Question 1
A gene mutation changes the sequence to .
What type of mutation has occurred?
A.
Insertion
B.
Frameshift
C.
Substitution
D.
Deletion
Question 2
What is an example of a mutagenic form of radiation?
A.
Ultraviolet radiation
B.
Infrared radiation from a warm surface
C.
Visible green light used in microscopy
D.
Low-frequency sound waves
Question 3
What is a knockout library in a model organism?
A.
A collection of strains in which different strains each have a different inactivated gene
B.
A group of organisms selected because all mutations have been removed from their genomes
C.
A set of guide RNA molecules used only to sequence unknown genes
D.
A database listing all proteins expressed by one normal strain under one condition
Question 4
A single-nucleotide polymorphism occurs within a coding sequence, but the amino acid sequence of the polypeptide is unchanged.
What explains this outcome?
A.
The genetic code is degenerate, so different codons can specify the same amino acid.
B.
The base substitution changes the reading frame only after the affected codon.
C.
The mutation must have occurred in a germ cell rather than a somatic cell.
D.
The ribosome repairs the changed codon before translation begins.
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Question 5
Bacteria are exposed to an antibiotic. A resistant mutant later appears in the population.
What statement best describes how the mutation arose?
A.
All bases in the genome had exactly the same probability of mutating.
B.
A random change in DNA occurred, and selection could then favour resistant cells.
C.
The antibiotic directed mutation towards the gene that would improve survival.
D.
The bacterium deliberately changed the base sequence needed for resistance.
Question 6
What is the original source of all new alleles in a population?
A.
Fertilization combining gametes from two parents
B.
Meiosis producing new combinations of existing alleles
C.
Natural selection increasing favourable allele frequencies
D.
Mutation changing the base sequence of genes
Question 7
A gene of unknown function is made inoperative in a strain of yeast. The knockout strain cannot grow when lactose is the only carbon source, but the normal strain can.
What is the most reasonable inference?
A.
The gene must code directly for lactose molecules.
B.
The knockout has proved that all yeast genes control lactose use.
C.
The gene has no function because it has been deliberately changed.
D.
The gene is probably involved in lactose use or growth on lactose.
Question 8
In CRISPR-Cas9 gene editing, what directs Cas9 to the target DNA sequence?
A.
Random diffusion of Cas9 until it cuts any exposed chromosome region
B.
DNA polymerase adding nucleotides to both strands of the target gene
C.
A ribosome matching anticodons to codons during translation
D.
Complementary base pairing between guide RNA and target DNA
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Question 9
The diagram shows three altered versions of the same short DNA sequence in a gene.
State the type of gene mutation represented by X.
[1]
Distinguish between an insertion and a deletion mutation.
[2]
Question 10
A single nucleotide is deleted near the start of the coding sequence of a gene.
What is the most likely consequence for the translated polypeptide?
A.
The codon changes but the same amino acid is always inserted.
B.
The sequence length is unchanged but one base pair is replaced.
C.
The reading frame changes, altering many amino acids after the mutation.
D.
Only one amino acid is removed and the rest of the sequence is unchanged.
Question 11
A treatment for sickle-cell disease removes a patient’s blood-forming stem cells, edits them with CRISPR-Cas9 to increase fetal haemoglobin production, and returns them to the same patient.
What statement about this treatment is correct?
A.
It produces inherited resistance to sickle-cell disease in all descendants.
B.
It is somatic cell editing, so the edited sequence is not intended to be inherited by offspring.
C.
It changes every cell in the patient’s body to produce adult haemoglobin only.
D.
It is germline editing because red blood cells later produce gametes.
Question 12
Scientists in different countries may work under different rules for genome editing.
Why are international efforts made to harmonize regulation of CRISPR applications?
A.
To remove the need for ethical review when the guide RNA is accurate
B.
To prevent all somatic gene editing for severe inherited disease
C.
To ensure all countries allow embryo editing for enhancement at the same rate
D.
To consider safety, consent, equity and environmental risk consistently across countries
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Question 13
A single base substitution occurs in the coding sequence of a gene.
Outline how the substitution could be a silent mutation.
