Sickle-cell anaemia is caused by the substitution of glutamic acid by valine. Which one of the following triplets codes for valine?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → GUG
In sickle-cell anaemia a single base is changed in the sixth codon of the β globin gene. The mRNA codon changes from GAG, which codes for glutamic acid, to GUG, which codes for valine. So the sixth amino acid of the β chain becomes valine instead of glutamic acid.
Option (1) GAG — Incorrect. GAG is the normal codon at this position. It codes for glutamic acid, the amino acid that is replaced in the disease.
Option (2) AUG — Incorrect. AUG is the initiation codon. It codes for methionine, not valine.
Option (3) GUG — Correct. GUG is the codon formed after the single base substitution (GAG to GUG). It codes for valine, which takes the place of glutamic acid.
Option (4) UGC — Incorrect. UGC is not the codon formed in sickle-cell anaemia. It codes for cysteine, not valine.
NCERT Extract:
Mendelian Disorders – “… 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. The mutant haemoglobin molecule undergoes polymerisation under low oxygen tension causing the change in the shape of the RBC from biconcave disc to elongated sickle like structure (Figure 4.15).”
Figure 4.15 Micrograph of the red blood cells and the amino acid composition of the relevant portion of β-chain of haemoglobin: (a) From a normal individual; (b) From an individual with sickle-cell anaemia