Stem cells: most of a cell's DNA is not translated - AQA A-Level Biology (Paper 2) Exam Questions

Practise 16 exam-style questions on Stem cells: most of a cell's DNA is not translated for AQA A-Level Biology (Paper 2). 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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16 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

Before cells made from pluripotent stem cells are transplanted into a patient, scientists check that none of the cells are still pluripotent. Explain why.

Model answer

Pluripotent cells can keep dividing in unlimited numbers, so any that remain in the transplant could form a tumour. They could also differentiate into types of cell that are not wanted in that part of the body.

Explanation

The property that makes pluripotent cells useful, unlimited division, is also a risk: undifferentiated cells left in a transplant can form tumours. Specialised cells made from them divide much less, so transplants should contain only fully differentiated cells. This is one of the main points to raise when evaluating stem cell treatments.

Mark scheme

1 mark
Pluripotent cells can divide in unlimited numbers / keep dividing (uncontrollably)
Max 2.
1 mark
(So) they could form a tumour
Accept 'mass of cells / growth that is not wanted'.
1 mark
They could differentiate into (many) other types of cell that are not wanted in that tissue / in the wrong place

Question 2

3 marks

Scientists isolated stem cells from the fat tissue of adult humans. In culture, these stem cells formed fat cells, bone cells and cartilage cells, but not muscle cells or neurones.

Name this type of stem cell.

Explain why these stem cells could not be used to replace neurones lost after a spinal cord injury, but iPS cells made from the same person's skin cells might be.

Model answer

They are multipotent stem cells. They can form only a limited number of cell types (fat, bone and cartilage cells), which do not include neurones, so they cannot replace lost neurones. iPS cells made from the person's skin cells are pluripotent: they can form almost any type of cell, so they could be made to differentiate into neurones.

Explanation

Multipotent cells in adults can form only a few related cell types, so their use in treatment is limited to those cells. Pluripotent cells, including iPS cells made from adult somatic cells, can form almost any cell type. Using the person's own cells also reduces the risk of rejection.

Mark scheme

1 mark
Multipotent
Reject unipotent, pluripotent or totipotent.
1 mark
These stem cells can only form a limited number of cell types, which do not include neurones (only some genes can be expressed)
Accept 'cannot differentiate into neurones'.
1 mark
iPS cells are pluripotent, so they can form (almost) any type of cell, including neurones
Accept 'can be made to differentiate into neurones'.

Question 3

4 marks

Limbal stem cells at the edge of the cornea divide to replace the cells lost from the surface of the cornea. They produce only this one type of cell. A chemical burn can destroy all the limbal stem cells in an eye, and the cornea then becomes cloudy.

Doctors treated 84 people who had a chemical burn to one eye. They grew limbal stem cells from a small sample taken either from the person's healthy eye (50 people) or from a donor (34 people). The people given donor cells also took drugs that suppress the immune system. After one year, the cornea was clear in 38 of the people given their own cells and in 14 of the people given donor cells.

Calculate the percentage of people in each group whose cornea was clear after one year.

Name the type of stem cell that limbal stem cells are.

Suggest one reason for the difference between the two groups, and one disadvantage of using cells from the person's own healthy eye.

Model answer

Own cells: 38 ÷ 50 × 100 = 76%; donor cells: 14 ÷ 34 × 100 = 41%.
Limbal stem cells are unipotent.
The donor cells have non-self antigens, so some are rejected by the immune system even though the immunosuppressant drugs reduce the response; the person's own cells are not rejected.
A disadvantage of using the person's own cells is that taking cells could damage the healthy eye, and the method cannot be used if both eyes are burned.

Explanation

Unipotent stem cells, found in mature mammals, produce only one type of cell. Cells from the patient's own body have self antigens and are not rejected, which is a major advantage in stem cell treatments; donor cells need immunosuppressant drugs, which do not always work and increase the risk of infection. Calculate a percentage for each group before comparing them, because the groups are different sizes.

Mark scheme

1 mark
76% (own cells) AND 41% (donor cells); correct answers with no working score this mark
Correct percentages of 76% and 41% with no working score 1 mark. Both percentages are needed. Accept 41.2 or 41.18 for the donor cells.
1 mark
Unipotent
Reject multipotent or totipotent.
1 mark
Donor cells have non-self antigens / are rejected by the immune system (even with immunosuppressant drugs, which do not suppress it completely)
Accept 'the person's own cells are not rejected'. List rule: if more than one reason (or more than one disadvantage) is given, a wrong or contradictory extra answer cancels the correct one.
1 mark
Removing cells could damage the healthy eye OR it cannot be used if both eyes are damaged
Accept 'takes longer because the cells must be grown first' only if linked to the person's own cells.

