Q. Since among the 64 codons of mRNA 61 codify amino acids that form polypeptide chains what are the functions of the three remaining codons?

Since there are 20 amino acids and 64 possibilities of mRNA codons, it is expected some amino acids to be codified by more than one codon. And that really happens.

Not all 64 codons however codify amino acids. Three of them, UAA, UGA and UAG, work on information that the last amino acid of a polypeptide chain under productions was already bound, i.e., they signal the end of the polypeptide synthesis. These codons are termed as terminal codons. The codon AUG codifies the amino acid methionine and at the same times it signals the beginning of the synthesis of a polypeptide chain (it is an initialization codon).

In prokaryotic cells there is a sequence called Shine-Dalgarno sequence in general AGGAGG in the position that antecedes the initialization codon AUG. The function of this sequence is distinctness between the initialization AUG and other AUG codons of the RNA.