Which of the following mutations, occurring just after the start codon in the mRNA is likely to have the most serious effects on the polypeptide product?

• A. Deletion of one codon.
• B. Deletion of one nucleotide.
• C. Insertion of three nucleotides.
• D. Substitution of one nucleotide.

Answer: (B)

Verified step by step guidance

1. Understand the role of the start codon in mRNA, which is typically AUG, and marks the beginning of translation for protein synthesis.
2. Recognize that mutations occurring just after the start codon can affect the reading frame of the mRNA, which is crucial for correct protein synthesis.
3. Consider the impact of a deletion of one nucleotide: this causes a frameshift mutation, altering the reading frame and potentially changing every subsequent amino acid in the polypeptide chain.
4. Compare this to a deletion of one codon, which removes three nucleotides but maintains the reading frame, resulting in the loss of one amino acid but not affecting the rest of the sequence.
5. Evaluate the effects of insertion of three nucleotides and substitution of one nucleotide, both of which do not cause a frameshift and have less severe impacts compared to a single nucleotide deletion.