IB Biology Quiz: Apply Gene Expression
11 questions · exam conditions
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Apply Gene ExpressionQuestion 1 of 11

Nutrients such as folate and vitamin B12 are crucial for the synthesis of S-adenosylmethionine (SAM), the body's primary methyl group donor. A severe deficiency in these nutrients would most directly impair which epigenetic process?

The ability to add acetyl groups to histone tails, leading to widespread gene silencing.
The establishment and maintenance of DNA methylation patterns, potentially leading to global hypomethylation.
The process of X-chromosome inactivation, causing both X chromosomes to become active in all female cells.
The recruitment of histone deacetylases (HDACs) to gene promoters, resulting in global hyperacetylation.
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IB Biology Quiz

IB Biology Quiz: Apply Gene Expression

Practice Apply Gene Expression in IB Biology with focused quiz questions that help you check what you know, review explanations, and build confidence with test-style prompts.

What this quiz covers

This quiz focuses on Apply Gene Expression, giving you a quick way to practice the rules, question types, and explanations that matter most for IB Biology.

How to use this quiz

Try each quiz question before looking at the correct answer. Use the explanations to review missed ideas, then come back to similar questions until the pattern feels familiar.

All questions

Question 1

Nutrients such as folate and vitamin B12 are crucial for the synthesis of S-adenosylmethionine (SAM), the body's primary methyl group donor. A severe deficiency in these nutrients would most directly impair which epigenetic process?

  1. The ability to add acetyl groups to histone tails, leading to widespread gene silencing.
  2. The establishment and maintenance of DNA methylation patterns, potentially leading to global hypomethylation. (correct answer)
  3. The process of X-chromosome inactivation, causing both X chromosomes to become active in all female cells.
  4. The recruitment of histone deacetylases (HDACs) to gene promoters, resulting in global hyperacetylation.
Explanation: DNA methylation requires the transfer of a methyl group to cytosine bases. This methyl group is supplied by the universal methyl donor, SAM. The synthesis of SAM is dependent on a metabolic pathway involving folate and vitamin B12. A deficiency in these nutrients would deplete the pool of SAM, thereby impairing the ability of DNA methyltransferases (DNMTs) to methylate DNA. This would most directly affect the maintenance of methylation patterns. Acetylation (A) uses acetyl-CoA, not SAM. HDAC recruitment (D) is not directly linked to methyl group supply. While X-inactivation (C) involves methylation, the most direct and widespread effect would be on all methylation processes (B).

Question 2

Studies on rats show that high levels of maternal grooming in early life lead to reduced anxiety in offspring. This is correlated with decreased methylation of the glucocorticoid receptor (GR) gene promoter. How does this epigenetic change lead to the observed phenotype?

  1. Decreased methylation leads to silencing of the GR gene, which in turn causes an increase in the stress response.
  2. Decreased methylation increases transcription of the GR gene, leading to a more effective negative feedback on the stress response. (correct answer)
  3. Increased methylation increases transcription of the GR gene, which enhances the feedback mechanism for the stress response.
  4. The maternal care directly alters the DNA sequence of the GR gene, creating a more active allele which is then inherited.
Explanation: Decreased DNA methylation is associated with increased gene transcription. The glucocorticoid receptor (GR) is essential for the negative feedback loop that shuts down the physiological stress response. More GR expression leads to a more efficient shutdown of the stress response, resulting in reduced anxiety. Therefore, decreased methylation leads to increased GR transcription and a calmer phenotype. Choice A incorrectly links decreased methylation to silencing. Choice C incorrectly links increased methylation to increased transcription. Choice D describes a genetic change, whereas the mechanism is epigenetic.

Question 3

A researcher identifies a compound that enhances the activity of histone acetyltransferases (HATs). If this compound is applied to a culture of human cells, what is the most likely widespread effect on gene expression?

  1. A general decrease in transcription, as added acetyl groups cause chromatin to condense and block RNA polymerase access.
  2. Specific silencing of genes containing CpG islands, as HATs are known to recruit DNA methyltransferases to these sites.
  3. A general increase in transcription, as acetyl groups on histone tails neutralize positive charges, leading to a looser chromatin structure. (correct answer)
  4. No significant change in transcription, as histone modification is secondary to DNA methylation in regulating gene expression.
Explanation: Histone acetyltransferases (HATs) add acetyl groups to lysine residues on histone tails. This neutralizes their positive charge, weakening the interaction between histones and negatively charged DNA. The result is a more open chromatin structure (euchromatin), which is more accessible to transcription machinery, leading to a general increase in gene expression. Acetylation causes decondensation, not condensation (A). HATs are associated with gene activation, not silencing via methylation (C). Histone modification is a primary and crucial mechanism of gene regulation (D).

