What specific molecular point mutation in the beta-globin gene causes normal hemoglobin (HbA) to be converted into sickle cell hemoglobin (HbS)?
Answer and explanation
Correct answer: A. substitution of valine for glutamic acid at codon position 6 of the beta-polypeptide chain
- Step 1: In sickle cell mutation, a single base substitution (GAG -> GTG) occurs in the sixth codon of the beta-globin gene. Step 2. This replaces a hydrophilic, negatively charged glutamic acid residue with a hydrophobic, neutral valine residue. Step 3. In deoxygenated conditions, hydrophobic interactions cause HbS molecules to polymerize into rigid sickle fibers. Answer: substitution of valine for glutamic acid at codon position 6 of the beta-polypeptide chain. Why not the others: Phenylalanine deletion describes cystic fibrosis; stop codon causes thalassemia.
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