Green Fluorescent Protein (GFP)

Green Fluorescent Protein (GFP) began as a curiosity of marine biology before becoming one of the most heavily used tools in modern molecular biology. It is a fairly small protein - 238 amino acid residues, about 26.9 kDa - first isolated from the jellyfish Aequorea victoria, and its defining property is that it glows bright green when illuminated with light in the blue-to-ultraviolet range, specifically around 395 nm. What makes GFP so exceptionally useful as a laboratory tool is how it achieves this glow: it forms its own internal light-emitting structure (a chromophore) entirely by itself, using nothing but ordinary molecular oxygen - it needs no external cofactor, no accessory enzyme, and no added substrate the way many other reporter systems do. Because of that self-sufficiency, GFP can simply be fused to, or co-expressed with, a gene of interest, and its resulting fluorescence then works as a direct, visible read-out of whether (and where, and how strongly) that gene is being expressed inside a living cell or organism - which is exactly why it is used so frequently as a reporter of gene expression in cell and molecular biology, and why modified versions of it have also been engineered into biosensors. This was demonstrated concretely by isolating the GFP gene from Aequorea victoria, altering its codons through gene-splicing techniques, and …