AQA GCSE Biology Biology Paper 2 (Foundation), June 2025: Question 5

✅ Correct Answer

Tick: Bases

💡 Key Knowledge

• DNA is a polymer made of repeating units called nucleotides.
• Each nucleotide contains a sugar molecule, a phosphate group, and one of four bases: Adenine (A), Cytosine (C), Guanine (G), Thymine (T).
• Complementary base pairing: A pairs with T, and C pairs with G.

❌ Common Errors

Confusing bases with the sugars (deoxyribose pentagons) or phosphates (circles) that form the DNA backbone.

✅ Correct Answer

3 (or three)

📐 Working Out

1. Count the total number of bases: A-G-T-A-A-G-T-A-C = 9 bases.
2. A sequence of 3 bases codes for 1 amino acid.
3. Divide total bases by 3: 9 ÷ 3 = 3 amino acids.
4. Grouped into triplets: [AGT] [AAG] [TAC] = 3 triplets.

🧠 Exam Technique

Always chunk long sequences into groups of three with a pencil on your exam paper to avoid miscounting.

✅ Correct Answer

Tick: Mutation

💡 Key Knowledge

• Mutation: A random change in the DNA base sequence of an organism.
• Differentiation: The process by which a cell changes to become specialised.
• Natural selection: Survival and reproduction of organisms better adapted to their environment.

❌ Common Errors

Ticking 'Natural selection'. A mutation is the event that alters the DNA sequence; natural selection is the mechanism that drives evolution over generations.

📐 Step-by-Step Calculation

1. Step 1: Read values from the graph (Mark 1)
   In 1993: 1 case per million people
   In 2005: 40 cases per million people
2. Step 2: Set up rate equation (Mark 2)
   Change in cases = 40 − 1 = 39
   Time period = 12 years
   Rate = 39 ÷ 12
3. Step 3: Calculate the final value (Mark 3)
   39 ÷ 12 = 3.25 cases per million people per year

✅ Final Value

3.25

(Units already provided on the answer line: cases per million people per year)

❌ Common Errors & Traps

• Misreading the starting value: Thinking 1993 starts at 0. Looking closely at the y-axis, 1 small square = 1 unit; 1993 is at exactly 1.
• Dividing by the wrong year span: The question gives the time period directly (12 years), so do not guess or divide by 13.
• Rounding incorrectly: Leaving it as a fraction without completing the division to a decimal.

✅ Correct Answer

Tick: Resistant

💡 Key Knowledge

• Bacteria become resistant to antibiotics via mutations.
• Humans/animals become immune to pathogens via antibodies and memory white blood cells.
• Golden rule: Bacteria are NEVER "immune" to antibiotics.

❌ Major Misconception

Selecting "Immune". This is one of the most common mistakes in GCSE Biology. Pathogens become resistant; hosts become immune.

✅ Accepted Answers (Any ONE)

• Better hygiene (e.g. more hand-washing in hospitals / alcohol gel dispensers)
• Isolation of patients infected with MRSA
• Discovery of a new antibiotic / drug / treatment
• Better / improved education or awareness about MRSA
• Vaccination / social distancing

🧠 Exam Technique

• Always be specific: write "better hand washing by doctors/nurses in hospitals" rather than just "cleaner".
• You only need one reason—giving multiple conflicting reasons risks losing the mark if one is incorrect.

❌ Answers Not Awarded

• "People became immune to MRSA" (MRSA does not readily produce permanent population immunity).
• "Used more of the same antibiotic" (this would actually select for more resistance!).