Why are stop and start codons signals necessary for protein synthesis

The start and stop codons —part of the mRNA's genetic code— are necessary because they indicate the exact length a protein should have.

Explanation:

The sequence of bases contained in an mRNA strand is the genetic code that allows the synthesis of specific proteins.  The mRNA information is organized into triplets or codons, whose function is :

The importance of the start and stop codons is that they establish the exact length that, genetically established, a specific protein must have.

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substitution is the answer in this case