[2]
Explain how a different base substitution in the same gene could prevent the protein from functioning.
[2]
Question 14
Mutations can arise during DNA replication or after exposure to mutagens.
Outline how an error in DNA replication can become a gene mutation.
[2]
Give two examples of mutagens that can increase the rate of gene mutation.
[2]
Question 15
A gene-like DNA sequence is identified in a model plant, but its function is unknown. Researchers produce plants in which this gene is made inoperative.
State the name of this research technique.
[1]
Explain how the technique can help infer the function of the gene.
[2]
Question 16
A laboratory studies gene function in yeast. A knockout library is available for this model organism.
Outline what is meant by a knockout library.
[2]
Suggest one advantage of using a knockout library in this investigation.
[2]
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Question 17
A researcher sequenced a short region of the same gene from four bacterial colonies after one round of growth. The reference sequence and the sequences from colonies A, B and C are shown.
Sample | DNA sequence () | Length / nt |
|---|---|---|
Reference | ATGCCTGAATCGAAC | 15 |
Colony A | ATGCCTGAGTCGAAC | 15 |
Colony B | ATGCCTGAAATCGAAC | 16 |
Colony C | ATGCCTGATCGAAC | 14 |
Identify the type of gene mutation shown in colony A.
[1]
Distinguish between the mutations in colonies B and C.
[2]
Suggest why the mutation in colony A is less likely than the mutation in colony B or C to alter many amino acids in the polypeptide.
[1]
Question 18
A DNA sequence within a gene is almost identical in mammals, birds and reptiles, although nearby non-coding sequences differ greatly.
What is the best hypothesis to account for the highly conserved sequence?
A.
The sequence has no biological role, so natural selection has preserved it unchanged.
B.
Mutations in the sequence are often harmful because the gene product has an important function.
C.
The sequence must have been recently copied from one species into all the others.
D.
All bases in the sequence mutate at a higher rate than bases in nearby regions.
Question 19
The diagram shows part of an mRNA molecule before and after a one-nucleotide deletion near the start of the coding sequence.
State the term used for the change in codon grouping caused by this deletion.
[1]
Explain why this deletion is likely to have a greater effect on the polypeptide than a base substitution.
[3]
Question 20
Bacteria are exposed to an antibiotic. A small number of bacteria in the population are resistant because of a gene mutation.
Explain why it is inaccurate to say that the bacteria deliberately mutated in order to become resistant.
[2]
State why mutations are not expected to occur at exactly the same rate at every base in the genome.
[2]
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Question 21
Mutations may occur in germ cells or in somatic cells.
Compare the consequences of a mutation in a germ cell with a mutation in a somatic cell.
[3]
State the original source of all new alleles in a population.
[1]
Question 22
The diagram shows a simplified CRISPR-Cas9 gene-editing system acting on a target DNA sequence.
State the role of the guide RNA in CRISPR-Cas9 gene editing.
[1]
Explain how CRISPR-Cas9 can alter a specific DNA sequence in a genome.
[3]
Question 23
CRISPR-Cas9 can be used to treat some blood disorders, including sickle-cell disease and transfusion-dependent beta-thalassaemia.
Describe one successful medical use of CRISPR-Cas9 for these disorders.
[3]
State whether this treatment is somatic or germline gene editing.
[1]
Question 24
The table shows three single-nucleotide polymorphisms in the coding strand of a human gene. The codon in the corresponding mRNA and the amino acid or stop signal are also shown.
SNP | Coding DNA codon | mRNA codon | Amino acid / stop |
|---|---|---|---|
SNP 1 | CTG CTA | CUG CUA | leucine leucine |
SNP 2 | GAA GAC | GAA GAC | glutamic acid aspartic acid |
SNP 3 | TGG TGA | UGG UGA | tryptophan stop codon |
State the SNP that is least likely to change the structure of the polypeptide.
[1]
Explain how the degeneracy of the genetic code accounts for the result for SNP 1.
[2]
Using the data, explain why SNP 3 is expected to have the greatest effect on the protein.
[2]
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Question 25
A model was used to predict the effect of small insertions and deletions near the start of a coding sequence. The figure shows the predicted number of amino acids that differ from the normal polypeptide before the first stop codon is reached.
