Mutation introduces G to form TGA stop codon (UGA in mRNA)

Steps:

• Protein shortens to two amino acids, so third codon mutates to stop UGA in mRNA (TGA in coding DNA).
• Original DNA CGAAAGTGAAAC has codons CGA-AAG-TGA-AAC; third codon positions 7-9 (TGA) causes premature stop.
• Mutation alters third codon to TGA, specifically changing position 8 to G.
• This creates UGA after gly-ser, halting synthesis at two amino acids.

Why C is correct:

• TGA requires G as the second base; mutation substitutes original base with G to form UGA stop per genetic code.

Why the others are wrong:

• A: Forms TAA (UAA stop), but question specifies UGA.
• B: C forms TCA (UCA serine), not stop.
• D: T forms TTA (UUA leucine), not stop.

Final answer: C