AQA AS Level Biology Paper 2, November 2020: Question 2

✅ Correct Answer

• Condensation reaction (involving the loss of water).
• Forms between a phosphate group and a deoxyribose sugar.
• Catalysed by the enzyme DNA polymerase.

💡 Key Knowledge

Nucleotides link together in a condensation reaction between the phosphate group attached to the 5' carbon of one nucleotide pentose sugar and the hydroxyl (OH) group attached to the 3' carbon of the next nucleotide.

🧠 Exam Technique

To secure both available marks, you must explicitly state what type of chemical reaction occurs and name the two specific molecular components joined together. Keep your wording concise and direct.

❌ Common Errors

Students frequently lose marks by stating that DNA polymerase joins complementary bases or forms hydrogen bonds. Remember: DNA polymerase joins adjacent nucleotides along the backbone via phosphodiester bonds, whereas DNA helicase/RNA polymerase handle bases and hydrogen bonding.

✅ Correct Answer

70

📐 Step-by-Step Calculation

1. Recall Chargaff's rules and DNA structure: In double-stranded DNA, Adenine equals Thymine ( A = T ) and Guanine equals Cytosine ( G = C ). Therefore, substitute C with G in the given equation since total G equals total C across the complementary strands.
2. Solve for A and T: The equation is given as G = 4(A + T) - C . Since G = C , we can write G = 4(A + T) - G , which simplifies to 2G = 4(A + T) , or G = 2(A + T) .
3. Input known values: We are given G = 168 . Therefore, 168 = 4(A + T) - C . Because G = C , C = 168 .
4. Rearranging: 168 + 168 = 4(A + T) → 336 = 4(A + T) → A + T = 84 . Since A = T , A = 42 and T = 42 .
5. Find total bases: Total bases = G + C + A + T = 168 + 168 + 42 + 42 = 420 total bases.
6. Calculate template strand length: A gene is double-stranded, so the template strand contains half the total bases: 420 / 2 = 210 bases.
7. Calculate amino acid count: Each amino acid is coded for by a triplet of 3 bases (codon). Number of amino acids = 210 / 3 = 70 .

❌ Common Errors

A major trap is dividing the total number of bases in the gene by 3 without accounting for the fact that only one strand acts as the template for transcription. Always remember to halve the total base count to find the template strand length!

✅ Correct Answer

Histone(s)

💡 Key Knowledge

Eukaryotic DNA is wound around alkaline proteins called histones to help package and condense DNA tightly into chromosomes within the nucleus. Prokaryotic DNA, by contrast, is naked and not associated with histones.

🧠 Exam Technique

This is a straightforward recall question worth 1 mark. Do not over-elaborate; simply state the correct protein name clearly.

✅ Correct Answer

• Action of the enzyme DNA helicase.
• Breaks the hydrogen bonds between complementary base pairs (or between the two polynucleotide strands).

💡 Key Knowledge

Before semi-conservative replication can begin, the double helix must "unzip". DNA helicase moves along the molecule, breaking the relatively weak hydrogen bonds holding the complementary nitrogenous bases (A-T and C-G) together without disrupting the strong covalent phosphodiester backbone.

🧠 Exam Technique

To capture both marks, ensure you name the specific enzyme (1 mark) and explicitly state that hydrogen bonds are broken (1 mark).

❌ Common Errors

Students frequently lose the second mark by stating that DNA helicase causes hydrolysis of hydrogen bonds. Examiners strictly reject the word "hydrolysis" in this context because hydrogen bonds are broken physically by enzyme action, not chemically hydrolysed.