Required practical 2: root tip squashes and mitotic index - AQA A-Level Biology (Paper 3) Exam Questions

Practise 15 exam-style questions on Required practical 2: root tip squashes and mitotic index for AQA A-Level Biology (Paper 3). Write your answer as you would in the exam and get it marked in seconds against the mark scheme, point by point, with the Study Helper on hand if you get stuck.

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15 questions, marked as you go

Sample questions with model answers

Here are 4 of the questions in this topic, each with a model answer, an explanation of why it earns the marks and the mark scheme an examiner would use.

Question 1

2 marks

In school laboratories, the stages of mitosis are usually studied using root tips from germinating seeds rather than tissue from animals.

Suggest two reasons why.

Model answer

1. Root tips contain meristem, where a high proportion of cells are dividing.
2. No animals need to be killed, so there are no ethical issues.

Explanation

Plant meristems at root tips are regions of rapid cell division, so many cells in a squash are in mitosis. Germinating seeds are cheap, quick to grow and raise no ethical concerns, while animal tissue would require an animal to be killed and carries a greater risk of infection. A reason must say why root tips are better, not just describe them.

Mark scheme

1 mark
Root tips/meristems contain a high proportion of cells that are dividing (cell division is concentrated there)
Max 2. One mark for each reason. List rule: if more than two reasons are given, each wrong or contradictory extra answer cancels one correct answer.
1 mark
Seeds are cheap/easy/quick to germinate and grow (in large numbers)
Accept "roots can be grown in a few days in water".
1 mark
No ethical issues / no animal has to be killed or harmed (plants are not sentient)
Accept "it is more ethical".
1 mark
Lower risk of infection/pathogens than handling animal tissue
Ignore "plants have cell walls".

Question 2

3 marks

Three students each prepared a stained squash of a root tip from a chickpea seedling. Each slide had a different problem when it was viewed with an optical microscope.

Slide A: the cells were in thick clumps, several layers deep.
Slide B: almost all the cells were long and narrow, and none of them were dividing.
Slide C: the cells were well spread out, but the chromosomes were very pale and hard to see.

For each slide, suggest one mistake in the preparation that could have caused the problem.

Model answer

Slide A: the root tip was not left in the warm acid for long enough, so the middle lamellae were not broken down and the cells stayed stuck together in clumps.
Slide B: the student cut tissue from too far back from the tip, where cells are elongating and no longer divide.
Slide C: too little stain was used, or it was not left on long enough, so the chromosomes did not take up enough stain.

Explanation

Each step of a root tip squash has a purpose: acid hydrolysis separates the cells, maceration and squashing spread them into a single layer, only the meristem (the first 1-3 mm) contains dividing cells, and the stain makes the chromosomes visible. Working back from an observation to the step that failed is a common exam task. Cells behind the meristem are elongated and have stopped dividing, so a slide made from them shows almost no mitosis.

Mark scheme

1 mark
Slide A: root tip not left in the (warm) acid for long enough (so the middle lamella was not broken down and cells did not separate) / tissue not broken up (macerated) or not squashed firmly enough / too large a piece of tissue used
One mark for each slide. Each point lists different wordings or causes for the same slide; credit any one. Ignore "did not use enough stain" for slide A.
1 mark
Slide B: the wrong part of the root was used, e.g. tissue cut from further back than the tip / the end without the tip / more than the first few mm (cells from the zone of elongation)
Accept "the root tip had already been cut off" or "used the end of the root nearest the seed". Insufficient: "used the wrong cells" with no indication of which part of the root.
1 mark
Slide C: too little stain / stain not left on for long enough / stain diluted (e.g. by water left after rinsing) / old stain
Accept "did not heat the stain if it needs warming (e.g. acetic orcein)". Insufficient: "did not stain it properly".

Question 3

3 marks

A student made a scientific drawing of one cell from a stained squash of a fenugreek root tip. She wrote these four notes beside her drawing, each with a line to the part it refers to:

1 'chromosome'
2 'chromosomes lined up across the equator of the cell'
3 'the cell is dividing'
4 'no nuclear envelope visible'

Explain the difference between a label and an annotation on a scientific drawing.

The student said that her notes show a reader that the cell is at metaphase. Evaluate her claim.

Model answer

A label just names a structure, whereas an annotation is a short note that describes or explains a feature of it.

Note 2 supports her claim, because chromosomes lined up across the equator are seen only at metaphase. However, note 4 does not show metaphase on its own, because there is also no nuclear envelope in late prophase and in anaphase, and note 3 is true of every stage of mitosis. Note 1 is only a label. So her claim is only partly supported: she should add an annotation such as 'each chromosome made of two chromatids joined at a centromere, attached to spindle fibres'.

