There are 3 types of genetic codon.
1. Sense codon: All the codons that provide the signal for the creation of amino acids are called sense codons. There are 61 sense codons for 20 amino acids.
2. Start codon: All the codons that initiate protein synthesis are called start codons or initial codons. The start codon is AUG. Amino acids that initiate protein synthesis are methionine (true cells) and formyl methionine (primitive cells).
3. Stop codon: Those codons that terminate protein synthesis are called termination or stop codons. The stop codons are UAA, UAG and UGA. They do not provide any signal for the formation of amino acids. Hence they are called non sense codons.