AQA A-Level Biology Paper 1, June 2024: Question 5

10 marks · Medium difficulty · Short Answer

Describe the appearance and behaviour of chromosomes during mitosis stages, calculate mitotic index, interpret a correlation coefficient, and analyze cell cycle distribution from root tip data.

Practise this question

Question

A four-part exam question about mitosis in onion root tips. Question 05.1 asks to describe chromosome appearance and behaviour during prophase and anaphase (4 marks). Question 05.2 asks for the word equation to determine mitotic index based on Figure 4 (1 mark). Question 05.3 provides a correlation coefficient r = -0.98 (P < 0.05) and asks what can be concluded (2 marks). Question 05.4 asks to conclude the effect of distance from the root tip on the proportion of cells in different stages of the cell cycle using Figure 4 (3 marks).
Question text

05.1 Describe the appearance and behaviour of chromosomes during prophase and during

anaphase of mitosis.

[4 marks]

Prophase

Anaphase

05.2 A scientist used an optical microscope to determine the mitotic index in cells at

different distances from the tip of onion roots.

Figure 4 shows the results.

Figure 4

Figure 4 not reproduced here due to third-party copyright restrictions

The figure is taken from the following website:

https://legacy.nimbios.org//~gross/bioed/webmodules/mitoticindex.htm#:~:text=

Interpretation%3A%20In%20general%2C%20the%20mitotic,of%20the%20root's

%20dividing%20cells

Complete the word equation used to determine each mitotic index in Figure 4.

[1 mark]

Mitotic Index =

05.3 The scientist used data from Figure 4 to calculate a correlation coefficient (r). The

scientist then used a statistical test to determine the probability (P) associated with the

value of r.

*14* r = –0.98 (P < 0.05)

What can you conclude from this result?

[2 marks]

05.4 What can you conclude about the effect that distance from the root tip has on the

proportion of cells in different stages of the cell cycle?

Use information in Figure 4.

[3 marks]

Mark scheme

Show the mark scheme The mark scheme provides detailed marking guidance for questions 05.1 through 05.4. For 05.1, it awards max 2 marks for prophase (chromosomes condense, become visible, attach to spindle) and max 2 marks for anaphase (centromeres divide, chromosomes pulled to opposite poles). For 05.2, it accepts number of cells in mitosis divided by total number of cells. For 05.3, it accepts strong negative correlation and probability occurring by chance is less than 0.05. For 05.4, it accepts descriptions of decreasing proportion of cells in mitosis and increasing proportion in interphase further from the tip.

Question Marking Guidance Mark Comments

Max two marks for prophase 1. Accept chromatin

for chromosomes

(In prophase)

1. Accept shorten OR

1. Chromosomes/chromatids (continue to)

thicken OR coils OR a

condense;

description of

condensing for

2. Chromosomes/chromatids (become/are) visible;

condense

3. Chromosomes attach to spindle (fibres)

4 max 2. Accept appear OR

OR form OR present OR

05.1 (4 x

Chromatids attach to spindle (fibres) distinct for visible

AO1)

OR

Centromeres attach to spindle (fibres);

(In anaphase)

5. Reject homologous

4. Centromeres divide/split;

chromosomes moved

5. Chromosomes/chromatids moved/pulled to to opposite sides

opposite poles/sides/ends;

Number of cells in mitosis ÷ Total number of cells Accept cells ‘with

(in field of view); visible chromosomes’

05.2 (1 x for ‘in mitosis’

AO1)

Reject if x100

1. Strong/significant negative (correlation);

2. (Because probability of) correlation occurring by 2. Accept relationship

chance is less than / < 0.05 / 5% Max 2 for correlation

05.3 OR (2 x 2. Reject ‘statistical

AO3) test’ OR ‘results’ OR

(Because) more than / > 0.95 / 95% (probability)

‘differences’ are

that correlation is not due to chance;

– A-LEVEL BIOLOGYsignificant/not due to––

3. Reject null hypothesis; chance

1. More/larger proportion of cells in mitosis closer Ignore references to

to tip cytokinesis

OR 1. and 3. Accept a

named stage of

Fewer/lower proportion of cells in mitosis further

mitosis

from tip;

2. and 3. Accept G1 11

2. More/larger proportion of cells in interphase

3 OR S OR G2 for

further from tip

05.4 (3 x interphase

OR AO3)

Fewer/smaller proportion of cells in interphase

closer to tip;

3. No cells in mitosis at/after 2 (mm)

OR

All cells in interphase at/after 2 (mm);

How to answer it

Mitosis, Mitotic Index and Statistical Data Analysis

What this question tests

This question assesses your knowledge of cell division stages (prophase and anaphase), your ability to calculate mitotic index formulas, and your competency in interpreting correlation coefficients (r-values) and biological data from root tip squashes.

