From gene to protein.

Step 1 of 6

Genes make proteins

DNA keeps instructions, but the work in a cell is done by proteins: they digest food, carry oxygen in the blood, build hair and muscle, and copy the DNA itself. A gene is a stretch of DNA holding the recipe for one protein.

A protein is a chain of amino acids, of twenty kinds. The order of the letters in the gene sets the order of the amino acids, and that order sets the protein’s shape and what it does. The Scene shows a short, made-up gene. Drag Look inside from Gene to Folded protein, and follow the three steps.

Step 2 of 6

Transcription

The DNA does not leave the cell’s nucleus. A copy goes out instead: a machine called RNA polymerase opens a short stretch of the helix and builds, against one of the strands, the template strand, a single strand of messenger RNA, letter against letter, by the pairing rule. This step is called transcription.

The RNA is the complement of the template strand, so it matches the other strand, the coding strand, with one difference: RNA has no T, and in its place comes U, uracil. That is why a gene is always written on its coding strand. Look inside now stands at the end of transcription: the letters below are the RNA.

Step 3 of 6

Reading in threes

The RNA reaches a ribosome, which reads it in groups of three letters. Each triplet is a codon. The ribosome does not start at the first letter: it slides to the start codon, AUG, and from there reads codon after codon. For each codon a transfer RNA (tRNA) arrives with the matching amino acid, and the ribosome joins it to the chain. This step is called translation.

Step 4 of 6

The chain

The ribosome carries on until it meets a stop codon. There the chain is complete. Look at the beads: each bead is one amino acid.

Step 5 of 6

Where the chain ends

Three codons match no amino acid: UAA, UAG and UGA. These are the stop codons. The letters after them are not translated, just like the letters before the AUG.

Step 6 of 6

The genetic code

The table matching codons to amino acids is the genetic code, and almost every living thing, from bacteria to people, uses the same table. That is one of the strongest signs that all of them share a common ancestor, and it is why a bacterium can make human insulin from a human gene.

The code has more codons than amino acids, so most amino acids have several codons, often differing only in the third letter. Look inside now stands at Folded protein: the chain has folded, and the amino acids that shun water have packed inside.