Gene expressionAQA A-Level Biology: Topic test

20 questions, 54 marks

AQA A-Level Biology

Gene expression topic test

Total 54 marks

Name

Class

Date

1
In the gene for one polypeptide, the mRNA codon GAG codes for the amino acid glutamic acid. One mutation changes this codon to GUG, which codes for valine. In another mutation the codon GAG changes to GAA, which also codes for glutamic acid.
(a)
What type of gene mutation changed GAG to GUG?
[1 mark]
ASubstitution
BDeletion
CInversion
DDuplication
(b)
Why did the change from GAG to GAA not alter the polypeptide?
[1 mark]
AThe mutation caused a frame shift
BThe mutation occurred outside the gene
CThe genetic code is degenerate, so more than one codon codes for the same amino acid
DGlutamic acid has no codon
(c)
Explain why a substitution of one base changes at most one amino acid, whereas the deletion of a single base near the start of a gene usually changes many amino acids.
[2 marks]
Total for question 1: 4 marks
2
Umbilical cord blood contains stem cells that divide to form red blood cells, platelets and several types of white blood cell, but they do not form nerve cells or muscle cells. Some parents pay to store cord blood in case their child later needs a transplant.
(a)
Which term describes the stem cells in cord blood?
[1 mark]
ATotipotent
BMultipotent
CUnipotent
DPluripotent
(b)
Which statement about totipotent cells is correct?
[1 mark]
AThey are found in cord blood
BThey can form only one type of cell
CThey are present throughout adult life
DThey occur only for a limited time in early mammalian embryos and can form any type of body cell
(c)
Explain why the stem cells in cord blood can form blood cells but not nerve cells.
[2 marks]
Total for question 2: 4 marks
3
Researchers studied the effect of maternal care on gene expression in rat pups. In the brains of pups reared by attentive mothers, 12% of the cytosine bases in the promoter region of the gene for a receptor protein were methylated, and the amount of mRNA for the receptor was 100 units. In pups reared by less attentive mothers, 68% of these cytosine bases were methylated and the amount of mRNA was 31 units. The base sequence of the gene was identical in both groups.
(a)
Explain how increased methylation of the promoter region could reduce the amount of receptor protein made.
[3 marks]
(b)
Calculate how many times greater the percentage of methylated cytosine bases is in pups of less attentive mothers than in pups of attentive mothers. Explain why this difference is an example of epigenetic control and not the result of a gene mutation.
[4 marks]
Total for question 3: 7 marks
4
A tumour taken from the lung of a patient has cells in which a proto-oncogene called RAS contains a mutation that changes one base. The protein made by the mutated gene is permanently active and sends signals that stimulate cell division. In the same tumour the promoter of a tumour suppressor gene, TP53, is heavily methylated. A 20-year study followed 12 000 smokers and 12 000 non-smokers. Lung cancer developed in 492 of the smokers and in 36 of the non-smokers.
(a)
Explain how the changes to RAS and TP53 could lead to the formation of a tumour.
[6 marks]
(b)
Evaluate the evidence from the study that smoking is a risk factor for lung cancer but is not the only cause.
[6 marks]
Total for question 4: 12 marks
5
A gene codes for a polypeptide of 150 amino acids. In one patient a mutation adds a single base between the 12th and 13th codons. In a second patient a mutation deletes exactly one whole codon, the 80th, and the rest of the sequence stays in frame.
(a)
The addition of a single base between the 12th and 13th codons is most likely to
[1 mark]
Achange only the 13th amino acid
Bchange many of the amino acids from the 13th onwards, because of a frame shift
Chave no effect because the genetic code is degenerate
Ddelete the 12th amino acid
(b)
What is the most likely effect of deleting exactly the 80th codon?
[1 mark]
AA polypeptide of 149 amino acids in which the other amino acids are unchanged
BA polypeptide of 150 amino acids with a changed sequence after the 80th
CA frame shift that changes every amino acid after the 79th
DA polypeptide of 151 amino acids
(c)
Explain why the addition of one base is likely to have a more serious effect than the deletion of one whole codon.
[2 marks]
Total for question 5: 4 marks
6
A research team wants to treat a patient whose pancreas has lost its insulin-secreting cells. Option 1 is to use pluripotent stem cells obtained from donated early embryos. Option 2 is to make induced pluripotent stem (iPS) cells from the patient's own skin cells and grow them into insulin-secreting cells.
(a)
Which statement about pluripotent stem cells is correct?
[1 mark]
AThey can form only one type of cell
BThey are found only in adult tissues
CThey can divide in unlimited numbers and form many types of body cell
DThey can form only a limited range of cell types, such as blood cells
(b)
How are iPS cells produced from adult body cells?
[1 mark]
ABy fertilising the cells with sperm
BBy exposing the cells to ultraviolet radiation
CBy culturing the cells with antibiotics
DBy treating the cells with protein transcription factors that switch on genes
(c)
Give two advantages of using iPS cells made from the patient's own skin cells rather than pluripotent cells from donated embryos.
[2 marks]
Total for question 6: 4 marks
7
Researchers are developing a treatment for a viral infection. Infected cells use their ribosomes to translate viral mRNA into a coat protein. The researchers introduced short double-stranded RNA molecules, with a base sequence matching part of the viral gene for the coat protein, into infected cells. Infected cells without the treatment made 240 units of coat protein per hour. Treated cells made 36 units per hour.
(a)
Explain how the short RNA molecules reduce the amount of coat protein made.
[3 marks]
(b)
Calculate the percentage reduction in coat protein made per hour after treatment. Explain how the way RNA interference reduces gene expression differs from the way a transcription factor controls gene expression.
[4 marks]
Total for question 7: 7 marks
8
All the cells in a person's body contain the same DNA. In the nerve cells of one person, the promoter region of a gene for a muscle protein is heavily methylated and the histones associated with the gene are deacetylated. In the muscle cells of the same person, the promoter is not methylated and the histones are acetylated. In a clinical trial, nerve cells derived from induced pluripotent stem (iPS) cells, made from each patient's own skin cells, were injected into the brains of 24 patients with a movement disorder. After 18 months, 17 patients had improved movement scores, 5 showed no change and 2 had developed tumours at the injection site. There was no control group.
(a)
Explain how nerve cells and muscle cells from the same person can make different proteins even though they contain the same DNA.
[6 marks]
(b)
Evaluate the use of iPS cells to treat this movement disorder, using the information about the trial.
[6 marks]
Total for question 8: 12 marks

End of questions

Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).