Explanation

Labels name structures; annotations add observations or explanations. An annotation that is meant to identify a stage must describe a feature found only at that stage: at metaphase the chromosomes lie on the equator, each still made of two chromatids attached to spindle fibres at the centromere. Features shared with other stages, such as 'no nuclear envelope', or general statements such as 'the cell is dividing', are not evidence of metaphase. In an evaluation, say which notes support the claim and which do not.

Mark scheme

1 mark
A label (only) names a structure; an annotation is a note that describes or explains a feature (e.g. its appearance or what it shows)
Max 2 if the answer gives only reasons for the claim or only reasons against it. Max 3. Point 1 is the difference between a label and an annotation. Points 2 to 6 evaluate the claim, 1 mark each, max 2: point 2 supports the claim and points 3 to 6 are against it. Insufficient for point 1: 'an annotation gives more information' with no idea of describing or explaining.
1 mark
Note 2 supports the claim: chromosomes lined up on the equator (of the cell) are seen only at metaphase
Accept 'only note 2 is evidence of metaphase'. Accept 'note 2 is an annotation that shows metaphase'.
1 mark
Note 4 does not show metaphase on its own: there is also no nuclear envelope in (late) prophase and in anaphase
Accept 'no nuclear envelope is true of other stages too'. Reject 'there is a nuclear envelope at metaphase'.
1 mark
Note 3 does not show metaphase: every stage of mitosis is part of cell division / it does not describe a visible feature of the cell
Accept 'the cell is dividing is true of prophase, anaphase and telophase as well'.
1 mark
Note 1 is a label, not an annotation / gives no information about the stage
Accept 'chromosome only names the structure'.
1 mark
She should add an annotation of another feature found at metaphase, e.g. 'each chromosome made of two chromatids joined at a centromere' or 'spindle fibres attached to the centromeres'
The feature must distinguish metaphase. Ignore 'chromosomes are visible / condensed / stained dark' (also true of prophase and anaphase).

Question 4

6 marks

Some scientists think that cells in root tips divide more at some times of day than at others.

Describe how you could use stained squashes of root tips from barley seedlings to investigate whether the mitotic index of the root tips changes during a 24-hour period.

Model answer

Germinate seeds from one batch in the same conditions, at a constant temperature with a 12-hour light/12-hour dark cycle, until the roots are about 2 cm long.
Every 3 hours over 24 hours, cut the tips from the roots of five seedlings and put each straight into warm hydrochloric acid to fix it.
Treat every tip the same way: the same time in acid, rinse, keep the first 2 mm, stain with toluidine blue, macerate and squash.
For each slide, count the cells in mitosis and the total number of cells in five randomly chosen fields of view (at least 200 cells).
Calculate the mitotic index (cells in mitosis ÷ total cells) for each tip and the mean for each time.
Plot mean mitotic index against time of day and use a statistical test to find whether the highest and lowest means differ significantly.

Explanation

The independent variable is the time of day, so root tips must be sampled at intervals across 24 hours, with temperature and other conditions kept the same so that only time changes. Fixing each tip immediately captures the proportion of dividing cells at that moment. Several seedlings per time, random fields and large cell counts make the means representative, and a statistical test shows whether differences are likely to be due to chance.

Mark scheme

1 mark
Grow all the seedlings from one batch of seed in the same conditions (temperature, light/dark cycle, water)
Max 4 if the answer does not describe taking root tips at different times of day. Max 6.
1 mark
Take root tips at regular intervals over 24 hours, e.g. every 2-4 hours
Accept a stated set of times covering 24 hours.
1 mark
Use several (at least 3) seedlings/root tips at each time
Accept "a different seedling for each sample".
1 mark
Put each root tip into (warm) acid immediately to fix it / stop division at that time, and treat every tip in the same way (same time in acid and stain)
1 mark
Use the same length of tip each time (e.g. the first 2 mm / the meristem)
1 mark
Count the cells in mitosis and the total number of cells in several randomly chosen fields of view (or a stated large number, e.g. at least 200 cells per tip)
Accept "count cells at each stage of mitosis and in interphase".
1 mark
Mitotic index = number of cells in mitosis ÷ total number of cells; calculate the mean mitotic index for each time
1 mark
Keep the temperature constant throughout (temperature affects the rate of cell division)
1 mark
Plot mean mitotic index against time of day and/or use a statistical test to see if differences between times are significant
Accept "calculate standard deviations and compare".

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