Question

In sickle cell anaemia, glutamic acid is replaced by valine. Which one of the following triplets codes for valine?
A. GGG
B. AAG
C. GAA
D. GUG

Answer 1

There are 20 amino acids and possible codons are 64, hence, every single amino acid has more than one codon. A point mutation is a mutation that affects the single nucleotide on a DNA molecule.

Complete answer: Haemoglobin consists of two alpha and two beta chains. In a person diagnosed with sickle cell anaemia, a point mutation occurs in the beta globin chain of haemoglobin.
The gene mutation is a single nucleotide of the β-globin gene, which results in glutamic acid substituted by valine at position 6. GAG codon coding for Glutamic acid changes to GUG codon for Valine. Thus, the hydrophilic amino acid is substituted by hydrophobic amino acid. Abnormal haemoglobin results in the change of the shape of RBCs from biconcave disc to sickle shape.RBCs attaining sickle-shaped becomes hard and is unable to move through small blood vessels. It slows down the blood flood and creates an anaemic condition. In sickle cell anaemia, when point mutation occurs on DNA, during transcription GUG codon develops on RNA and it substitutes GAG codon. GUG codon when translated, it forms the amino acid valine.
So, the correct answer is option D.

Note: The allele responsible for sickle cell anaemia can be located on the short arm of chromosome 11. Sickle cell anaemia is a recessive autosomal disorder. A codon is a tri-nucleotide sequence of DNA or RNA which corresponds to the specific amino acid.