A Levels Biology (9700)•9700/12/O/N/20

Explanation
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
Topic: Protein synthesis
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