AQA A-Level Biology Paper 1, June 2022: Question 7

8 marks · Medium difficulty · Short Answer

Complete a table to give three differences between DNA and tRNA molecules, describe how to remove large organelles from a cell suspension by centrifugation, and explain the positions of ribosome bands in tubes A and B after ultracentrifugation with and without detergent.

Practise this question

Question

Question text

07.1 Complete Table 3 to give three differences between DNA molecules and

tRNA molecules.

[3 marks]

Table 3

DNA molecules tRNA molecules

Scientists investigated ribosomal RNA in liver cells.

Figure 10 shows the method they used to isolate the ribosomes from the liver cells.

The detergent dissolves lipids.

Figure 10

07.2 The scientists broke open the cells to produce a suspension of cell contents.

Describe how the scientists would remove large organelles from this suspension of

cell contents.

[2 marks]

07.3 Explain the position of the bands of ribosomes in tubes A and B in Figure 10.

[3 marks]

Mark scheme

Show the mark scheme

Question Marking Guidance Mark Comments

DNA v tRNA 3 max Apply list rule

1. Deoxyribose v ribose; (3 x 2. Accept double helix

2. Double-stranded v single-stranded; AO1) for double stranded

3. Many nucleotides v few ; 3 Accept longer v

4. Thymine v uracil; shorter

5. Linear v clover leaf (structure) 4. Ignore T and U

OR 5. Accept description

07.1 Double helix v clover (leaf structure); of clover leaf

6. Does not bind to amino acid v does bind to structure.

amino acid; 5. Accept straight for

7. No exposed bases v anticodon; linear

6. Accept

attach/carry/have

binding site for ‘bind

to’

1. Use centrifuge/centrifugation at 2 Ignore homogenate

slow/low/increasing (sequence of) speed(s); (1 x OR filtering

2. Large/dense organelles (removed) in (first/early) AO2, 1 Ignore cold, isotonic,

x AO3) buffered

pellet

2. Accept descriptions

of supernatant and

Less dense organelles (removed) in supernatant pellet, eg liquid and

sediment/bottom of

OR tube

07.2 Small organelles (removed) in supernatant;

2. Accept light for

small

2. Accept ribosomes

for small organelles

2. Accept more dense

OR heavy for large

2 Accept named large

– A-LEVEL BIOLOGYorganelle in pellet, eg––

mitochondria

(Tube A) 3 Award marks across

1. (Ribosomes bound to) rough endoplasmic (3 x A and B

reticulum; AO2) 1. and 3 Do not credit

2. (Are) denser/heavier so move further; ER/rER the first time

used if endoplasmic

(Tube B) reticulum not given

3. (Only free ribosomes because) 2. Accept, free

membrane/phospholipids/endoplasmic reticulum ribosomes are lighter

dissolved (by detergent); so form a band higher

in tube OR do not

move as far

07.3 2. For ‘heavier/lighter’

accept ‘denser/less

dense’.

2. Accept settle lower

OR are lower down

OR at bottom OR in

pellet, for move

further

3. Accept broken

down for dissolved

3. Ignore damaged for

dissolved

How to answer it

Nucleic Acids, Cell Fractionation & Ultracentrifugation

What this question tests

This question assesses your knowledge of molecular biochemistry (contrasting the structure of DNA and tRNA) alongside practical biological skills (ultracentrifugation, cell fractionation, and data interpretation from experimental diagrams). You must apply theoretical knowledge of organelles and membrane structures to explain experimental outcomes.

Question 07.1 [3 marks]

Differences Between DNA and tRNA Molecules

✅ Correct Answers (Any 3)

Deoxyribose sugar vs Ribose sugar
Double-stranded vs Single-stranded (or double helix vs cloverleaf)
Many nucleotides vs Few nucleotides
Contains thymine vs Contains uracil
Linear structure vs Cloverleaf structure
Does not bind to amino acids vs Binds to a specific amino acid
No exposed bases/anticodon vs Has an anticodon

💡 Key Knowledge

When comparing nucleic acids, always structure your answers as direct contrasts (e.g., "DNA has... whereas tRNA has..."). Avoid vague statements like "DNA is bigger"—use specific structural terminology such as "many vs few nucleotides".

🧠 Exam Technique

This is marked using the list rule. If you write more than three differences, the examiner will mark your first three answers. However, if any incorrect statement is included within your first three points, it will negate a correct mark. Stick strictly to three clear, unambiguous points!

❌ Common Errors

Students frequently lose marks by writing "has T instead of U" without specifying what "T" and "U" stand for. Always write out full biochemical names (Thymine and Uracil) or ensure single-letter abbreviations are unambiguously presented.

🎯 Mark Allocation: 3 marks awarded for 3 distinct, valid structural comparisons (3 × AO1).

Question 07.2 [2 marks]

Removing Large Organelles from Cell Suspensions

✅ Correct Answers

Mark 1: Use a centrifuge / centrifugation at a slow / low / increasing speed.
Mark 2: Large/dense organelles form a pellet at the bottom, OR smaller/less dense organelles remain in the supernatant.

💡 Key Knowledge

Cell fractionation relies on differential centrifugation. Because different organelles have different densities and masses, applying a lower gravitational field ($g$-force) forces the heaviest, densest structures (like nuclei) to pellet first, leaving lighter components suspended in the liquid supernatant.

❌ Common Errors

Do not mention steps involving filtration or homogenate preparation unless specifically asked—the stem already states the cells have been broken open. Avoid confusing the terms pellet and supernatant.

🎯 Mark Allocation: 1 mark for method (centrifugation at low speed) + 1 mark for outcome (large organelles pelleted / small organelles in supernatant). (1 × AO2, 1 × AO3)

Question 07.3 [3 marks]

Explaining Ribosome Bands in Tubes A and B

✅ Correct Answers

Tube A (Lower band): Contains ribosomes bound to the rough endoplasmic reticulum (RER), which are denser/heavier and therefore move further down during ultracentrifugation.
Tube B: Contains only free ribosomes because the detergent dissolved the phospholipid membranes of the endoplasmic reticulum.

💡 Key Knowledge

Detergents are amphipathic molecules that disrupt lipid bilayers. Treating cell suspensions with a detergent dissolves biological membranes (such as the ER membrane), releasing bound proteins/ribosomes into free suspension, altering their sedimentation behavior during ultracentrifugation.

🧠 Exam Technique

Read the diagram carefully. Figure 10 shows Tube A has two bands (free and bound), whereas Tube B (with detergent) has only one band. Your explanation must explicitly contrast what happens to membrane-bound structures when lipids are dissolved.

❌ Common Errors

A common mistake is writing "ER" without stating "endoplasmic reticulum" on its first mention, or referring to organelles as being "damaged" or "broken down" instead of membranes being dissolved by the detergent.

🎯 Mark Allocation: 3 marks awarded for identifying RER-bound ribosomes in A, explaining their greater density/mass movement, and accounting for the single free-ribosome band in B due to membrane dissolution. (3 × AO2)

Topics

Biology · 3.1 Biological molecules · 3.2 Cells

Question and mark scheme from the AQA A-Level Biology examination, Paper 1, June 2022. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.