A Level Biology H3 Genetics Inheritance Quiz

Answer Key: A-Level Biology H3 Quiz - Genetics Inheritance

Total Marks: 75

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Section A: Multiple-Choice Questions (20 marks)

Q	Answer	Mark
1	B	1
2	A	1
3	B	1
4	B	1
5	B	1
6	A	1
7	B	1
8	C	1
9	B	1
10	B	1

Explanations:

Q1. Answer: B

• Concept: SCNT involves transferring the nucleus from a differentiated somatic cell into an enucleated egg cell. The egg cytoplasm contains factors that reprogram the donor nucleus back to a pluripotent state.
• Why A is wrong: The egg cell provides the cytoplasm and mitochondria, not the nuclear genome. The donor nucleus provides the nuclear genome.
• Why C is wrong: SCNT uses differentiated somatic cells (e.g., skin fibroblasts), not embryonic stem cells.
• Why D is wrong: The cloned organism is genetically identical to the nuclear donor, not the egg donor.

Q2. Answer: A

• Method: The recognition site is 6 base pairs long. The probability of a specific 6-base sequence occurring at any given position is (1/4)^6 = 1/4096. For a 10,000 bp linear molecule, the expected number of sites is approximately 10,000 / 4096 ≈ 2.44. This is closest to 2.
• Why C is wrong: 24 is incorrect because it would require a much higher frequency of the recognition sequence.

Q3. Answer: B

• Concept: Ribozymes are RNA molecules with catalytic activity. Their three-dimensional structure, formed by folding, creates an active site that can bind substrates and lower activation energy.

Q4. Answer: B

• Concept: Eukaryotic genes contain introns that bacteria cannot splice out. Using a cDNA copy (synthesised from mature mRNA) ensures the gene lacks introns and can be correctly expressed in E. coli.

Q5. Answer: B

• Concept: DNA methylation at CpG islands in promoter regions is typically associated with gene silencing. Methylation can recruit proteins that compact chromatin or block transcription factor binding.

Q6. Answer: A

• Concept: Imprinted genes are expressed from only one parental allele. If the maternal allele is the expressed one and the mother is heterozygous for a loss-of-function mutation, all offspring who inherit the mutant allele from her will lack a functional copy and show the mutant phenotype.

Q7. Answer: B

• Concept: Plant tissue culture uses somatic cells to produce genetically identical clones rapidly, preserving desirable traits.

Q8. Answer: C

• Concept: DNA ligase catalyses the formation of phosphodiester bonds between the sugar-phosphate backbone of the gene and the plasmid, sealing the recombinant DNA molecule.

Q9. Answer: B

• Concept: Sexual selection arises from competition for mates or mate choice, often leading to traits that differ between sexes.

Q10. Answer: B

• Concept: The microbiota provides colonisation resistance against pathogens and stimulates the development and maturation of immune tissues, including both innate and adaptive components.

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Section B: Structured Questions (30 marks)

Q11. (a) Definition of epigenetics [2]

• Answer: Epigenetics refers to heritable changes in gene expression that do not involve changes to the underlying DNA sequence. (1 mark for "heritable changes in gene expression", 1 mark for "without changes to DNA sequence").

Q11. (b) Two molecular mechanisms [4]

• Mechanism 1: DNA methylation (2 marks): Methylation of cytosine residues in CpG dinucleotides, particularly in promoter regions, can recruit methyl-CpG-binding proteins that compact chromatin or block transcription factor binding, leading to gene silencing.
• Mechanism 2: Histone modification (2 marks): Acetylation of histone tails (e.g., by histone acetyltransferases) neutralises the positive charge on lysine residues, loosening chromatin structure and promoting transcription. Deacetylation (by histone deacetylases) has the opposite effect, compacting chromatin and silencing genes. (Other modifications like methylation or phosphorylation are also acceptable.)

