Rucete ✏ AP Biology In a Nutshell
15. Regulation and Mutations — Practice Questions 2
This chapter explores how organisms control gene expression through various regulatory mechanisms, and how mutations can introduce genetic variation or disrupt normal biological function.
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(Multiple Choice — Click to Reveal Answer)
1. A bacterial operon is normally transcribed unless the end product of the operon is present and binds to a regulatory protein. What type of operon is this most likely describing?
(A) Inducible operon
(B) Repressible operon
(C) Eukaryotic enhancer
(D) RNA interference pathway
Answer
(B) — A repressible operon is typically on but can be turned off when the end product (corepressor) activates the repressor.
2. Which of the following mechanisms contributes to cell differentiation in multicellular organisms despite all cells having identical DNA?
(A) Alternative splicing of ribosomal RNA
(B) Variation in transcription factor expression
(C) Differences in amino acid sequences
(D) Variation in operon structures
Answer
(B) — Cell differentiation is driven by selective gene expression regulated by transcription factors.
3. What is the likely effect of removing a silencer region from a eukaryotic gene?
(A) Transcription of the gene decreases
(B) The gene becomes inducible
(C) The gene may become overexpressed
(D) RNA polymerase will not bind to the promoter
Answer
(C) — Silencers suppress transcription; removing them can increase gene expression.
4. What is the function of small interfering RNA (siRNA) in eukaryotic gene regulation?
(A) Enhances RNA polymerase activity
(B) Binds mRNA to prevent translation
(C) Inserts mutations into DNA
(D) Attaches to promoter regions
Answer
(B) — siRNA binds to specific mRNAs and targets them for degradation, preventing translation.
5. Which of the following best describes the impact of histone acetylation on gene expression?
(A) DNA becomes more tightly packed, reducing transcription
(B) RNA polymerase is degraded
(C) Transcription factors are blocked from enhancers
(D) Chromatin structure loosens, increasing transcription
Answer
(D) — Acetylation reduces histone-DNA interaction, loosening chromatin and allowing access for transcription machinery.
6. Which term describes the regulation of gene expression at the level of chromatin structure?
(A) Epistasis
(B) Epigenetics
(C) Post-translational modification
(D) Frameshift regulation
Answer
(B) — Epigenetic mechanisms like DNA methylation and histone modification influence gene expression without altering DNA sequence.
7. What type of mutation changes an amino acid codon into a stop codon?
(A) Missense
(B) Silent
(C) Nonsense
(D) Frameshift
Answer
(C) — A nonsense mutation introduces a premature stop codon, halting protein synthesis early.
8. Which protein binds to enhancer sequences to increase transcription in eukaryotic cells?
(A) RNA polymerase
(B) Corepressor
(C) Activator protein
(D) Ribosome
Answer
(C) — Activators bind enhancers to increase the rate of transcription by helping RNA polymerase bind more effectively.
9. Which of the following best describes a silent mutation?
(A) A mutation that deletes an entire gene
(B) A mutation that changes an amino acid
(C) A mutation that has no effect on the amino acid sequence
(D) A mutation that causes a frameshift
Answer
(C) — Silent mutations change a nucleotide but do not change the encoded amino acid.
10. What component is common to both inducible and repressible operons in prokaryotes?
(A) Enhancer sequences
(B) Spliceosome binding sites
(C) Operator region
(D) Poly-A tail
Answer
(C) — Both types of operons have an operator region that binds a repressor to regulate transcription.
11. Which of the following regulatory mechanisms occurs after transcription but before translation?
(A) DNA methylation
(B) RNA splicing
(C) Protein folding
(D) Histone acetylation
Answer
(B) — RNA splicing removes introns from pre-mRNA in eukaryotic cells before translation occurs.
12. Which mutation is most likely to result in a completely nonfunctional protein?
(A) Silent
(B) Missense
(C) Nonsense
(D) Insertion of three nucleotides
Answer
(C) — Nonsense mutations cause premature stop codons, truncating the protein and usually rendering it nonfunctional.
