Step by Step
I
Initiation — RNA polymerase binds the promoter
Transcription begins when RNA polymerase (with transcription factors in eukaryotes) binds to the promoter region — a specific DNA sequence upstream of the gene. In prokaryotes, the sigma factor guides RNA polymerase to the promoter. The DNA unwinds locally to expose the template strand.
Key promoter sequences in prokaryotes: TATAAT (–10 box) and TTGACA (–35 box). In eukaryotes: the TATA box (~–25).
E
Elongation — RNA polymerase builds the mRNA
RNA polymerase reads the template strand 3'→5' and synthesizes the mRNA strand 5'→3', adding complementary RNA nucleotides (A, U, G, C). Unlike DNA polymerase, RNA polymerase does NOT need a primer — it can start a new strand from scratch.
Memory trick: RNA polymerase = no primer needed. DNA polymerase = always needs a primer.
T
Termination — transcription stops
In prokaryotes: either a rho-independent terminator (hairpin loop in RNA causes polymerase to stall and dissociate) or rho-dependent (rho protein chases and catches RNA polymerase). In eukaryotes: cleavage and polyadenylation signal (AAUAAA) triggers termination and poly-A tail addition.
EU
Eukaryotic mRNA processing (pre-mRNA → mature mRNA)
In eukaryotes, the initial transcript (pre-mRNA or hnRNA) is processed before leaving the nucleus: (1) 5' cap added (7-methylguanosine — protects mRNA and aids ribosome binding). (2) 3' poly-A tail added (protects from degradation). (3) Splicing: introns (non-coding) removed, exons (coding) joined.
Memory trick: Introns = INtervening sequences (stay IN the nucleus, removed). Exons = EXpressed sequences (EXit and get translated).
Applied Walkthrough
1
Transcription factors bind the TATA box promoter. RNA polymerase II assembles at the transcription start site.
2
RNA polymerase unwinds the DNA and begins synthesizing pre-mRNA 5'→3', reading the template strand 3'→5'.
3
The pre-mRNA is capped at the 5' end, a poly-A tail is added at the 3' end, and introns are spliced out.
4
Mature mRNA exits the nucleus through nuclear pores and is translated by ribosomes in the cytoplasm.
Exam Application
Exams test the three stages of transcription, the difference between prokaryotic and eukaryotic transcription, and mRNA processing steps. Know that RNA polymerase doesn't need a primer, the 5' cap function (ribosome binding + protection), poly-A tail function (stability), and intron vs exon distinction.
⚠ Common Trap
Students forget that eukaryotic transcription and translation are separated in space — transcription in the nucleus, translation in the cytoplasm. In prokaryotes they happen simultaneously (no nucleus). Also: the template strand is read 3'→5', so the mRNA is built 5'→3' — same direction as the coding strand of DNA (just with U instead of T).
✓ Quick Self-Check
1. What are the three stages of transcription?
Initiation (RNA polymerase binds promoter), Elongation (mRNA synthesized 5'→3'), Termination (transcription stops).
Tap to reveal / hide
2. Does RNA polymerase need a primer?
No — unlike DNA polymerase, RNA polymerase can initiate a new strand without a primer.
Tap to reveal / hide
3. What are the three mRNA processing steps in eukaryotes?
5' cap addition, 3' poly-A tail addition, and splicing (introns removed, exons joined).
Tap to reveal / hide
4. What is the function of the 5' cap?
Protects the mRNA from degradation and helps the ribosome recognize and bind the mRNA.
Tap to reveal / hide
5. What is the difference between introns and exons?
Introns are non-coding sequences removed during splicing. Exons are coding sequences that remain and are translated into protein.
Tap to reveal / hide