Q11. (c) Inheritance of epigenetic marks [4]

• Answer:
  • During DNA replication, the methylation pattern on the parental strand is copied to the newly synthesised daughter strand by maintenance methyltransferases (e.g., DNMT1), which recognise hemimethylated CpG sites. (2 marks)
  • Histone modifications can be copied by the recruitment of histone-modifying enzymes to newly assembled nucleosomes, although this is less well understood. (1 mark)
  • Implications: Epigenetic marks can be stably inherited through mitotic divisions, allowing cell identity to be maintained. In some cases, they can also be transmitted transgenerationally through meiosis, potentially affecting the phenotype of offspring without changing the DNA sequence. This challenges the traditional view of heredity and has implications for understanding disease risk and evolution. (1 mark)

Q12. (a) Reason for using cDNA [1]

• Answer: The genomic DNA contains introns that E. coli cannot splice out. Using cDNA (synthesised from mature mRNA) ensures the gene is intron-free and can be correctly expressed in bacteria.

Q12. (b) Blue-white screening [3]

• Answer:
  • The plasmid contains the lacZ gene, which encodes β-galactosidase. This enzyme can cleave the substrate X-gal to produce a blue product. (1 mark)
  • When a foreign gene is inserted into the lacZ gene (at the multiple cloning site), the lacZ gene is disrupted, and β-galactosidase is not produced. (1 mark)
  • Bacteria transformed with recombinant plasmids will form white colonies on X-gal medium, while bacteria with non-recombinant (intact lacZ) plasmids will form blue colonies. (1 mark)

Q12. (c) Potential problem and solution [2]

• Answer:
  • Problem: E. coli may not perform post-translational modifications (e.g., glycosylation) correctly, leading to a non-functional or improperly folded protein. (1 mark)
  • Solution: Use a eukaryotic expression system (e.g., yeast, insect, or mammalian cells) that can carry out the required modifications. Alternatively, engineer the protein to be secreted into the periplasm to allow disulfide bond formation. (1 mark)

Q13. (a) Role of reverse transcriptase [2]

• Answer: Reverse transcriptase is used to synthesise a complementary DNA (cDNA) strand from an mRNA template. (1 mark) This cDNA copy lacks introns and can be cloned into a vector for expression in bacteria. (1 mark)

Q13. (b) Self-ligation explanation [2]

• Answer: The plasmid and foreign DNA are both cut with the same restriction enzyme, producing complementary sticky ends. (1 mark) If the foreign DNA is present in low concentration or the ligation reaction is not optimised, the complementary ends of the linearised plasmid may re-anneal and be ligated by DNA ligase, reforming a circular plasmid without the foreign insert. (1 mark)

Q13. (c) Experimental control [2]

• Answer: Perform a control ligation where the digested plasmid is incubated with ligase in the absence of foreign DNA. (1 mark) If colonies are obtained from this control, they will contain only self-ligated plasmids. Comparing the number of colonies from the experimental (with insert) and control (without insert) plates allows estimation of the proportion of recombinant vs. self-ligated plasmids. (1 mark)

Q14. (a) Two properties of plasmids [2]

• Answer: (Any two of the following, 1 mark each)
  • Small size, allowing easy manipulation and transformation.
  • Contain an origin of replication (ori) for autonomous replication in bacteria.
  • Contain selectable markers (e.g., antibiotic resistance genes) to identify transformed cells.
  • Have multiple cloning sites (MCS) with unique restriction enzyme recognition sites.

Q14. (b) Ribozyme catalysis [3]

• Answer: Ribozymes are RNA molecules that fold into complex three-dimensional structures. (1 mark) This folding creates an active site with specific functional groups (e.g., 2'-OH groups, nitrogenous bases) that can participate in catalysis. (1 mark) The active site can bind specific substrates, orient them correctly, and lower the activation energy of the reaction (e.g., by stabilising the transition state or by acting as a general acid/base catalyst). (1 mark)

Q14. (c) Application of ribozymes [1]

• Answer: (Any one of the following, 1 mark)
  • Gene therapy: Design ribozymes to specifically cleave and inactivate disease-causing mRNAs (e.g., viral RNA or oncogene transcripts).
  • Biosensors: Engineer ribozymes to change their catalytic activity in response to a specific ligand, allowing detection of that ligand.
  • Synthetic biology: Use ribozymes as components of genetic circuits or as tools for RNA processing.

