A point mutation changes the codon UGC (cysteine) to UAA (stop codon) in the mRNA of a human beta‑globin gene. Discuss the molecular consequences of this nonsense mutation on the synthesized hemoglobin protein and its possible phenotypic effect in an individual.
Answer
The mutation produces a premature stop codon, leading to a truncated beta‑globin chain that cannot assemble into functional hemoglobin, resulting in beta‑thalassemia with anemia, microcytosis, and short stature.
Step-by-step solution
- Identify the codon change: UGC (cysteine) → UAA (stop).
- During translation, ribosomes recognise UAA as a termination signal, so synthesis of the beta‑globin chain stops prematurely.
- The truncated protein lacks essential amino acids required for proper folding and tetramer assembly.
- Such incomplete chains are unstable and are rapidly degraded, reducing the amount of functional hemoglobin.
- Clinically, the reduced hemoglobin leads to microcytic hypochromic anemia, often manifesting as beta‑thalassemia, with symptoms like pallor, fatigue, and skeletal deformities.
Common mistake: Assuming the truncated protein functions normally or ignoring nonsense‑mediated decay
Concept tested: Effect of nonsense mutations
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