Question 4

Researchers study a gene, GeneX, in individuals subjected to chronic stress. The levels of GeneX mRNA are significantly lower than in controls. Further analysis reveals no difference in the DNA sequence of GeneX between groups, but the stressed group shows reduced histone H3 lysine 9 acetylation (H3K9ac) at the GeneX promoter. What can be deduced?

  1. Chronic stress causes a deletion mutation in the enhancer region of GeneX.
  2. The individuals in the stressed group inherited a less active allele of GeneX from their parents.
  3. The GeneX protein product is being degraded more rapidly in the stressed group, leading to lower mRNA levels via feedback.
  4. The reduced mRNA levels are likely due to increased activity of histone deacetylases (HDACs) at the GeneX promoter. (correct answer)
Explanation: The data show lower mRNA levels (less transcription) are correlated with reduced histone acetylation (a mark for active transcription). This suggests a shift towards a more repressive chromatin state. Reduced acetylation can be caused by either decreased activity of histone acetyltransferases (HATs) or increased activity of histone deacetylases (HDACs). Therefore, increased HDAC activity is a very plausible cause. Choices A and D are ruled out because the DNA sequence is unchanged. Choice C proposes a post-translational mechanism, but the evidence (histone acetylation) points to a transcriptional control mechanism.

Question 5

In certain types of cancer, tumour suppressor genes are found to be transcriptionally silent despite having no mutations in their coding sequences. What epigenetic modification is the most likely explanation for this observation?

  1. Increased acetylation of histones near the gene's promoter, leading to a more open and accessible chromatin structure.
  2. Decreased methylation of CpG islands within the gene's promoter, allowing transcription factors to bind more easily.
  3. Increased methylation of CpG islands within the gene's promoter, preventing the binding of transcription machinery. (correct answer)
  4. A frameshift mutation in the gene's promoter region, altering the binding site for RNA polymerase.
Explanation: Increased or 'hypermethylation' of CpG islands in the promoter region of a gene is a common epigenetic mechanism for silencing gene expression. This modification blocks transcription factors and RNA polymerase from accessing the DNA. Since the stem specifies that a tumour suppressor gene is silenced, this is the most plausible mechanism. Acetylation (A) activates genes. Decreased methylation (B) would also activate the gene. A mutation (D) is a genetic, not epigenetic, change and is excluded by the premise.

Question 6

Exposure to a certain environmental fungicide is linked to health problems in the offspring of exposed mice, even when the offspring are not directly exposed. This effect is associated with hypermethylation of a key metabolic gene, MetG. Which statement provides the most plausible explanation?

  1. The fungicide acts as a mutagen, causing a permanent deletion in the MetG gene that is passed to the offspring.
  2. The fungicide increases histone acetylation at the MetG locus, causing a permanent increase in its expression.
  3. The offspring learn behaviours from the exposed parent that lead to the silencing of the MetG gene postnatally.
  4. The fungicide induces an epigenetic change in the germline cells of the parent, which is then inherited by the offspring. (correct answer)
Explanation: This scenario describes transgenerational epigenetic inheritance. For an effect to be passed to offspring that were not directly exposed, the change must occur in the germ cells (sperm or egg) of the parent. The fungicide likely alters the methylation patterns in these cells, and this altered epigenome is then transmitted to the zygote. Choice A describes a genetic, not epigenetic, mechanism. Choice C is a behavioural explanation, not a molecular one. Choice D describes a contradictory epigenetic effect (activation via acetylation).

Question 7

Monozygotic twins are genetically identical at birth, yet they often show increasing phenotypic differences as they age, such as in their susceptibility to certain diseases. Which statement best explains this phenomenon of phenotypic discordance?

  1. Spontaneous mutations accumulate at different rates in each twin, leading to significant genetic divergence over their lifetime.
  2. The twins' differing environmental exposures and lifestyles lead to divergent patterns of DNA methylation and histone modifications. (correct answer)
  3. The initial zygote contained mosaicism, so the twins were never truly genetically identical and these differences become more pronounced.
  4. One twin consistently expresses the dominant alleles while the other expresses recessive alleles due to random mitotic events.
Explanation: The primary reason for increasing phenotypic differences between monozygotic twins as they age is the accumulation of different epigenetic marks. Environmental factors (diet, stress, toxins) and stochastic (random) events cause their epigenomes to diverge over time. This leads to different gene expression profiles and, consequently, different phenotypes. While somatic mutations (A) do occur, they are not considered the main driver of this widespread, progressive divergence. Mosaicism (C) is rare and cannot explain the general trend. The mechanism in (D) is not biologically correct.

