cis-acting (same strand) or trans-acting (different strand).

Post-Transcriptional Processing in Eukaryotes

• $5'$ Capping: Addition of a $7\text{-methylguanosine}$ cap to protect mRNA from exonucleases and assist in translation initiation.

• $3'$ Poly(A) Tail: mRNA is cleaved $10$ to $35$ nucleotides downstream of the $AAUAAA$ recognition site; Poly(A) polymerase adds up to $200$ Adenine residues.

• Intron Splicing:     

• Introns: Non-coding intervening sequences removed in the nucleus.     

• Spliceosome: A complex of five snRNAs ($U1$, $U2$, $U4$, $U5$, and $U6$) and over $60$ proteins (forming snRNPs or "snurps") that facilitate splicing.