alternative splicing
/ all-TER-nuh-tiv SPLY-sing /
Suppose a single instruction manual could be assembled into a bicycle, a scooter, or a wheelbarrow, just by choosing which pages to keep and which to skip. That is the trick alternative splicing pulls off with genes. From the same primary transcript, a cell can include or skip particular exons, and so produce more than one kind of finished mRNA — and more than one kind of protein — out of a single gene.
It works because splicing is not forced to use every exon every time. The spliceosome can choose to keep an exon in one cell and leave it out in another, or pick between alternative start and end boundaries. Each different combination of kept exons yields a different mRNA, and therefore a related but distinct protein, often called an isoform. The decision is regulated: cells use special proteins that bias the spliceosome one way or another depending on the cell type, developmental stage, or signals it is receiving.
This is a big deal for understanding how complex life is built from a surprisingly small number of genes. Humans have only around twenty thousand protein-coding genes — not many more than a tiny worm — yet we make far more proteins, because the great majority of our multi-exon genes are alternatively spliced. Alternative splicing is one of the main ways a modest genome generates a rich, varied set of proteins, and getting it wrong contributes to numerous diseases.
The same antibody gene gives rise to two versions through alternative splicing: one that anchors the antibody to the surface of an immune cell, and one that floats free to be secreted into the blood. Same gene, two jobs, decided at the splicing step.
Alternative splicing lets one antibody gene make both a membrane-bound and a secreted form.
Alternative splicing is why gene count is a poor measure of an organism's complexity — one gene can yield many proteins. It is mainly a eukaryotic capability and is itself tightly regulated.