Sickle cell anemia is caused by a point mutation in the beta-globin chain. Which amino acid substitution occurs at position 6?

Answer and explanation

Correct answer: D. Glutamic acid replaced by Valine

  1. Step 1: Sickle cell anemia is an autosomal recessive hemoglobinopathy caused by a single base-pair substitution ($GAG \to GUG$) in the sixth codon of the -globin gene.
  2. Step 2: This transversion results in the replacement of hydrophilic glutamic acid with hydrophobic valine at position 6 of the adult -globin polypeptide chain.
  3. Step 3: Under deoxygenated conditions, this exposed hydrophobic valine interacts with adjacent hemoglobin molecules, causing insoluble polymerization and erythrocyte sickling.
Answer
Glutamic acid replaced by Valine.
Why not the others: Valine replaced by Glutamic acid reverses the wild-type and mutant residues, describing the reverse mutation; Lysine replaced by Glycine is not the molecular lesion of sickle cell anaemia; Alanine replaced by Aspartate represents an unrelated amino acid substitution.

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