In the DNA, 3 nucleotide pairs sequence corresponds to one amino acid.
The code of 3 bases is called "Codon", and its order determines what kind of amino acid is made.
The 3 nucleotide pairs sequence is also called "Triplet".

DNA is made up of 4 bases, A T C G. Triplet is determined by its kinds and order.
3 bases can make 64 kinds of amino acids. 4×4×4=64
If it was 2 bases, it couldn't make enough kinds of amino acids for keeping the life. 4×4=16

Let's assume that the DNA sequence is "ATGAGACAT".
Then, it is divided according to codon, you'll find "ATG-AGA-CAT".
If ATG corresponds to A amino acid, AGA does to B amino acid, CAT does to C amino acid, the sequence can make the protein which its structure is "A amino acid - B amino acid - C amino acid".

In short, amino acids, are determined by the triplet, are connected in accordance with the codon.

As you can see, in 64 kinds of codon, 61 kinds of it correspond to the amino acid. one sort of amino acid is made from not only one codon.

Especcially, AUG corresponds "methionine" and "start codon", is the marker of starting the amino acid code.
Also, other 3 kinds of codon correspond "stop codon", is the end of the code, as if it was a piriod.

In other words, gene is a sentence written by the words of codon.

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