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Biology Unit 4
Topic 6: Infection, Immunity and Forensics
4. Explain how one gene can give rise to more than
one protein through post-transcriptional changes to
It was originally thought that each gene coded for one protein. We now know this is not correct.
Most genes code for many proteins and this is achieved by post-transcriptional changes in the
mRNA. These changes are made to the mRNA before it is used in translation. We now think of the
mRNA made in the nucleus as pre-mRNA.
It contains RNA copied from the entire DNA in the gene, including nonsense sections which
are not used to code for the protein. These areas are known as introns.
The rest of the RNA is a copy of the areas of the DNA which code for the polypeptide chain
these areas are known as exons.
When the mRNA is first transcribed, it is not quite finished. A number of processes take place
before it lines up on the ribosomes, such as capping the ends of the RNA strand, so it is not
attacked by enzymes, and the removal of the introns.
The remaining exons are joined together to form a single long molecule during RNA splicing.
This is carried out by large enzyme complexes known as spliceosomes.
Sometimes some exons are removed as well, so that the code on the final mRNA is clearly
different from the code on the DNA.
As the strands of mRNA transcribed from the same bit of DNA may not be the same after
these processes are complete, they may code for polypeptide chains containing slightly
different amino acids, which in turn produce different proteins.
Text Book: p. 77