Question

consider the following sequence:
aug gcu cga uac
which mutation would result in no change to the phenotype?

aug gcu cga uag
cug gcu cga uac
aug gau cga uac
aug gcu aga uac

Explanation:

To determine which mutation causes no phenotypic change, we analyze codons (each codes for an amino acid). A silent mutation (same amino acid despite codon change) occurs when the new codon codes for the original amino acid.

• Original codons: AUG (Met), GCU (Ala), CGA (Arg), UAC (Tyr).
• Option 1: UAG is a stop codon (change from Tyr), so phenotype changes.
• Option 2: CUG (Leu) replaces AUG (Met) – amino acid changes, phenotype changes.
• Option 3: GAU (Asp) replaces GCU (Ala) – amino acid changes, phenotype changes.
• Option 4: AGA also codes for Arg (same as CGA). So the amino acid (Arg) remains, leading to no phenotypic change (silent mutation).

Answer:

AUG GCU AGA UAC (the fourth option)