RNA synthesis, splicing, and post-transcriptional regulation
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RNA
translation
protein
RNA polymerase reads the template 3′→5′, synthesizes mRNA 5′→3′, needs no primer, and lacks proofreading. Eukaryotes use three polymerases: Pol I (rRNA), Pol II (mRNA), Pol III (tRNA and 5S rRNA); prokaryotes use one with a sigma factor.
Eukaryotic mRNA Processing
Three required steps: a 5' 7-methylguanosine cap (protection, ribosome recognition), a 3' poly-A tail added after cleavage at AAUAAA, and spliceosomal removal of introns at conserved GU...AG junctions via a lariat intermediate.
Alternative splicing lets one gene encode many proteins — roughly 20,000 human genes yield well over 100,000 proteins; the calcitonin gene makes calcitonin in thyroid and CGRP in neurons.
Prokaryotes skip processing entirely and couple transcription to translation because there is no nuclear envelope.
Regulation and Drugs
TFIID's TATA-binding protein anchors initiation; enhancers act at a distance through DNA looping; the Mediator complex bridges activators to Pol II. mRNA lifespan is tuned by 3' UTR elements and miRNAs.
Rifampin inhibits bacterial RNA polymerase selectively; alpha-amanitin poisons eukaryotic Pol II.
Key Takeaways
RNA polymerases initiate de novo — the key contrast with DNA polymerases.
Cap, tail, and splicing are mandatory before nuclear export.
Alternative splicing is the main source of proteome diversity.
Know which transcription inhibitor hits which polymerase.