AQA AS Level Biology Paper 1, June 2022: Question 2

Mark scheme

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Question Marking Guidance Mark Comments

02.1 1. RNA/rRNA; Reject tRNA and

2 mRNA

2. Protein;

(2 x AO1) Ignore amino acids

02.2 1. DNA has deoxyribose, mRNA has ribose; Must be comparisons

2. DNA has thymine, mRNA has uracil; Ignore splicing/introns

3. DNA long, mRNA short;

4. DNA is double stranded, mRNA is single 4. Accept ‘double

stranded; 4 max helix’ for ‘double

5. DNA has hydrogen bonds, mRNA has no (4 x AO1) stranded’ and ‘single

hydrogen bonds helix’ for ‘single

stranded’

OR

DNA has (complementary) base pairing,

mRNA does not;

How to answer it

Ribosomes and Nucleic Acid Structure Study Guide

AQA AS Level Biology — Nucleotides and Nucleic Acids

What this question tests

This question assesses your factual recall and comparative understanding of molecular biology components (AO1). Specifically, it targets the composition of ribosomes and the fundamental structural differences between deoxyribonucleic acid (DNA) and messenger RNA (mRNA).

Question 02.1: Composition of a Ribosome

Give the two types of molecule from which a ribosome is made. [2 marks]

✅ Correct Answers

RNA (accept rRNA / ribosomal RNA)
Protein

💡 Key Knowledge

Ribosomes are non-membrane-bound organelles found in all living cells. They act as the site of translation during protein synthesis and consist of a large and small subunit made entirely of ribosomal RNA and structural proteins.

🧠 Exam Technique

Keep your answers direct and unambiguous. Write single terms or concise phrases. Do not over-elaborate or write full sentences for list-style questions.

❌ Common Errors

Writing tRNA or mRNA instead of rRNA (these are specific types of RNA not used to build ribosomes).
Stating "amino acids" instead of "protein" (amino acids are monomers; the ribosome is built from fully polymerized protein chains).

Mark Allocation: 2 marks total (1 mark per correct molecule type, assessed via AO1).

Question 02.2: Structural Differences Between DNA and mRNA

Complete Table 2 to give four structural differences between a DNA molecule and an mRNA molecule. [4 marks]

✅ Correct Answer Options (Any 4 valid comparisons)

1. Pentose sugar: DNA has deoxyribose, mRNA has ribose.
2. Pyrimidine base: DNA has thymine, mRNA has uracil.
3. Length: DNA is long, mRNA is short.
4. Strands: DNA is double-stranded (or double helix), mRNA is single-stranded (or single helix).
5. Hydrogen bonds / Base pairing: DNA has hydrogen bonds / complementary base pairing, mRNA does not.

💡 Key Knowledge

You must know the biochemical and structural differences between nucleic acids inside out. Remember that differences must always be comparative—stating "mRNA has ribose" is insufficient unless you explicitly contrast it with DNA.

🧠 Exam Technique

Comparison rule: Ensure every row in your table clearly contrasts DNA against mRNA. If you mention a property for DNA, you must state the corresponding property for mRNA in the same row.

❌ Common Errors

Failing to make direct comparisons (e.g., writing "DNA is double-stranded" on one line and "mRNA has uracil" on another without matching counterparts).
Including irrelevant biological details like splicing or introns, which are not structural features of the whole molecules being compared here.

Mark Allocation: 4 marks max (4 x AO1). One mark for each valid, complete comparative point up to a maximum of four rows.

Topics

Biology · 3.1 Biological molecules · 3.2 Cells · 3.4 Genetic information, variation and relationships between organisms