Question 05.1 (4 Marks)

Appearance and Behaviour of Chromosomes in Mitosis

✅ Correct Answer / Mark Scheme

Prophase (Max 2 marks):

  • Chromosomes/chromatids continue to condense / shorten / thicken / coil.
  • Chromosomes/chromatids become visible / distinct.
  • Chromosomes/chromatids/centromeres attach to spindle fibres.

Anaphase (Max 2 marks):

  • Centromeres divide / split.
  • Chromosomes/chromatids are moved / pulled to opposite poles / sides / ends.

💡 Key Knowledge

  • Prophase: DNA supercoils, making chromosomes visible under a light microscope as sister chromatids joined by a centromere.
  • Anaphase: Spindle fibres contract, pulling sister chromatids apart toward opposite poles of the cell following centromere cleavage.

🧠 Exam Technique

Structure your answer clearly with subheadings for Prophase and Anaphase. Ensure you include both appearance (condensing/visible) and behaviour (moving/attaching) to lock in all available marks.

❌ Common Errors

  • Describing homologous chromosomes separating (this happens in Meiosis I, not mitosis).
  • Forgetting to mention centromeres dividing during anaphase.
Mark allocation: 4 marks total (Max 2 for prophase, Max 2 for anaphase, all AO1 recall).
Question 05.2 (1 Mark)

Word Equation for Mitotic Index

✅ Correct Answer

Number of cells in mitosis ÷ Total number of cells (in field of view)

💡 Key Knowledge

The mitotic index measures the proportion of cells undergoing active division. Cells in mitosis are identified by visible chromosomes (prophase, metaphase, anaphase, telophase).

❌ Common Errors

Multiplying the ratio by 100 (percentage). The question asks for a word equation to determine the mitotic index itself as a ratio/decimal, so including ×100 will lose you the mark.

Mark allocation: 1 mark for the correct ratio (AO1).
Question 05.3 (2 Marks)

Interpreting Correlation Coefficients

✅ Correct Answer / Mark Scheme

Any two of the following:

  • Strong / significant negative correlation (as distance increases, mitotic index decreases).
  • Probability of correlation occurring by chance is less than 0.05 / < 5% (p < 0.05).
  • Reject the null hypothesis.

🧠 Exam Technique

When given an r-value close to -1 alongside a p-value, immediately check two things: the strength and direction of the correlation, and whether it is statistically significant (p < 0.05 means we reject the null hypothesis).

❌ Common Errors

Vaguely stating "there is a significant difference" or "results are not due to chance". Statistical tests determine the probability of a correlation occurring by chance, not general differences between groups.

Mark allocation: 2 marks (AO3 evaluation/interpretation).
Question 05.4 (3 Marks)

Relating Cell Cycle Proportions to Root Tip Distance

✅ Correct Answer / Mark Scheme

Any three of the following points:

  • More / larger proportion of cells in mitosis closer to the tip (or fewer further away).
  • More / larger proportion of cells in interphase further from the tip (or fewer closer to the tip).
  • No cells in mitosis at / after 2 mm (or all cells in interphase at / after 2 mm).

💡 Key Knowledge

Meristematic tissue is located right at the root tip (apical meristem), where cells actively divide by mitosis. Further away from the tip, cells elongate and undergo differentiation rather than division.

🧠 Exam Technique

Always anchor your answers using specific data trends from the graph/figure. Reference the specific distance threshold (e.g., 2 mm) mentioned in the mark scheme to secure full marks.

Mark allocation: 3 marks (AO3 data interpretation).

Topics

Biology · Practical skills · Required Practicals · 3.2 Cells · Data analysis · AS practicals (1–6)

Question and mark scheme from the AQA A-Level Biology examination, Paper 1, June 2024. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.