Identify the mutation class predicted to alter the greatest number of amino acids.
[1]
Describe the difference in predicted effect between a one-base deletion and a three-base deletion.
[1]
Suggest why insertions or deletions of one or two bases usually have a greater effect than insertions or deletions of three bases.
[2]
Question 26
Cultures of yeast cells were exposed to different treatments. After growth, colonies were screened for mutations in a marker gene. The results are shown.
State the treatment that produced the highest mutation frequency.
[1]
Describe the effect of increasing ultraviolet radiation on mutation frequency.
[1]
Explain why mutations were detected in the control culture.
[1]
Suggest one way in which ultraviolet radiation can increase the mutation rate.
[1]
Question 27
DNA samples were taken from different tissues of one individual with a skin tumour and from the individual's child. The samples were tested for two mutations in the same gene. Mutation P was detected in the tumour only. Mutation Q was detected in sperm cells and in the child.
Sample | Mutation P | Mutation Q |
|---|---|---|
Skin tumour cells | Yes | No |
Sperm cells | No | Yes |
Child | No | Yes |
Identify which mutation is most likely to have occurred in a somatic cell.
[1]
Explain why mutation Q could be inherited by the child.
[2]
Suggest how mutation P could contribute to tumour formation.
[2]
Question 28
A knockout library of Saccharomyces cerevisiae contains strains in which different genes have been made inoperative. The growth of four knockout strains was measured on normal medium and on medium with low glucose.
Identify the knockout strain that gives the strongest evidence that its inactivated gene is needed for growth on low glucose.
[1]
Explain how the data support this conclusion.
[2]
State one advantage of using a knockout library in this investigation.
[1]
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Question 29
Blood-forming stem cells from patients with sickle-cell disease were edited outside the body using CRISPR-Cas9 and then returned to the same patients. The aim was to increase fetal haemoglobin production. The figure shows results before treatment and 12 months after treatment.
Calculate the decrease in the mean number of severe pain episodes per patient per year after treatment.
[1]
Explain how CRISPR-Cas9 can target a specific DNA sequence in these stem cells.
[2]
Using the data, explain why the treatment is considered somatic gene editing rather than germ-line gene editing.
[2]
Question 30
A proposal is made to use CRISPR-Cas9 to edit human embryos so that all cells of the resulting child would carry the edit.
Discuss two ethical or regulatory concerns that should be addressed before this application is implemented.
[4]
Question 31
The figure shows an alignment of part of the same gene from several distantly related vertebrate species. A short region is highly conserved, while neighbouring regions vary more.
Species | 1–15 bp | 16–27 bp | 28–42 bp |
|---|---|---|---|
Human | ATGCCAGTTGACGAA | GCTTACGGAATC | CCGTTAAGCTTACGA |
Mouse | ATGCCCGTTGATGAG | GCTTACGGAATC | CCATTAAGCCTACGG |
Chicken | ATGCTGGTTGACGAT | GCTTACGGAATC | CCGTTAAGTTTACGA |
Frog | ATGCCAGCTGACGAC | GCTTACGGAATC | TCGTTAAGCATACGA |
Lizard | ATGACAGTTGATGAG | GCTTACGGAATC | CCGTCAAGCTTACGG |
Zebrafish | ATGCCGGTTGACGAA | GCTTACGGAATC | CCTTTAAGCTTACAA |
State what is meant by a highly conserved sequence.
[1]
Evaluate two hypotheses that could account for the highly conserved region shown in the alignment.
[3]
Question 32
A fluctuation test was performed using identical small bacterial cultures. Each culture was grown without antibiotic and then plated on agar containing antibiotic. The number of resistant colonies on each plate was counted.
Describe the pattern in the numbers of resistant colonies among the parallel cultures.
[2]
Explain how this pattern supports the conclusion that resistance mutations occurred before exposure to the antibiotic.
[2]
State what is meant by random mutation in this context.
[1]
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Question 33
Researchers investigated gene X in mice by comparing wild-type mice with mice in which gene X had been knocked out. Three physiological measurements were made after exercise.