Question 4

8 marks

Scientists used viruses to insert genes for four different transcription factors, W, X, Y and Z, into mouse skin cells (fibroblasts) in different combinations. Each dish contained 2.0 × 10⁵ fibroblasts. After three weeks, they counted the colonies of induced pluripotent stem cells (iPS cells) in each dish. Each colony grew from one fibroblast that had been reprogrammed. The table shows the mean number of colonies per dish.

Calculate the percentage of fibroblasts that were reprogrammed when all four transcription factors were inserted. Give your answer in standard form.

What can you conclude about the importance of each transcription factor in producing iPS cells?

Transcription factor Z also stimulates cell division and is found at high concentrations in some tumours. Suggest one advantage and one disadvantage of producing iPS cells for treating patients without transcription factor Z.

Explain why the scientists included dishes in which no genes were inserted.

Table with columns "Transcription factors inserted", "Mean number of colonies of iPS cells per dish". Transcription factors inserted: W, X, Y and Z; Mean number of colonies of iPS cells per dish: 84 | Transcription factors inserted: X, Y and Z (no W); Mean number of colonies of iPS cells per dish: 0 | Transcription factors inserted: W, Y and Z (no X); Mean number of colonies of iPS cells per dish: 0 | Transcription factors inserted: W, X and Z (no Y); Mean number of colonies of iPS cells per dish: 5 | Transcription factors inserted: W, X and Y (no Z); Mean number of colonies of iPS cells per dish: 21 | Transcription factors inserted: none; Mean number of colonies of iPS cells per dish: 0.

Model answer

Percentage = 84 ÷ (2.0 × 10⁵) × 100 = 0.042% = 4.2 × 10⁻² %

W and X are essential: no colonies formed without either of them. Y is not essential but is needed for efficient reprogramming: only 5 colonies formed without it. Z is not essential but increases the number of iPS cells about four times (21 colonies without Z, 84 with all four). All four factors together gave the most iPS cells.

Without Z: the advantage is a lower risk that the iPS cells, or cells made from them, divide uncontrollably and form a tumour; the disadvantage is that only about a quarter as many iPS cells are produced, so reprogramming is less efficient.

The dishes with no inserted genes are a control: they show that fibroblasts do not form iPS colonies on their own, so the colonies were caused by the transcription factors.

Explanation

iPS cells are made from adult somatic cells using particular protein transcription factors, which switch on the genes needed for pluripotency. Reading the table factor by factor (what happens when each one is left out) shows which are essential and which only increase efficiency. In the calculation, divide by the number of fibroblasts and multiply by 100 to get a percentage. Factors that stimulate cell division raise the risk of tumours, which is one reason why pluripotent cells must be used with care in treatment.

Mark scheme

1 mark
Correct method: 84 ÷ (2.0 × 10⁵) × 100; or a correct answer of 4.2 × 10⁻² (%) with no working
Max 8. Points 1 and 2 are the calculation: a correct answer of 4.2 × 10⁻² % with no working scores 2 marks. Accept for point 1 only: 0.042 (correct value but not in standard form) or 4.2 × 10⁻⁴ (not multiplied by 100).
1 mark
4.2 × 10⁻² (%)
1 mark
W and X are essential / no iPS cells form without W or without X
Points 3 to 6: max 3. Each point must be supported by the data (the number of colonies) or a clear comparison.
1 mark
Y is not essential but is needed for efficient reprogramming: only 5 colonies form without Y (compared with 84)
1 mark
Z is not essential but increases the number of iPS cells: 21 colonies without Z, so four times as many with Z
Accept 'a quarter as many without Z'.
1 mark
All four together produce the most iPS cells / the factors act together
1 mark
Advantage: lower risk of the iPS cells (or cells made from them) dividing uncontrollably / forming a tumour
Points 7 and 8 are the advantage and the disadvantage. List rule: if more than one advantage or more than one disadvantage is given, each wrong or contradictory extra answer cancels the correct one. Accept 'safer because fewer cells divide uncontrollably'.
1 mark
Disadvantage: fewer iPS cells are produced / reprogramming is less efficient (about a quarter as many), so more fibroblasts or more time are needed
1 mark
(Control) to show that fibroblasts do not form iPS cell colonies without the inserted genes / that the colonies are caused by the transcription factors
Do not accept 'as a control' or 'for comparison' unless the answer says what the control shows.

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