AQA AS Level Biology Paper 2, June 2023: Question 10

10 marks · Medium difficulty · Extended Answer

Outline the similarities and differences in structure between DNA and RNA, and between chloroplasts and mitochondria.

Practise this question

Question

An exam question split into two parts: Question 10.1 asks to outline the similarities and differences between the structures of DNA and RNA molecules (6 marks), with lines provided for an answer. Question 10.2 asks to outline the similarities and differences between the structures of chloroplasts and mitochondria (4 marks), with lines provided for an answer.
Question text

10.1 Outline the similarities in, and the differences between, the structures of DNA and

RNA molecules.

[6 marks]

10.2 Outline the similarities in, and the differences between, the structures of chloroplasts

and mitochondria.

[4 marks]

Mark scheme

Show the mark scheme A mark scheme table containing marking guidance for questions 10.1 and 10.2. For 10.1, similarities include being polymers of nucleotides, containing pentose sugar, base, and phosphate, having cytosine/guanine/adenine, and phosphodiester bonds; differences include deoxyribose vs ribose, thymine vs uracil, length, and double vs single stranded (6 max marks). For 10.2, similarities include double membrane, circular DNA, and ribosomes; differences include thylakoids/grana vs cristae, stroma vs matrix, pigments, and starch grains (4 max marks).

Question Marking Guidance Mark Comments

Similarities

1. Polymers of nucleotides; 1. Accept ‘chain’ for

2. (Nucleotide has) pentose, (nitrogen-containing polymer

organic) base and a phosphate (group); 2. Accept in correct

context

3. Cytosine, guanine and adenine (as bases); ‘ribose/deoxyribose’

4. Have phosphodiester bonds; for pentose

6 max

10.1 (6 x

Differences AO1)

5. Deoxyribose v ribose;

6. Thymine v uracil;

7. Long v short; 7. Accept DNA longer

8. Double helix/stranded v single stranded; 7. Ignore ‘large’ and

‘small’

Similarities Max 3 marks for

differences

1. Double membrane;

2. Both contain (circular) DNA;

3. Ignore numbers in

3. Both contain ribosomes;

4 max front of ribosomes

10.2 Differences (4 x

AO1)

4. Thylakoids/lamellae/grana v cristae;

5. Stroma v matrix;

6. Pigments v no pigments; 6. Accept ‘chlorophyll

v no chlorophyll’

7. Starch grains v no starch grains;

How to answer it

Comparing Nucleic Acids and Organelle Structures

What this question tests

This revision guide covers core biological knowledge (AO1) regarding the molecular structures of DNA and RNA, as well as the ultrastructure of chloroplasts and mitochondria. You must be able to systematically compare similarities and differences without confusing chemical components or organelle sub-compartments.

Question Part (10.1) - [6 marks]

DNA vs RNA Structural Comparison

Outline the similarities in, and the differences between, the structures of DNA and RNA molecules.

✅ Correct Marking Points (Max 6)

  • Similarities: Both are polymers/chains of nucleotides.
  • Both contain a pentose sugar, a phosphate group, and nitrogen-containing organic bases (adenine, cytosine, guanine).
  • Both have phosphodiester bonds linking nucleotides together.
  • Differences: DNA contains deoxyribose; RNA contains ribose.
  • DNA contains thymine; RNA contains uracil.
  • DNA is longer/double-stranded (double helix); RNA is shorter/single-stranded.

💡 Key Knowledge

  • Remember the mnemonic ATP for shared nitrogenous bases: Adenine, Thymine/Uracil, Cytosine, Guanine. Wait—Thymine is DNA only, Uracil is RNA only! Shareable bases are A, C, G.
  • Phosphodiester bonds form the sugar-phosphate backbone in both nucleic acids via condensation reactions.

🧠 Exam Technique

  • When a question asks for "similarities AND differences", explicitly use subheadings or comparative language (e.g., "Both have...", whereas "DNA has... while RNA has...").
  • Avoid vague terms like "DNA is big and RNA is small." Examiners accept "longer" vs "shorter", but reject "large" and "small".

❌ Common Errors

  • Stating that RNA contains thymine instead of uracil.
  • Confusing sugar types (saying RNA has deoxyribose).
  • Failing to mention that both contain adenine, cytosine, and guanine, leading to incomplete similarity points.
Mark Scheme Breakdown: 6 marks maximum awarded entirely for AO1 (Knowledge and Understanding). Must hit a balanced mix of similarities and differences.
Question Part (10.2) - [4 marks]

Chloroplast vs Mitochondria Ultrastructure

Outline the similarities in, and the differences between, the structures of chloroplasts and mitochondria.

✅ Correct Marking Points (Max 4)

  • Similarities: Both possess a double membrane.
  • Both contain circular DNA.
  • Both contain ribosomes (for protein synthesis).
  • Differences: Chloroplasts have thylakoids/lamellae/grana; mitochondria have cristae.
  • Chloroplasts contain stroma; mitochondria contain a matrix.
  • Chloroplasts contain photosynthetic pigments (chlorophyll); mitochondria do not.
  • Chloroplasts may contain starch grains; mitochondria do not.

💡 Key Knowledge

  • Both organelles are involved in energy transformation and share evolutionary origins (Endosymbiotic Theory), which explains why they retain their own circular DNA and 70S ribosomes.
  • Keep fluid-filled spaces straight: Stroma is to chloroplasts what Matrix is to mitochondria.

🧠 Exam Technique

  • Watch out for caps: The mark scheme specifically states a Maximum of 3 marks for differences. This means you cannot just list four differences and get full marks—you must include at least one similarity!
  • Always pair your contrasting points clearly (e.g., "Chloroplasts have grana, whereas mitochondria have cristae").

❌ Common Errors

  • Listing four differences and zero similarities, capping the final score at 3 out of 4.
  • Mixing up internal terms (e.g., stating mitochondria have a stroma or cristae are found in chloroplasts).
  • Using imprecise language like "they both have membranes" instead of specifying a double membrane.
Mark Scheme Breakdown: 4 marks maximum (4 × AO1). Rule enforcement: Max 3 marks can be awarded from the differences category. Ensure at least one similarity is stated to unlock full marks.

Topics

Biology · 3.1 Biological molecules · 3.2 Cells

Question and mark scheme from the AQA AS Level Biology examination, Paper 2, June 2023. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.