genetic code
/ juh-NET-ik KODE /
Think of a recipe written in a four-letter alphabet — A, U, G, C (the bases of RNA) — and a cook who can only read the letters three at a time. The recipe spells out a protein, but proteins are not made of those four letters; they are made of twenty kinds of building block called amino acids. So there has to be a lookup table that says 'this three-letter chunk means add this amino acid.' That lookup table is the genetic code.
Concretely, the genetic code is the set of rules that maps each three-letter group of bases in messenger RNA (a triplet called a codon) to a specific amino acid, or to a 'stop' instruction. With four possible letters in each of three positions there are sixty-four possible codons (4 x 4 x 4), but only twenty amino acids plus a stop signal to assign. So the code is redundant: most amino acids are spelled by more than one codon (for example, both GAA and GAG mean glutamate). The code is also nearly universal — a bacterium, an oak tree, and a human read almost exactly the same table, which is powerful evidence that all life shares a common ancestor.
The genetic code matters because it is the actual translation key between the language of genes and the language of proteins, and it lets us predict the protein that a stretch of DNA will produce. A common misconception is that the code is in DNA letters (A, T, G, C); by convention the code is written in the RNA the cell actually reads, so it uses U (uracil) where DNA has T (thymine). Another subtlety: the code itself is just the rulebook — it does not by itself say where to start reading, which is why the reading frame and the start codon matter so much.
The mRNA fragment AUG-GCC-AAA reads as: start (methionine) - alanine - lysine. Notice that if the cell instead started reading one letter later, it would chop the message into A-UGG-CCA-AA and spell a completely different protein — same letters, wrong frame.
Reading the same letters in groups of three is what turns RNA into a protein recipe.
The code is 'nearly' universal, not perfectly so: a few organisms and our own mitochondria reassign a handful of codons. So 'universal genetic code' is a useful rule of thumb with documented exceptions.