reading frame

A reading frame is how a continuous string of bases gets carved into three-letter words. The same letters can spell different messages depending on where you start grouping them — just as “THEFATCAT” read in threes from the first letter gives THE-FAT-CAT, but shifted by one letter becomes nonsense.

Because the genetic code is read in non-overlapping triplets, the ribosome must lock onto one frame and keep it. That frame is set by the start codon: once translation begins there, every following codon is counted from it, three bases at a time, all the way to a stop codon.

Choosing the wrong frame is disastrous. Inserting or deleting a number of bases not divisible by three shifts the frame from that point onward — a frameshift mutation — so every downstream codon is misread and the protein is usually garbled and prematurely terminated. This is why frameshifts are often far more damaging than a single swapped base.