AQA AS Level Biology Paper 1, June 2025: Question 4

✅ Correct Answers

• Locus: (Fixed) position of a gene on a DNA molecule or chromosome.
• Exon: (Base sequence) coding for amino acid sequence(s).
• Universal genetic code: The same triplet / codon (always) codes for the same amino acid (in all organisms).

💡 Key Knowledge

• Locus is simply gene "address" on a homologous chromosome pair.
• Exons are coding regions. In pre-mRNA splicing, introns (non-coding sequences) are removed, leaving exons.
• Universal means from bacteria to humans, AUG always codes for methionine (with very rare minor exceptions).

🧠 Exam Technique

• Always specify "amino acid sequence" when defining an exon. Examiners specifically state to ignore "polypeptide chain/protein".
• For universal, mention both parts of the relationship: the codon/triplet AND the amino acid.

❌ Common Errors

• Defining locus as "where an allele lives" without stating on DNA or chromosome.
• Confusing exon with intron (introns are non-coding intervening sequences).
• Saying "all organisms have the same DNA" instead of stating that identical triplets code for the same amino acids.

✅ Completed Table 3

📐 Step-by-Step Decoding

1. Transcribe DNA to mRNA (Complementary Pairing):
   • G pairs with C
   • C pairs with G
   • T pairs with A
   • A pairs with U (Uracil, not Thymine!)
   DNA: GCT | CTA | CAG → mRNA: CGA | GAU | GUC
2. Read Table 2 for Amino Acids:
   • CGA : 1st base C, 2nd base G, 3rd base A → Arg
   • GAU : 1st base G, 2nd base A, 3rd base U → Asp
   • GUC : 1st base G, 2nd base U, 3rd base C → Val

❌ Common Errors

• Writing T in the mRNA strand (mRNA never contains Thymine).
• Looking up the DNA triplets directly in the mRNA table rather than transcribing to mRNA codons first.
• Misaligning rows/columns in the table (e.g., reading column 1 as 1st base instead of row 1).

✅ Correct Marking Points (Max 3 marks)

• Point 1 (Deletion effect): Deletion causes a frameshift (or changes all subsequent codons/triplets)
  OR changes the second codon from GAU (Asp) to AUG (Met). [1 mark]
• Point 2 (Substitution codon identity): Substitution of base 3 (T replaced in DNA) results in codon 1 being CGU , CGG , or CGC . [1 mark]
• Point 3 (Substitution effect): All these alternative codons still code for the same amino acid (Arg / arginine) OR the genetic code is degenerate. [1 mark]

💡 Key Knowledge

• Frameshift: Because the code is non-overlapping and read in triplets, removing 1 base shifts the reading frame for every subsequent codon, changing the entire downstream amino acid sequence.
• Degenerate Code: Multiple different triplet codons can code for the same amino acid. In Table 2, all codons starting with CG_ ( CGG, CGA, CGC, CGU ) code for Arg.

🧠 Exam Technique: Structuring Comparison Questions

Always address both halves of the prompt distinctly:

1. Part 1 — Deletion: Use the term "frameshift" or explicitly show what the new triplets become: original sequence GCT | CTA | CAG without the 3rd base becomes GCC | TAC | AG... (mRNA: CGG | AUG | UC... ).
2. Part 2 — Substitution: Name the alternative codons that could form ( CGU / CGC / CGG ) and use the technical keyword: degenerate.

❌ Common Misconceptions & Slip-ups

• Losing marks on wording: Writing that codons "produce" or "make" amino acids — the mark scheme explicitly states Reject 'produce' amino acids. Always write "codes for".
• Failing to cite evidence from Table 2: The question explicitly says "Use information in Table 2 to explain why". Top students showed that all 4 CG_ combinations specify Arg.