Transcription
High-Yield Summary
- Helicase unwinds the DNA double helix; topoisomerase relieves tension ahead of the fork. RNA polymerase reads the template (antisense) strand 3'→5' and builds RNA 5'→3'; the coding (sense) strand matches the RNA product except T-for-U.
- Transcription starts at the promoter region (often containing a TATA box in eukaryotes), aided by transcription factors that help RNA polymerase bind.
- Three eukaryotic RNA polymerases: Pol I → rRNA (nucleolus); Pol II → mRNA (+ snRNA for splicing) — the one that matters most here; Pol III → tRNA (+ some rRNA).
- Synthesis begins at the plus one site, producing raw hnRNA (heterogeneous nuclear RNA) that must be processed into mature mRNA.
- Three processing steps: splicing (spliceosome removes introns, joins exons), 5' capping (7-methylguanosine — ribosome recognition + degradation protection), 3' polyadenylation (~100-250 adenines — protection + nuclear export).
- Alternative splicing: a single hnRNA transcript spliced different ways yields multiple mature mRNAs (and proteins) from one gene — expands protein diversity without more genes.
Key Terms
- Template (antisense) strand
- DNA strand RNA polymerase reads to build RNA; complementary to the RNA product.
- Coding (sense) strand
- DNA strand not read directly, but matches the RNA product's sequence (T instead of U).
- TATA box
- A/T-rich eukaryotic promoter element that helps position RNA polymerase at the start site.
- hnRNA
- Heterogeneous nuclear RNA — the raw, unprocessed transcript before splicing/capping/polyadenylation.
- Spliceosome
- snRNA + protein complex that removes introns and joins exons during splicing.
- Poly-A tail
- ~100-250 adenines added to the 3' end of pre-mRNA; protects RNA and assists nuclear export.
The Three RNA Polymerases
| Polymerase | Product / Note |
|---|---|
| RNA Polymerase I | rRNA — active in the nucleolus |
| RNA Polymerase II | mRNA — also makes snRNA (assists splicing) |
| RNA Polymerase III | tRNA — also produces some rRNA |
hnRNA → Mature mRNA Processing
- 1Splicing: spliceosome (snRNA + proteins) recognizes intron boundaries, removes introns, joins exons into a continuous coding sequence.
- 25' capping: 7-methylguanosine added to the 5' end — aids ribosome recognition and protects from degradation.
- 33' polyadenylation: ~100–250 adenines added to the 3' end — protects from degradation and assists nuclear export.
- 4Mature mRNA exits the nucleus, ready for translation in the cytoplasm.
Common MCAT Trap
- The coding (sense) strand is NOT read by RNA polymerase — it's just a same-sequence reference (T instead of U). The template (antisense) strand is what's actually transcribed.
- Only RNA Polymerase II makes mRNA — Pol I makes rRNA, Pol III makes tRNA. A question naming a polymerase is testing this table directly.
- Alternative splicing means one gene ≠ one protein — a single hnRNA transcript can produce multiple distinct mature mRNAs/proteins depending on which exons are kept.
Quick Recall
Which DNA strand does RNA polymerase actually read, and in which direction?
What eukaryotic promoter element helps position RNA polymerase at the transcription start site?
What are the three post-transcriptional processing steps that convert hnRNA to mature mRNA?
What complex carries out splicing, and what is it made of?