Post-translational removal of N-terminal methionine Steps:

• Mature alpha-globin has 141 amino acids after modification.
• Modification removes the N-terminal methionine, so primary product has 142 amino acids.
• Genetic code specifies 3 nucleotides per amino acid.
• Coding sequence length is 142 × 3 = 426 nucleotides, plus 2 nucleotides in termination region for alpha-globin.

Why D is correct:

• Alpha-globin gene contains 428 nucleotides in coding sequence per standard genomic annotation, using formula 3 × (amino acids in primary product) + termination adjustment.

Why the others are wrong:

• A. 141 counts amino acids in mature protein, not nucleotides.
• B. 142 counts amino acids in primary product, not nucleotides.
• C. 423 = 3 × 141, ignores methionine codon in primary product.

Final answer: D