STEPTRACKERBIOCHEMISTRY + IMMUNOLOGY | MOLECULAR BIOLOGY | HIGH-YIELD REVIEW

RNA Processing & Splicing

Pre-mRNA becomes mature mRNA in the nucleus — capped, tailed and spliced — and small RNAs later tune how much protein it makes.

FOCUS ON
Three steps in the nucleus: 5′ 7-methylguanosine cap, 3′ poly-A tail (signal AAUAAA), splicing out introns by the spliceosome (snRNPs). Anti-Smith antibodies (SLE) target snRNPs; anti-U1 RNP = mixed connective tissue disease. Alternative splicing makes several proteins from one gene. miRNA silences mRNA after transcription.
STEP · RNA
WHAT HAPPENS
WHY IT MATTERS
CLUE · DISEASE
5′ capping
7-methylguanosine added by a 5′–5′ triphosphate bond
Protects from degradation; ribosome recognition
Happens first, co-transcriptionally
3′ polyadenylation
Poly-A polymerase adds ~200 A after the AAUAAA signal (no template)
Stability, nuclear export
Histone mRNAs lack poly-A
Splicing
snRNPs + proteins form the spliceosome; introns removed as lariats (GU…AG rule)
Joins exons into a continuous coding sequence
Splice-site mutations: β-thalassemia, some Gaucher
Alternative splicing
Different exon combinations from one pre-mRNA
One gene → several proteins
Membrane vs secreted antibody; calcitonin vs CGRP
snRNPs
Small nuclear RNA + proteins (U1, U2, U4, U5, U6)
Core of the spliceosome
Anti-Smith (SLE, very specific) · anti-U1 RNP (MCTD)
mRNA export & P-bodies
Mature mRNA leaves via nuclear pores; cytoplasmic processing bodies store and degrade mRNA
Regulates mRNA lifespan
Contain decapping enzymes, exonucleases, miRNA machinery
microRNA (miRNA)
Small noncoding RNA in RISC; base-pairs with the 3′ UTR of target mRNA
↓ translation or degradation of the target
Loss of a tumor-suppressor miRNA → ↑ oncogene protein
siRNA / RNA interference
Double-stranded RNA designed against one mRNA → cleaved
Gene knockdown in research and therapy
Patisiran (hereditary TTR amyloidosis)
Antisense oligonucleotides
Single-stranded oligos alter splicing or block translation
Restore or silence protein
Nusinersen (SMA: SMN2 exon inclusion) · eteplirsen (DMD exon skipping)
RNA types
rRNA (most abundant) · mRNA (longest) · tRNA (smallest)
—
Pol I → rRNA, pol II → mRNA, pol III → tRNA

Mechanism — pre-mRNA to mature mRNA

Pre-mRNAExons + introns
→
5′ cap7-methyl-G
→
Poly-A tailAfter AAUAAA
→
SpliceosomeIntrons out as lariats
→
Mature mRNAExport to cytoplasm

All three steps happen in the nucleus — bacteria have none of them (coupled transcription–translation).

Antibodies to the splicing machinery

ANTIBODY
TARGET
DISEASE
Anti-Smith
snRNP core proteins
SLE (specific)
Anti-U1 RNP
U1 snRNP
Mixed connective tissue disease
Anti-dsDNA
DNA (contrast)
SLE, tracks nephritis

How it's tested

COMMON CONFUSIONmiRNA occurs naturally and fine-tunes many genes; siRNA is designed to cut one specific mRNA.
THE TRAPBacterial (or cloned) genes lack introns — so a eukaryotic gene expressed in bacteria needs cDNA, made by reverse transcriptase from mature mRNA.
YOUR APPROACHLocate the step (nucleus vs cytoplasm) → name the RNA or particle → link to antibodies, splice mutations or RNA drugs.

If you see… think…

Very specific antibody for SLE against splicing proteins → anti-Smith
Overlapping SLE, scleroderma, myositis + anti-U1 RNP → MCTD
Intron removed as a loop → lariat (spliceosome)
Signal for poly-A tail → AAUAAA
One gene making secreted vs membrane IgM → alternative splicing
Small RNA binding 3′ UTR to lower translation → microRNA
SMA treated by changing SMN2 splicing → nusinersen
siRNA drug for TTR amyloidosis → patisiran
Most abundant RNA in a cell → rRNA
Splice-site mutation causing anemia → β-thalassemia