Mouse type | Time to exhaustion / min | Mitochondrial enzyme activity / % WT | Blood lactate / mmol dm |
|---|---|---|---|
Wild-type | 42 | 100 | 4 |
Gene X knockout | 18 | 45 | 7 |
Describe two differences between wild-type and gene X knockout mice shown in the data.
[2]
Explain the logic of using a gene knockout to investigate the function of gene X.
[2]
Suggest a likely function of gene X based on the data.
[1]
Question 34
A section of the same gene was compared in six vertebrate species. The figure shows the percentage similarity of three regions: a coding region for an active site, an intron and a regulatory sequence active during early development.
Identify the region that is least conserved among the species.
[1]
Suggest why the active-site coding region is highly conserved.
[2]
Suggest why a non-coding regulatory sequence can also be highly conserved.
[1]
Question 35
A section of a coding sequence from a gene and three variants found in a population are shown. The variants each differ from the original sequence at one nucleotide position.
Variant | Coding DNA sequence (5'3') | Amino acid sequence |
|---|---|---|
Original | ATG GAA TTT CAG GGC | Met–Glu–Phe–Gln–Gly |
Variant 1 | ATG GAG TTT CAG GGC | Met–Glu–Phe–Gln–Gly |
Variant 2 | ATG GAA TCT CAG GGC | Met–Glu–Ser–Gln–Gly |
Variant 3 | ATG GAA TTT TAG GGC | Met–Glu–Phe–Stop |
I.
Identify the molecular type of mutation shown in each variant.
[1]
II.
Explain how the three variants could have different effects on the polypeptide.
[3]
Evaluate the use of a commercial genetic test that reports one of these variants as being associated with future disease risk.
[3]
Question 36
A culture of bacteria is exposed to ultraviolet radiation and then transferred to a medium containing an antibiotic. A small number of colonies grow on the antibiotic medium.
I.
Define mutagen.
[1]
II.
Explain how mutations could arise in the bacterial culture.
[3]
Evaluate the claim that the antibiotic caused the bacteria to make the exact mutation needed for resistance.
[3]
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Question 37
In an island population of insects, a new allele changes the colour of the body surface. Birds more easily detect one colour form than the other.
I.
Explain why mutation is the original source of the new allele.
[2]
II.
Explain why most mutations are neutral or harmful but mutation remains essential in the long term.
[2]
Discuss how natural selection could affect variation within this population and biological diversity between populations.
[3]
Question 38
Four possible guide RNAs were designed for CRISPR-Cas9 editing of a disease-associated allele. Computer analysis predicted on-target cutting and possible off-target cutting. The proposed uses of the edit are also shown.
Guide RNA | On-target cutting / % | Coding off-target sites | Non-coding off-target sites | Proposed use |
|---|---|---|---|---|
A | 84 | 4 | 1 | Embryo editing |
B | 96 | 0 | 1 | Somatic treatment |
C | 81 | 1 | 3 | Gene drive |
D | 89 | 2 | 5 | Enhancement |
Identify the guide RNA that would be the best technical choice for somatic treatment, based only on editing efficiency and off-target risk.
[1]
Explain why a guide RNA with several off-target sites in coding sequences may be unsafe.
[2]
Evaluate why international harmonization of regulation is especially important for the proposed uses shown in the table.
[2]
Question 39
Two hypotheses were tested to explain why a gene sequence is highly conserved in several mammal species. Region M is part of the gene that encodes a binding domain. Region N is nearby and has no known function. The table shows mutation-rate estimates from family sequencing and the proportion of new variants predicted to reduce fitness.
Region | DNA sequence similarity across mammals / % | Fixed differences across mammals / number | Mutation rate / per site per generation | New variants predicted to reduce fitness / % |
|---|---|---|---|---|
M (binding domain) | 99.8 | 1 | 1.8 | 84 |
N (no known function) | 98.7 | 5 | 1.2 | 18 |
Compare the evidence for conservation of regions M and N.
[2]
Discuss which hypothesis is better supported for the high conservation of region M.
[3]
Question 40
A gene contains a coding sequence that is translated from a fixed start point. Two mutant versions of the gene are shown relative to the original sequence.
I.
Distinguish between substitution, insertion and deletion mutations at the molecular level.