Q15. (a) Distinguish between CpG islands and histone acetylation [2]

• Answer:
  • CpG islands are regions of DNA with a high density of CpG dinucleotides, often found in promoter regions. Methylation of cytosine in CpG islands is associated with gene silencing. (1 mark)
  • Histone acetylation is a post-translational modification of histone proteins. Acetylation of lysine residues neutralises their positive charge, loosening chromatin structure and promoting gene expression. (1 mark)

Q15. (b) Hypermethylation in cancer [2]

• Answer: Hypermethylation of the promoter region of a tumour suppressor gene can silence its expression. (1 mark) Since tumour suppressor genes normally inhibit cell proliferation or promote DNA repair, their silencing removes a critical brake on cell growth, contributing to cancer development. (1 mark)

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Section C: Free-Response Questions (25 marks)

Q16. Significance of genetic engineering [12]

• Marking scheme:
  • Food sustainability (4 marks): Discuss genetic modification of crops for increased yield, pest resistance (e.g., Bt crops), herbicide tolerance, drought tolerance, and enhanced nutritional content (e.g., Golden Rice). Benefits: reduced pesticide use, increased food production, improved nutrition. Risks: potential environmental impacts (gene flow to wild relatives), development of resistant pests, ethical concerns about corporate control of seeds.
  • Disease treatment (4 marks): Discuss production of therapeutic proteins (e.g., insulin, growth hormone, clotting factors) in engineered organisms. Benefits: large-scale production of safe, human-compatible proteins. Risks: potential for contamination, high cost, need for rigorous quality control.
  • Drug design (4 marks): Discuss use of genetic engineering in drug discovery (e.g., target identification, high-throughput screening) and development of gene therapies (e.g., using viral vectors to deliver functional genes). Benefits: targeted therapies, potential cures for genetic diseases. Risks: off-target effects, immune responses, ethical issues with germline editing.
  • Overall evaluation (up to 2 additional marks): Balanced discussion of benefits and risks, recognition of the need for regulation and ethical oversight.

Q17. Cellular reprogramming [13]

• Marking scheme:
  • Explanation of reprogramming (5 marks): Describe the process of induced pluripotent stem cell (iPSC) generation. Introduction of four transcription factors (Oct4, Sox2, Klf4, c-Myc) into differentiated cells (e.g., fibroblasts) using viral vectors or other methods. These factors reprogram the cell to a pluripotent state, similar to embryonic stem cells. Explain the concept of pluripotency.
  • Potential applications (5 marks):
    • Disease modelling: Create iPSCs from patients with genetic diseases to study disease mechanisms in relevant cell types.
    • Drug screening: Test potential drugs on patient-specific iPSC-derived cells.
    • Cell replacement therapy: Generate healthy cells (e.g., neurons, cardiomyocytes, pancreatic beta cells) for transplantation to treat degenerative diseases (e.g., Parkinson's, heart disease, diabetes).
  • Ethical considerations (3 marks):
    • Avoids the ethical issues associated with using human embryos (unlike embryonic stem cells).
    • Concerns about the use of viral vectors (risk of insertional mutagenesis).
    • Potential for tumour formation (especially if c-Myc is used).
    • Issues of consent and ownership of patient-derived cells.
    • Concerns about the safety and efficacy of iPSC-derived therapies.

