Learn & Understand

Cracking the Code: How Three Letters Spell a Protein

A gene written in DNA's four-letter alphabet must somehow specify a protein built from twenty different amino acids. The translation between them — the genetic code — was one of the great puzzles of twentieth-century biology, and cracking it revealed that life reads its instructions three letters at a time.

From Four Letters to Twenty Amino Acids

The puzzle was a mismatch. DNA has four bases, but proteins are built from about twenty amino acids. A single base could specify only four amino acids, and pairs of bases only sixteen — still too few. Only groups of three bases could provide enough combinations to name all twenty amino acids with room to spare. But was the code really read in threes?

The Triplet Code

Experiments confirmed that the genetic code is read in triplets called codons — each three-base group specifying one amino acid. With four possible letters in each of three positions, there are sixty-four possible codons, comfortably more than enough for twenty amino acids. The surplus means several codons can specify the same amino acid, giving the code some redundancy.

Reading the Dictionary

Researchers led by Marshall Nirenberg cracked the code by feeding cells synthetic sequences and seeing which amino acids resulted — building, codon by codon, a complete dictionary translating every triplet into its amino acid. This dictionary is nearly universal across all life, from bacteria to humans, powerful evidence of a shared origin for every living thing.

Start, Stop, and Length

The code includes punctuation. A start codon marks where a protein begins, and stop codons signal where translation ends without coding for any amino acid. This is why a protein's length in amino acids is slightly fewer than the number of codons in its coding sequence — the stop codon takes up a triplet but adds no amino acid to the finished chain.

Calculating Protein Length

To convert a coding sequence length into a protein length, use the Protein Length From Gene Calculator. Analyze the sequence's composition with the GC Content Calculator, and its weight with the DNA Molecular Weight Calculator.