01 / 24

mRNA Processing
Capping, Splicing & Polyadenylation

Explore the three critical modifications that transform pre-mRNA into a mature transcript ready for translation in eukaryotic cells.

Biochemistry

Why Does mRNA Need Processing?

In eukaryotes, the initial transcript from RNA polymerase II is called pre-mRNA. It is not ready for translation. Before it can leave the nucleus, three modifications must occur:

5' Capping
A modified guanine nucleotide is added to the 5' end to protect the transcript.
Splicing
Non-coding introns are removed and exons are joined together.
3' Polyadenylation
A poly-A tail of ~200 adenines is added to the 3' end.

From Pre-mRNA to Mature mRNA

Pre-mRNA Exon 1 Intron 1 Exon 2 Intron 2 Exon 3 Processing Mature mRNA Cap Exon 1 Exon 2 Exon 3 AAAAAA 5' Cap Poly-A Tail

Why Processing is Essential

Without proper processing, mRNA would be degraded in the cytoplasm, mistranslated, or unable to exit the nucleus. Each modification serves a distinct protective and functional role:

1
Stability: The 5' cap and poly-A tail shield mRNA from exonuclease degradation.
2
Nuclear export: Properly processed mRNA is recognized by export factors that transport it through nuclear pores.
3
Translation initiation: Ribosomes recognize the 5' cap to begin translating the coding sequence.
4
Protein diversity: Alternative splicing allows one gene to produce multiple protein isoforms.

The 5' Cap Structure

7-methyl Guanosine 5'-5' triphosphate Nt 1 mRNA coding sequence →
\(\text{CH}_3\)
 Cap 0 (minimal cap) Cap 1 (2'-O-methyl) 5' 3'

Capping: Enzymatic Steps

1
RNA triphosphatase removes the terminal phosphate from the 5' end of the pre-mRNA, leaving a 5'-diphosphate.
2
Guanylyltransferase adds a GMP in a 5'-to-5' linkage, creating the distinctive inverted cap structure.
3
Methyltransferase adds a methyl group to the N-7 position of the guanine, yielding the m7G cap.

RNA Splicing: Removing Introns

Pre-mRNA with Intron Exon 1 GU Intron A Branch Point AG Exon 2 Step 1 Lariat Intermediate Exon 1 A Lariat Exon 2 Step 2 Spliced mRNA Exon 1 Exon 2 Degraded

The Spliceosome Machinery

Splicing is catalyzed by the spliceosome, a large ribonucleoprotein complex composed of five small nuclear RNAs (snRNAs: U1, U2, U4, U5, U6) and associated proteins.

U1 snRNP
Recognizes the 5' splice site (GU) through base pairing.
U2 snRNP
Binds the branch point sequence, bulging out the adenosine.
U4/U6 + U5
Join to form the catalytic core that executes the two transesterification reactions.

Splice Site Consensus Sequences

Exon GU 5' splice site A Branch point Py tract AG 3' splice site Exon 5' ────────────────────────────→ 3'

Alternative Splicing: One Gene, Many Proteins

Pre-mRNA Exon 1 Exon 2 Exon 3 Exon 4 Isoform A 1 2 3 4 Isoform B 1 3 4 Isoform C 1 2 4 Exon Skipping: most common form of alternative splicing

3' Polyadenylation: Adding the Tail

AAUAAA Poly-A Signal Cleavage site (10-30 nt downstream) GU-rich Cleavage + PAP AAUAAA AAAAAAA...AAA ~200 adenines 5' 3'

Exons vs. Introns: The Key Distinction

Feature Exons Introns
Meaning Expressed sequences Intervening sequences
In Mature mRNA? ✓ Yes ✕ No
Coding Potential Contain protein-coding regions + UTRs Non-coding (usually)
Boundary Signals Flanked by splice sites Begin with GU, end with AG
Fate Joined together in mRNA Excised as lariat, degraded
Average Count per Gene ~8 per human gene ~7 per human gene

