Non-coding DNA, mRNA processing and alternative splicing

Molecules (Continuity and change) · Protein synthesis · note 6 of 8

Spec D1.2.14, D1.2.15, D1.2.16

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Non-coding DNA, mRNA processing and alternative splicingSpec D1.2.14, D1.2.15, D1.2.16

In short

Non-coding sequences are DNA that does not code for polypeptides, such as regulators of gene expression, introns, telomeres and genes for rRNA and tRNA. In eukaryotes, introns are removed from pre-mRNA, exons are spliced together, and a 5' cap and 3' polyA tail stabilise it. Alternative splicing joins different exons, giving several polypeptides from one gene.

Non-coding sequences

In eukaryotes most of the DNA does not code for polypeptides. Non-coding sequences include:

Regulators of gene expression, such as promoters, where proteins bind to control whether a gene is transcribed.
Introns: sequences within a gene that are transcribed but removed from the mRNA before translation.
Telomeres: repetitive sequences at the ends of chromosomes that protect the coding DNA from being lost during replication.
Genes for rRNA and tRNA: these are transcribed, but the RNA is used directly in ribosomes and in carrying amino acids, so it is never translated into a polypeptide.

Post-transcriptional modification

In eukaryotic cells the RNA made by transcription is pre-mRNA. It is modified in the nucleus to form mature mRNA:

A 5' cap (a modified guanine nucleotide) is added to the 5' end.
A 3' polyA tail (a long chain of adenine nucleotides) is added to the 3' end.
Introns are removed and the exons are spliced together to give a continuous coding sequence.

The cap and tail stabilise the mRNA transcript, protecting it from breakdown in the cytoplasm. Prokaryotes do not do this: their genes generally have no introns and mRNA can be translated while it is still being transcribed.

Alternative splicing

Alternative splicing is the splicing together of different combinations of exons from the same pre-mRNA. Some exons may be left out in one cell type but included in another. This allows one gene to code for different polypeptides, so a genome can produce many more proteins than it has genes.

Pre-mRNA with a 5′ cap, exons E1 to E4 separated by grey introns, and a 3′ polyA tail; splicing removes the introns to give mature mRNA E1-E2-E3-E4, while alternative splicing also removes E2 to give E1-E3-E4, and the two mature mRNAs are translated into two different polypeptides. — Alternative splicing joins different combinations of exons, so one gene can code for different polypeptides.

Exam tip:

Linking question: how does the diversity of proteins produced contribute to the functioning of a cell? Alternative splicing lets different cells make different variants of a protein from the same gene.

Written and checked against the IB Biology HL specification · Updated October 2026

Frequently asked questions

What is the difference between transcription and translation?

Transcription is the synthesis of RNA using a DNA template, carried out by RNA polymerase in the nucleus of eukaryotes. Translation is the synthesis of a polypeptide from mRNA at a ribosome, where tRNA anticodons pair with mRNA codons and amino acids are joined by peptide bonds. Transcription copies the sequence; translation decodes it.

Why is the genetic code a triplet code?

The genetic code is a triplet code because there are four bases and twenty amino acids. Pairs of bases would give only 4² = 16 combinations, too few for twenty amino acids. Triplets give 4³ = 64 codons, enough for every amino acid plus start and stop signals, which is why the code is also degenerate.

How does sickle-cell anaemia change the haemoglobin protein?

Sickle-cell anaemia is caused by a base substitution in the haemoglobin beta-chain gene. The mRNA codon changes from GAG to GUG, so valine replaces glutamic acid at the sixth amino acid. This hydrophobic valine makes haemoglobin S molecules stick together into fibres at low oxygen, distorting red blood cells into sickle shapes.

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