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D1.2.16 (HL)—Alternative splicing

Alternative splicing allows different exon combinations from one transcript to generate multiple mature mRNAs and protein variants in eukaryotic cells.

Syllabus
First assessment 2025
Objective
D1.2.16
Level
HL

Exam analysis

Chance of appearing2%of analysed past papers
Latest appearanceNovember 2020
Most common paperPaper1
Typical marks1

Scoring notes

Common mistake
Assuming one gene can produce only one protein product in all circumstances.

Recent exam appearances

November 2020Paper1 ["HL"] · TZ026[ 1 ]D1.2.16 (HL)—Alternative splicing
November 2016Paper1 ["HL"] · TZ028[ 1 ]D1.2.16 (HL)—Alternative splicing
Practice this objective

Coverage 2016–2020 · Updated 16 Jul 2026

Alternative Splicing Produces Multiple Messages

HL only

Alternative splicing joins different combinations of exons from one pre-mRNA, allowing one gene to code for different polypeptides.

After introns are removed, selected exons can be retained or omitted in different mature mRNAs. Each exon combination creates a different codon sequence for translation.

One DNA gene → one pre-mRNA containing exons/introns → introns removed → different exon combinations joined → different mature mRNAs → polypeptide variants.

If mature mRNA A contains exons 1–2–3 and mature mRNA B contains exons 1–3, translation can produce two polypeptides from the same gene.

Alternative splicing changes RNA processing, not the DNA gene. Specific named protein examples or detailed splicing-factor mechanisms are not required.

Alternative splicing

HL only

Assessment in practice

1 marks
How it is assessed

This objective is assessed through multiple choice.

What earns marks

Build the answer around this relationship: Alternative splicing joins different exon combinations.

Watch for

Assuming one gene can produce only one protein product in all circumstances.

Representative question

Question 1

[Maximum number: 1]

The number of protein-coding genes in the human genome is estimated to be about 20000 , which is much less than the size of the proteome. What is one reason for this?

A

Exons are removed from RNA before translation.

B

There are more types of amino acids than nucleotides.

C

mRNA can be spliced after transcription.

D

Base substitutions occur during transcription.

HL Protein Synthesis Details

HL only

RNA polymerase reads template DNA 3' to 5' and synthesizes RNA 5' to 3'; ribosomes translate mRNA codons in the 5' to 3' direction. Promoters mark transcription start regions and orientation; transcription factors help RNA polymerase bind and initiate in eukaryotes. Non-coding DNA does not code for polypeptide amino acid sequences and includes introns, regulatory sequences, telomeres, rRNA genes, and tRNA genes. Eukaryotic pre-mRNA is modified before export and translation by adding a 5' cap and poly-A tail and removing introns by splicing. Alternative splicing joins different exon combinations from one pre-mRNA, so one gene can produce multiple protein variants in different cells or stages. Translation initiation assembles ribosomal subunits at the start codon AUG; initiator tRNA enters the P site and A, P, and E sites organize tRNA movement. Newly made polypeptides may be folded, cleaved, or chemically modified; preproinsulin processing to active insulin is a key example. Proteasomes degrade tagged, damaged, or unneeded proteins; amino acid recycling supports new protein synthesis and proteome quality control.

Concept essentials

  • Alternative splicing joins different exon combinations.
  • One pre-mRNA can produce multiple mature mRNAs.
  • A single gene can contribute to several protein variants.
  • Alternative splicing helps the proteome exceed the number of protein-coding genes.