[3]
II.
Explain why the single-nucleotide deletion is likely to have a more severe effect than the deletion of three nucleotides.
[2]
Compare the likely consequences of insertions or deletions with those of base substitutions for protein function.
[3]
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Question 41
A patient has a tumour in lung tissue. Sequencing of the tumour cells shows mutations in genes involved in DNA repair and control of the cell cycle. The same mutations are not detected in blood cells from the patient.
I.
Compare the inheritance of mutations in germ cells and somatic cells.
[2]
II.
Explain how somatic mutations can lead to cancer.
[3]
Discuss whether the tumour sequencing result alone can be used to predict the genetic risk to the patient’s future children.
[3]
Question 42
A gene encodes an enzyme. A mutation changes the sequence near the enzyme’s active site. The enzyme is a protein polymer made from amino acid subunits.
I.
Compare the possible effects of a base substitution and a deletion in this region of the gene.
[3]
II.
Explain how variation in the subunit sequence of DNA or protein can change enzyme function.
[2]
Evaluate the statement: “A single base substitution near an active site will always destroy enzyme function.”
[3]
Question 43
Researchers studying a newly sequenced gene in mice produce a knockout strain in which the gene is inoperative. They compare normal mice, knockout mice and knockout mice in which a normal copy of the gene has been restored.
I.
Define gene knockout.
[1]
II.
Explain how the data could be used to infer the function of the gene.
[3]
Evaluate the usefulness of knockout libraries in model organisms for investigating gene function.
[4]
Question 44
A plant model organism has two similar genes, Gene A and Gene B. Researchers make single knockouts and a double knockout to investigate their roles in root development.
I.
Deduce what the knockout results suggest about the functions of Gene A and Gene B.
[2]
II.
Suggest two controls or comparisons that would strengthen the conclusion.
[2]
Discuss why a gene knockout may fail to reveal the full function of a gene.
[3]
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Question 45
A treatment for a blood disorder removes blood-forming stem cells from a patient, edits them using CRISPR-Cas9 so that fetal haemoglobin production increases, and returns the edited cells to the same patient.
I.
Explain the roles of guide RNA, CRISPR sequences and Cas9 in gene editing.
[3]
II.
State the role of hydrogen bonding in targeting the DNA sequence.
[1]
Discuss why this treatment is considered a successful somatic use of CRISPR-Cas9 rather than an inherited genome edit.
[4]
Question 46
A DNA sequence from the same gene is compared in several vertebrate species. One short region is almost identical in all species, while nearby regions contain many differences.
Species | region (nt) | Central region (nt) | region (nt) |
|---|---|---|---|
Human | ATGCTA | CCGTACGA | TTAAGC |
Mouse | GTCGAA | CCGTACGA | CCTTGT |
Chicken | ATCCGA | CCGTACGA | GGAATC |
Lizard | TTTGCA | CCGTACGA | ACTGGA |
Zebrafish | GACGTT | CCGTATGA | TGCATA |
I.
Identify the central block in the alignment using the appropriate term.
[1]
II.
Explain the functional requirement hypothesis for the conservation of this sequence.
[3]
Compare and contrast the functional requirement hypothesis with the slower mutation rate hypothesis for conserved sequences.
[4]
Question 47
A research group proposes using CRISPR-Cas9 to edit embryos to introduce a variant associated with increased muscle strength. The work would be allowed under regulations in one country but prohibited in several others.
I.
Compare somatic cell editing with embryo editing in terms of inheritance and ethical risk.
[3]
II.
Explain why international efforts to harmonize regulation of genome editing are important.
[2]
Evaluate whether the proposed embryo editing should be implemented before broad international agreement is reached.
[2]
Question 48
A conserved non-coding sequence near a developmental gene is identified in fish, birds and mammals. Researchers plan to use CRISPR-Cas9 to disrupt the sequence in a mouse model and observe embryo development.
I.
Explain why disrupting this conserved sequence could affect development.
[3]
II.
Suggest how a knockout experiment could be designed to test the sequence’s function.
[2]
Evaluate the claim that conservation of the sequence proves that it has an essential function in mice.
[3]
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