Q18. CRISPR-Cas9 gene editing [10]

• Marking scheme:
  • Description of process (4 marks): Explain that CRISPR-Cas9 is a bacterial adaptive immune system adapted for gene editing. It consists of two components: the Cas9 nuclease and a guide RNA (gRNA). The gRNA is designed to be complementary to a target DNA sequence. The gRNA binds to Cas9 and directs it to the target site. Cas9 creates a double-strand break (DSB) at the target site. The cell repairs the DSB via non-homologous end joining (NHEJ) or homology-directed repair (HDR).
  • Creating a knockout mouse model (4 marks): Design a gRNA targeting the gene of interest. Inject Cas9 protein and gRNA into a fertilised mouse egg (zygote). The Cas9-gRNA complex creates a DSB in the target gene. NHEJ repair often introduces small insertions or deletions (indels) that disrupt the gene's reading frame, creating a knockout allele. The edited zygote is implanted into a surrogate mother. Offspring are screened for the desired mutation. Heterozygous mice can be bred to generate homozygous knockouts.
  • Potential limitation (2 marks): Off-target effects: Cas9 may cleave at sites with partial complementarity to the gRNA, causing unintended mutations. This can lead to unpredictable phenotypes or toxicity. Other limitations include mosaicism in edited embryos, difficulty in delivering components to specific cell types, and ethical concerns about germline editing.

Q19. Mitochondrial vs. autosomal dominant inheritance [10]

• Marking scheme:
  • Comparison (4 marks):
    • Mitochondrial inheritance: Maternal inheritance (only passed from mother to all offspring). Affected males do not pass the trait to their children. High heteroplasmy (mixture of mutant and wild-type mitochondria within a cell).
    • Autosomal dominant inheritance: Equal transmission from males and females. Affected individuals have a 50% chance of passing the trait to each child. Homogeneous genotype (one mutant allele, one wild-type allele).
  • Variable expressivity in mitochondrial diseases (3 marks): Explain that heteroplasmy levels vary between cells and tissues due to random segregation of mitochondria during cell division. The threshold effect: a certain proportion of mutant mitochondria is needed to cause dysfunction. Different tissues have different energy requirements and thresholds. This leads to variable symptoms even within the same family.
  • Challenges in treatment (3 marks): Difficulty in delivering therapeutic agents to mitochondria. The need to correct a high proportion of mutant mtDNA to achieve a therapeutic effect. Lack of effective gene therapy tools for mtDNA. Complexity of mitochondrial genetics and the interplay with nuclear genes. Limited understanding of the full spectrum of mitochondrial diseases.

Q20. Epigenetics in cancer development [10]

• Marking scheme:
  • Role of epigenetic modifications (4 marks): Explain that cancer is driven by both genetic mutations and epigenetic alterations. Key epigenetic changes include:
    • Global DNA hypomethylation: Can lead to genomic instability and activation of oncogenes.
    • Promoter hypermethylation of tumour suppressor genes: Silences genes that normally control cell growth, DNA repair, and apoptosis (e.g., BRCA1, p16, MLH1).
    • Histone modifications: Altered patterns of acetylation and methylation can affect chromatin structure and gene expression.
  • Novel therapeutic strategies (4 marks):
    • DNA methyltransferase inhibitors (e.g., 5-azacytidine, decitabine): Reactivate silenced tumour suppressor genes by reversing DNA methylation.
    • Histone deacetylase inhibitors (e.g., vorinostat, romidepsin): Increase histone acetylation, opening chromatin and promoting expression of silenced genes.
    • Combination therapies: Using epigenetic drugs in combination with traditional chemotherapy or immunotherapy to enhance efficacy.
  • Advantages over genetic approaches (2 marks):
    • Epigenetic changes are potentially reversible, making them attractive drug targets.
    • Epigenetic drugs can affect multiple genes simultaneously, potentially targeting the entire cancer epigenome.
    • May be effective against cancers with few targetable genetic mutations.
    • Can potentially overcome drug resistance by reactivating silenced genes.