In sickle-cell anaemia, which amino acid substitution causes the disease?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Glutamic acid by valine
Sickle-cell anaemia is an autosome-linked recessive disorder controlled by the alleles HbA and HbS. A single base change in the sixth codon of the beta globin gene, GAG to GUG, changes one amino acid. At the sixth position of the beta globin chain, glutamic acid (Glu) is replaced by valine (Val). The changed haemoglobin polymerises when oxygen is low, and the red blood cells turn from biconcave discs into sickle shapes.
NCERT Extract:
Mendelian Disorders – “… Sickle-cell anaemia : This is an autosome linked recessive trait that can be transmitted from parents to the offspring when both the partners are carrier for the gene (or heterozygous). The disease is controlled by a single pair of allele, HbA and HbS. … The defect is caused by the substitution of Glutamic acid (Glu) by Valine (Val) at the sixth position of the beta globin chain of the haemoglobin molecule. The substitution of amino acid in the globin protein results due to the single base substitution at the sixth codon of the beta globin gene from GAG to GUG.”