Question 8

A muscle cell and a neuron from the same individual contain identical genomes but have vastly different structures and functions. Which statement best explains this observation from an epigenetic perspective?

  1. During development, neurons lose the genes required for muscle function, while muscle cells lose the genes for neural function.
  2. Neurons utilize a different genetic code than muscle cells to translate mRNA, resulting in different proteins from the same genes.
  3. The DNA in muscle cells is replicated with higher fidelity than in neurons, leading to fewer mutations and a different proteome.
  4. Different sets of genes are transcribed in each cell type due to distinct patterns of DNA methylation and histone modifications. (correct answer)
Explanation: Cellular differentiation is the process by which a single genome gives rise to many different cell types. This is achieved through differential gene expression, which is controlled by the epigenome. Each cell type establishes and maintains a unique pattern of epigenetic marks (like DNA methylation and histone modifications) that silence genes not needed for that cell type and activate those that are. Gene loss (A) is not the mechanism. Replication fidelity (C) and the genetic code (D) are generally consistent across all somatic cells.

Question 9

Histone deacetylase (HDAC) inhibitors are a class of drugs used in cancer therapy. They are often effective in cancers where tumour suppressor genes have been silenced. What is the molecular mechanism by which these drugs exert their therapeutic effect?

  1. They prevent the removal of acetyl groups from histones, maintaining a transcriptionally active chromatin state at tumour suppressor gene loci. (correct answer)
  2. They add methyl groups to the DNA of oncogenes, leading to their transcriptional silencing and preventing cancer cell proliferation.
  3. They directly bind to and activate silenced tumour suppressor proteins, restoring their function without altering gene expression patterns.
  4. They inhibit the synthesis of histones, causing a catastrophic failure of chromatin packaging and leading to apoptosis of cancer cells.
Explanation: HDACs remove acetyl groups from histones, leading to chromatin condensation and gene silencing. In some cancers, HDACs are overactive, silencing tumour suppressor genes. HDAC inhibitors block this action. As a result, acetyl groups accumulate on histones, chromatin remains open (euchromatin), and the transcription of tumour suppressor genes can resume, helping to control cell growth. Choice B describes a different epigenetic mechanism (DNA methylation). Choice C describes post-translational regulation, not the epigenetic action of these drugs. Choice D describes an incorrect mechanism.

Question 10

In female mammals, one of the two X chromosomes in each somatic cell is randomly inactivated to ensure proper gene dosage. Which combination of epigenetic mechanisms is primarily responsible for maintaining the silenced state of the resulting Barr body?

  1. Extensive histone acetylation and low levels of DNA methylation on the inactive X chromosome.
  2. The binding of specific activator proteins that recruit RNA polymerase to the active X chromosome only.
  3. Widespread histone deacetylation and high levels of DNA methylation on the inactive X chromosome. (correct answer)
  4. The physical removal of the inactive X chromosome from the nucleus before each mitotic division.
Explanation: The inactive X chromosome, or Barr body, is a classic example of facultative heterochromatin. Its silent state is maintained by a combination of repressive epigenetic marks. These include the removal of activating acetyl groups from histones (deacetylation) and the addition of repressive methyl groups to DNA (hypermethylation), both of which contribute to a highly condensed and transcriptionally inert chromatin structure. Choice A describes marks for active chromatin. Choice C is an incomplete explanation focusing only on activation. Choice D is factually incorrect.

Question 11

Which statement provides the best analogy for the relationship between the genome and the epigenome?

  1. The genome is the hardware of a computer, and the epigenome is the software that tells the hardware how to function. (correct answer)
  2. The genome is a book, and the epigenome is a single, permanent correction made to a spelling error in the book.
  3. The genome is a musical score, and the epigenome is the instrument on which the music is played.
  4. The genome is the blueprint for a house, and the epigenome is the supply of bricks and mortar.
Explanation: This is a common and effective analogy. The genome (hardware) is the fixed, underlying information. The epigenome (software) is a layer of instructions that is dynamic, changeable, and dictates how, when, and where the genomic information is read and used by the cell. A correction to a book (B) is more like fixing a mutation. The instrument (C) is more like the cell's transcriptional machinery. The building materials (D) are more like the nucleotides and amino acids.