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Exons and Introns

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A human gene was initially identified as having three exons and two introns. The exons are 456, 224, and 524 bp, while the introns are 2.3 kb and 4.6 kb.
(a) Draw the gene, showing promoter, 5' UTR, 3' UTR, introns, exons.

(b) Surprisingly, it is found that this gene encodes not one but two mRNAs that have only 224 nucleotides in common. The original mRNA is 1,204 nucleotides, while the new mRNA is 2,524 nucleotides. Explain how it is possible for this one region of DNA to encode these two transcripts. How does this change what was believed to be the original order of exons.

Codon table added (see attached file).

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Solution Summary

The solution discusses the exons and introns.

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The first part of the question asks us about the structure of the gene, giving the lengths of 3 exons (presumably in order) and 2 introns (also presumably in order). First, let's discuss what all of these components are.

Exons are the parts which are kept in the mRNA - the pieces of genetic code that are actually used in the translation process to create a functional protein. Introns were previously thought to be "junk DNA", parts which are cut out of the freshly made mRNA which do not contribute to the target protein's translation. It turns out, introns are fairly useful (more on that later).

The promoter region is a region which specifically allows for ...

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