International A Level Biology / Development of organisms
One gene, more than one protein revision notes
Edexcel International A Level Biology (YBI11), chapter 9: Development of organisms, specification point 3.19. Free lesson.
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A single gene can produce different proteins because its pre-mRNA can be spliced in different ways. This lets cells make different proteins from one gene.
How pre-mRNA becomes active mRNA
In a eukaryotic cell, the whole gene is transcribed first. The first RNA copy is called pre-mRNA. It contains both exons and introns, so it is not ready to be translated. Before it leaves the nucleus, spliceosomes change it to make active, or mature, mRNA. Exons are sections that can remain in the active mRNA. Introns are sections between exons that are removed.
How alternative splicing makes different proteins
Alternative splicing means that exons can be joined in different combinations. All the introns are removed, but some versions of the active mRNA can also have particular exons removed. Different exons mean different codons, which can give a different amino acid sequence and therefore a different protein. In one example, active mRNA A contains exons 1, 2, 3 and 4. Active mRNA B contains exons 1, 2 and 4 because exon 3 is removed. The two active mRNAs come from the same pre-mRNA, and therefore the same gene.
How to calculate amino acid numbers
Each amino acid is coded for by three bases. A stop codon marks the end of translation but does not code for an amino acid, so subtract its three bases before dividing the number of coding bases by three.
How to compare pre-mRNA and active mRNA
A comparison needs both similarities and differences. Both molecules are single-stranded RNA and contain exons. Pre-mRNA also contains introns, while active mRNA does not, so active mRNA is shorter. It may also lack some exons.
How to answer a question about one gene
If a question asks how one gene can produce different proteins, explain splicing, not epigenetics. Epigenetic changes affect which genes are switched on. They do not explain how one gene's pre-mRNA is spliced into different active mRNAs. State that introns are removed and exons are joined. Explain that some exons can also be removed, creating different active mRNAs. Link the different mRNAs to different proteins.