Biology · Molecular Basis of Inheritance · NEET
It is a point mutation, not a frameshift. Only a single base pair is changed (substituted). No base is inserted and no base is deleted, so the reading frame stays the same. This is why NCERT calls it the classical example of a point mutation. Frameshift needs an insertion or deletion, which shifts all codons after it.
The sixth codon of the beta-globin mRNA changes from GAG to GUG. On the DNA template a single base pair changes, and this shows up in the mRNA as A becoming U in the middle of the codon. NEET 2026 directly asked this: the mutant sixth codon is GUG.
Glutamate (glutamic acid, Glu) at the sixth position of the beta-globin chain is replaced by Valine (Val). Short form: Glu to Val at position 6. GAG codes for Glu; GUG codes for Val. This single amino acid swap is enough to make the whole disease.
It affects the beta-globin chain. Haemoglobin has two alpha chains and two beta chains. The mutation is in the gene for the beta chain only, at the sixth amino acid. The alpha chains are normal.
The new valine is water-hating (hydrophobic), while glutamate was water-loving. Under low oxygen, the mutant haemoglobin (HbS) molecules stick together and polymerise. This changes the round red blood cell into a sickle (curved) shape. Sickled cells block small blood vessels and break easily, so one amino acid causes big trouble.
The original change is in the DNA gene for beta-globin (a single base pair substitution). Because DNA is transcribed into mRNA, the mRNA sixth codon becomes GUG. When you read a codon like GUG, you are reading the mRNA, since codons are written in RNA bases (A, U, G, C).
The sixth mutant codon of beta globin gene causing polymerization of Haemoglobin and change in RBC shape is ______.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
A point mutation — a single base-pair substitution in the beta-globin gene. It is the classical NCERT example of a point mutation.
The sixth codon changes from GAG to GUG in the mRNA.
Glutamate (Glu) is replaced by Valine (Val) at the sixth position of the beta-globin chain.
HbS is the mutant (sickle) haemoglobin formed because of this amino acid change. It polymerises in low oxygen and deforms the red blood cell.
NEET repeatedly tests the exact codon (GUG), the amino acid change (Glu to Val), the position (sixth), and that it is a point mutation. NEET 2026 asked for the mutant codon directly.
