⚡ Core Concept: The Flow of Genetic Information: From Gene to Polypeptide
Gene expression converts digital nucleotide sequences into biological function through transcription (promoter recognition, pre-mRNA synthesis by RNA Polymerase II, $5’$ capping, splicing, and polyadenylation) and translation (decoding triplet codons on the 80S ribosome through aminoacylated tRNAs).
1. The Eukaryotic Central Dogma Pipeline
Unlike prokaryotic coupled transcription-translation, eukaryotes spatially segregate transcription within the nucleus and translation in the cytoplasm:
Figure 3: Compartmentalized Central Dogma Processing in Eukaryotic Cells
Gene Expression
Figure 3: The unified gene expression pathway: RNA Polymerase II transcription, nuclear pre-mRNA processing (capping, splicing, polyadenylation), export through NPCs, and ribosomal translation.
2. Transcription: Pre-Initiation Complex (PIC) & mRNA Maturation
Basal transcription by RNA Pol II requires assembly of general transcription factors (GTFs) on core promoters:
- TFIID (TBP + TAFs): TATA-binding protein binds the minor groove of the TATA box, sharply bending DNA by ≈80°.
- TFIIB & TFIIF: Bridge TBP to RNA Pol II and accurately position the transcription start site (+1).
- TFIIH: Contains dual enzymatic activities: helicase (XPB/XPD) that melts promoter DNA into an open transcription bubble, and CDK7 kinase that phosphorylates Ser-5 of the Pol II C-Terminal Domain (CTD), triggering promoter clearance.
Sequential CTD phosphorylation coordinates co-transcriptional processing: Ser-5 phosphorylation recruits the $m^7G$ capping complex; Ser-2 phosphorylation by CDK9/P-TEFb recruits the spliceosome (U1, U2, U4/U6, U5 snRNPs) and cleavage/polyadenylation factors (CPSF/CstF).
3. Ribosomal Translation: The Elongation Cycle
The eukaryotic 80S ribosome (60S large + 40S small subunits) coordinates translation across three distinct tRNA binding sites:
- A (Aminoacyl) Site: Accepts incoming cognate aminoacyl-tRNA escorted by eEF1A·GTP. Codon-anticodon base pairing induces GTP hydrolysis and proofreading.
- P (Peptidyl) Site: Harbors the tRNA bearing the growing nascent polypeptide chain. The 28S rRNA catalyzes peptidyl transfer via RNA-mediated ribozyme catalysis.
- E (Exit) Site: Deacylated tRNA translocates to the E site via eEF2·GTP hydrolysis and dissociates into the cytosol.
Antibiotic & Toxin Translation Inhibitors
Cycloheximide specifically inhibits eukaryotic 60S peptidyl transferase translocation. Puromycin mimics tyrosyl-tRNA, entering the A site to cause premature peptide chain termination. Diphtheria Toxin ADP-ribosylates diphthamide residues in eukaryotic eEF2, completely halting cellular protein synthesis.
