Lesson 5 of 6 / DNA replication and gene expression
Process a eukaryotic RNA message
Why can one gene produce more than one polypeptide?
In this lesson: Explain capping, polyadenylation and splicing of pre-mRNA.
About 6 min
The key ideaEukaryotic pre-mRNA is capped, polyadenylated and spliced; alternative exon combinations can produce different mature messages.
Explore the idea
One DNA sequence, two mature messages
Tissue A retains exon 2 in this permitted splice pattern. The underlying DNA and exon order are unchanged. If the different exon contains coding sequence, the resulting polypeptide can differ.
The cap and poly-A tail help protect and use RNA. The tail is added after cleavage rather than copied from the displayed sequence. Exons can also contain untranslated regions. Splicing is regulated; this example does not permit arbitrary exon rearrangement or guarantee that every combination preserves a reading frame.
Explanation
A newly transcribed eukaryotic pre-mRNA contains exons and introns. Splicing removes introns and joins exons into a continuous mature transcript. Exons are the regions retained in mature RNA; some exon sequence can be untranslated, so exon does not mean every base codes for an amino acid.
A modified nucleotide cap is added at the 5-prime end, and a poly-A tail is added to the 3-prime end after cleavage. These modifications help protect the RNA, support export and contribute to recognition for translation. The poly-A tail is not simply copied from a long matching run of template DNA.
Alternative splicing can join different permitted exon combinations, producing related mRNAs and polypeptides from one gene. This changes the RNA product without changing the underlying DNA sequence. It is regulated, not arbitrary rearrangement of every exon into any order.
Mature mRNA must still contain the appropriate signals and reading frame for its product. Translation does not normally read through removed introns. Distinguish RNA processing from gene mutation: both can alter a product, but only one necessarily changes DNA sequence.
Step by step
- 1
Mark retained regions
Identify exons before deleting introns.
- 2
Add end modifications
Place cap and tail on the correct ends.
- 3
Compare products
A different allowed exon combination can change the polypeptide.
Worked example
Work through the evidence
Pre-mRNA has exons 1, 2, 3 and 4 separated by introns. One tissue retains 1-2-4, another 1-3-4. What explains this?
One way to explain it
Regulated alternative splicing produces different mature mRNAs from the same pre-mRNA sequence, potentially giving different polypeptides.
Why this answer works
- The genomic exon order need not change.
- Introns are removed in each mature transcript.
Is this true? "Splicing removes unwanted genes from DNA."
Splicing acts on RNA, removing introns from a transcript while the DNA remains.