Step-by-Step: Tracing mRNA Processing

1
Transcription begins: RNA Pol II starts synthesizing pre-mRNA from the DNA template.
2
5' Capping (co-transcriptional): After ~20-30 nucleotides, capping enzymes add the m7G cap via a 5'-5' triphosphate bond.
3
Splicing (co/post-transcriptional): The spliceosome recognizes GU...AG boundaries, excises introns as lariats, and joins exons.
4
3' Cleavage + Polyadenylation: CPSF recognizes AAUAAA, cleaves the pre-mRNA, and PAP adds ~200 A residues.
5
Export: The mature mRNA (capped, spliced, polyadenylated) is exported through nuclear pore complexes to the cytoplasm for translation.

Example 1: Predicting mRNA Length

A gene contains 3 exons (200, 350, and 150 nucleotides) and 2 introns (1,200 and 800 nucleotides). What is the length of the pre-mRNA and the mature mRNA?

200 1,200 350 800 150 = 2,700 nt Pre-mRNA: 200 350 150 = 700 nt (coding) + cap + ~200 A tail Mature mRNA:

Example 2: Alternative Splicing Outcomes

A gene has 4 exons. Exon 2 can be included or skipped, and Exon 3 can be included or skipped independently. How many distinct mRNA isoforms are possible?

1
Exons 1 and 4 are always included (constitutive exons).
2
Exon 2: 2 choices (included or skipped).
3
Exon 3: 2 choices (included or skipped).
4
Total isoforms: \( 2 \times 2 = 4 \) distinct mRNAs: (1-2-3-4), (1-3-4), (1-2-4), (1-4).

Example 3: Mutation Effects on Splicing

A point mutation changes the 5' splice site of intron 2 from GU to AU. What happens to the mRNA?

Scenario A
Intron retention: The spliceosome fails to recognize the mutated site. Intron 2 remains in the mature mRNA, likely introducing a premature stop codon.
Scenario B
Cryptic splice site: A nearby GT sequence within the exon or intron is used instead, producing an mRNA with partial intron inclusion or exon truncation.

Common Mistakes to Avoid

✕ Confusing Where Processing Occurs
All three processing steps happen in the nucleus, not the cytoplasm. The mRNA must be fully processed before export.
✕ Thinking All Introns Are "Junk"
Introns contain regulatory elements (enhancers, silencers) and some encode functional non-coding RNAs like snoRNAs.
✕ Forgetting Poly-A Is Template-Free
PAP adds adenines without a DNA template. Students often confuse this with normal transcription.

Processing Summary

Feature 5' Capping Splicing 3' Polyadenylation
When Co-transcriptional Co/Post-transcriptional Post-transcriptional
What m7G added to 5' end Introns removed ~200 A's added to 3' end
Key Signal CTD Ser-5 phosphorylation GU...AG at intron borders AAUAAA hexamer
Machinery Capping enzymes Spliceosome (snRNPs) CPSF + PAP
Function Stability, translation init. Remove non-coding regions Stability, export, translation

Key Takeaways

1
Eukaryotic pre-mRNA undergoes three modifications: 5' capping, splicing, and 3' polyadenylation before export from the nucleus.
2
The 5' cap (m7G) protects against degradation and recruits translation factors.
3
Splicing removes introns via two transesterification reactions catalyzed by the spliceosome (a ribozyme).
4
Polyadenylation adds ~200 A residues template-independently after the AAUAAA signal.
5
Alternative splicing produces multiple protein isoforms from a single gene, vastly expanding the proteome.

Question 1 of 5

What type of bond connects the 5' cap to the first nucleotide of the mRNA?

Question 2 of 5

The consensus sequence AAUAAA in pre-mRNA signals which process?

Question 3 of 5

Which snRNP recognizes the 5' splice site (GU) at the beginning of an intron?

Question 4 of 5

A gene with 5 exons produces a pre-mRNA of 8,000 nucleotides. The exons total 1,500 nucleotides. Approximately what percentage of the pre-mRNA consists of intronic sequence?

Question 5 of 5

Why is the spliceosome considered a ribozyme?