The molecule that comes off a eukaryotic gene is not ready to be translated. It is fragile, it still has non-coding sections in the middle of it, and it has no protection at either end. Before it is allowed out of the nucleus, the cell edits it — and that editing is where one gene learns to make several different proteins.
📘 What you need to know
The immediate product of transcription in a eukaryote is pre-mRNA. It must be modified to become mature mRNA.
Three events occur: a methylated cap is added to the 5′ end, a poly-A tail to the 3′ end, and introns are removed by splicing.
The cap protects the molecule from exonucleases; the tail gives further protection and helps the transcript leave the nucleus.
Exons are the coding sections that are kept; introns are the non-coding sections that are cut out.
The whole gene is transcribed first — introns and all — and only then edited.
Prokaryotes do not do significant modification. With no nucleus, translation can begin straight away.
Alternative splicing lets one gene produce several different mature mRNAs, and so several different polypeptides. This is one reason the proteome is bigger than the genome.
Why the transcript needs work first
mRNA is single-stranded, which makes it much less stable than double-stranded DNA. It is also surrounded by enzymes called exonucleases that chew molecules from their ends. Left alone, a fresh transcript would not last long.
Eukaryotes have a convenient advantage here: transcription happens in the nucleus and translation happens in the cytoplasm, so there is a gap between the two in which the transcript can be repaired, protected and edited. Prokaryotes have no nucleus, so ribosomes can begin translating an mRNA while it is still being made. Speed protects the molecule instead.
Two compartments, one opportunity. The separation of transcription and translation in eukaryotes is exactly what makes post-transcriptional modification possible. That is a great one-line answer to “why do eukaryotes modify mRNA but prokaryotes do not?”
The three modifications
🧩 Turning pre-mRNA into mature mRNA
Add a methylated cap to the 5′ end. This protects the transcript from being broken down by exonucleases.
Add a poly-A tail to the 3′ end — a long run of adenine nucleotides. It gives further protection and helps the molecule get out of the nucleus.
Splice out the introns. The non-coding sections are cut out and the exons are joined together into one continuous coding sequence.
Together these three changes prevent degradation, make protein synthesis more efficient, and in eukaryotes increase the range of proteins a genome can produce.
Remember the shortcut: EXons are EXpressed. Introns are the ones that stay in the nucleus.
Alternative splicing: one gene, several proteins
Here is the clever part. The exons of a gene do not have to be joined in only one way. A particular exon may be included in the mature mRNA, or left out of it. Different combinations produce different mature mRNAs from exactly the same pre-mRNA.
Those different mRNAs are then translated into polypeptides with different amino acid sequences, which means different structures and different functions. So a single eukaryotic gene can code for several distinct proteins.
Same gene, same pre-mRNA, two different finished proteins — simply because exon 2 was kept in one version and exon 3 in the other.
Think of the exons as verses of a song. The band always records all of them, then the producer decides which verses go on the single. Two different edits, two different tracks, one recording session.
Genome, proteome, and why they are not the same size
Your genome is the full set of genes you carry. Your proteome is the full set of proteins that can be made. The proteome is far larger, and alternative splicing is one of the main reasons why. A single gene with several optional exons can supply a whole family of related proteins, each suited to a different tissue or a different job.
The relationship
one gene → several mature mRNAs → several polypeptides
Worked examples
WORKED EXAMPLE
Counting the possibilities
A gene has six exons. Three of them are always included, and the other three may each be either included or left out during splicing. How many different mature mRNAs could be produced?
Step 1: Identify the choices
Only the three optional exons matter. Each has 2 possibilities: in or out.
Step 2: Combine the choices2 × 2 × 2 = 23 = 88 different mature mRNAsOne gene, eight possible polypeptides — exactly why the proteome outgrows the genome.
WORKED EXAMPLE
How much gets cut out?
A pre-mRNA molecule is 6000 bases long. After splicing, the mature mRNA is 1200 bases long. Calculate the length of RNA removed and express it as a percentage of the original.
Step 1: Length removed6000 − 1200 = 4800 basesStep 2: As a percentage of the pre-mRNA(4800 ÷ 6000) × 100 = 80%4800 bases removed, which is 80% of the transcriptIn many human genes the introns really are much longer than the exons.
WORKED EXAMPLE
Comparing prokaryotes and eukaryotes
Explain why prokaryotic mRNA needs little or no post-transcriptional modification.
Step 1: Identify the structural difference
Prokaryotes have no nucleus, so transcription and translation happen in the same compartment.
Step 2: Work out the consequence
Ribosomes can start translating the mRNA immediately, often before transcription has even finished.
Step 3: Link it back to the purpose of modification
The mRNA is used before it can be degraded, so the protective cap and tail are not needed.
No nucleus, so translation is immediate and the transcript needs no protectingProkaryotic genes also contain few or no introns, so there is little to splice out.
💡 Exam tip
Learn the three modifications as a set: cap at the 5′ end, poly-A tail at the 3′ end, introns removed. Questions often ask for all three.
Get the ends the right way round. Cap 5′, tail 3′. Mixing them up costs the mark.
Use the exact words pre-mRNA and mature mRNA — saying just “mRNA” can make an answer ambiguous.
Remember the whole gene is transcribed first, introns included. Editing comes afterwards.
For alternative splicing, always finish the chain: different mRNA → different amino acid sequence → different structure → different function.
If a question mentions the proteome being larger than the genome, alternative splicing is almost certainly the answer they want.
⚠ Common mix-up
Swapping exons and introns. EXons are EXpressed. Introns are removed.
Thinking introns are skipped during transcription. They are transcribed, then cut out of the pre-mRNA afterwards.
Saying splicing happens in the cytoplasm. It happens in the nucleus, before the transcript leaves.
Describing the poly-A tail as “a protein”. It is a long run of adenine nucleotides.
Claiming prokaryotes have no mRNA. They do — it just does not need much modification.
Confusing alternative splicing with mutation. Splicing changes nothing in the DNA; it only changes which parts of the transcript are kept.
Up next: Translation & the Proteome — the mature mRNA finally reaches a ribosome. We look at translation in HL detail, then at what happens to a polypeptide after it is released, and how the cell recycles the ones it no longer needs.
Want this explained one-to-one?
Book a free session with an experienced IB Biology tutor and get your trickiest topics made simple.
