Let us consider that the human $\alpha$-globin polypeptide is 141 amino acids long. How long is the coding portion of the human $\alpha$-globin mRNA? (Consider that methionine from the start position is removed and is therefore not present in the polypeptide chain)

Each amino acid is specified by a codon containing three nucleotides. Therefore, the 141 amino acids in $\alpha$-globin will be specified by 423 (141x3) nucleotides. However, a termination codon must be present at the end of the coding sequence, bringing the length to 423+3 = 426 nucleotides.
In this case of $\alpha$-globin, the aminoterminal methionine (specified by the initiation codon AUG) is removed from the $\alpha$-globin during synthesis. Adding the initiation codon increases the coding sequence of the $\alpha$-globin mRNA to 429 nucleotides (426+3).