Start and stop codons
All mRNA strands begin with start codons and end with stop codons.
The first codon of an mRNA strand is always the start codon.
The start codon is usually AUG and it codes for the amino acid methionine. In eukaryotes, methionine is always the first amino acid in the polypeptide chain.
Out of the 64 codons, three do not code for any amino acids. These are the stop codons. The stop codons signal the end of the mRNA strand, where the translation into protein ends.
UAG, UAA and UGA are the stop codons.
Start and stop codons are found in the DNA strands from which mRNA is transcribed. They are found at the beginning and end of the templates from which mRNA is transcribed. In DNA, thymine is used instead of uracil.