Shuffling exons Alternative gene splicing

DNA sequences are transcribed into RNA sequences. A subset of these RNA sequences, termed messenger RNA (or mRNA for short), are then translated into sequences of amino acids, called proteins.

The most sensible way for an organism to make proteins would be to have just enough DNA to code for the length and type of protein being made. And often that's what we find, especially in things like bacteria. But many times, the sequence of DNA, and thus mRNA, is much longer than necessary to make the desired protein.

In this too-long sequence, some sections of the RNA nucleotides are translated to amino acids (they're called exons), and others aren't (they're called introns). To make the amino acid, the mRNA is "processed" — the introns get spliced out, leaving just the exons all strung together. This new, shorter piece of mRNA codes for the amino acids that make the protein, and everything's hunky-dory.

Yet when scientists look at the details of this process, they find that sometimes there's more that one way to process the same mRNA.

11 Sometimes all the exons get strung together 1 Other times just some of the exons get strung together.

In the second case, when the exons are sewn back together to make the final piece of RNA, some of them get left out, and the result is that different proteins are made from a single gene. This is important, because using different combinations of exons from the same sequence of DNA can result in cells with different functions. This process is called alternative splicing or, more informally, exon shuffling.

In exon shuffling, a gene with four exons, for example, might be spliced differently to create several different types of mRNA. One obvious one would be an mRNA made up of all 4 exons. But mRNAs could also be made from just a subset of the exons — say exons 1, 2, and 4 in one case, and exons 1, 3, and 4 in another. In each of these cases, the protein produced from this mRNA could have a different function. In mammals, for example, the calcitonin gene produces a hormone in one cell type and a neurotransmitter in another cell type, due to alternative splicing.

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Alternative splicing suggests one way that new functions can arise. A mutation that resulted in exons being spliced one way sometime and another way another time would create two protein products from the same DNA. In short, through exon shuffling, it would be possible to gain a new protein while still being able to make the original one. If the new protein were selectively advantageous, then the new mutant would increase in future generations.

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