13. Which molecule can bind to DNA and physically block the action of RNA polymerase in prokaryotes?
(A) Helicase
(B) Enhancer protein
(C) Repressor protein
(D) Small interfering RNA
Answer
(C) — Repressor proteins bind to the operator and block RNA polymerase from transcribing the operon.
14. What describes the ability of a single gene to produce multiple proteins in eukaryotes?
(A) Gene duplication
(B) Operon rearrangement
(C) Alternative splicing
(D) Histone methylation
Answer
(C) — Alternative splicing allows different combinations of exons to be joined, producing multiple proteins from a single gene.
15. Which of the following is most likely to be heritable across cell divisions without changing the DNA sequence?
(A) Histone acetylation
(B) A silent point mutation
(C) A transcription error
(D) Ribosome skipping
Answer
(A) — Epigenetic marks like histone acetylation can be inherited through mitosis and influence gene expression.
16. What is the name of the segment of DNA that RNA polymerase binds to in both prokaryotes and eukaryotes?
(A) Terminator
(B) Operator
(C) Promoter
(D) Enhancer
Answer
(C) — The promoter is the site where RNA polymerase binds to initiate transcription.
17. Which of the following would likely decrease transcription in a eukaryotic cell?
(A) Increased histone acetylation
(B) Decreased DNA methylation
(C) Loss of activator binding
(D) Opening of chromatin structure
Answer
(C) — Activators are needed to promote transcription; their absence would reduce gene expression.
18. Which type of gene mutation involves a shift in the reading frame?
(A) Point mutation
(B) Silent mutation
(C) Missense mutation
(D) Frameshift mutation
Answer
(D) — Frameshift mutations result from insertions or deletions that disrupt the triplet reading frame of the genetic code.
19. What is the role of enhancer elements in gene expression?
(A) Inhibit translation of mRNA
(B) Provide a site for RNA splicing
(C) Increase transcription rate when bound by activators
(D) Repress gene transcription when methylated
Answer
(C) — Enhancers are regulatory sequences that, when bound by activators, increase transcription efficiency.
20. What would be the effect of a mutation that removes the stop codon from a gene?
(A) Transcription would never begin
(B) The protein would be shorter than normal
(C) The ribosome would continue translating until it reaches the next available stop signal, creating an extended protein
(D) Splicing would remove the entire gene
Answer
(C) — Without a stop codon, translation would proceed beyond the intended end, producing a longer, often nonfunctional protein.
21. Which of the following regulatory strategies is most commonly used to turn genes off in eukaryotes?
(A) Use of polycistronic mRNA
(B) Binding of transcription activators
(C) Methylation of DNA near the promoter
(D) Translation by ribosomes
Answer
(C) — Methylation of promoter regions typically silences gene expression by making the DNA less accessible to transcription machinery.
22. What is the effect of inserting or deleting one or two nucleotides in a coding region?
(A) Silent mutation
(B) Frameshift mutation
(C) Point mutation
(D) Epigenetic modification
Answer
(B) — Insertion or deletion of one or two bases shifts the reading frame, changing every codon after the mutation.
23. What best defines the term “mutation”?
(A) A change in RNA structure
(B) A regulated expression of a gene
(C) A permanent change in the DNA sequence
(D) A breakdown of proteins in a cell
Answer
(C) — A mutation is a permanent alteration in the nucleotide sequence of DNA.
24. What determines whether a mutation will alter the phenotype of an organism?
(A) Its location in an intron
(B) Whether it occurs during transcription
(C) Whether it affects the structure or function of a protein
(D) Whether it is inherited
Answer
(C) — Only mutations that alter protein function or expression typically affect the phenotype.
25. What is the primary effect of histone deacetylation on gene expression?
(A) Enhances translation
(B) Promotes RNA splicing
(C) Condenses chromatin and reduces transcription
(D) Opens DNA for easier replication
Answer
(C) — Removing acetyl groups from histones tightens chromatin, making DNA less accessible and reducing gene expression.
26. A point mutation occurs in the operator sequence of a repressible operon, preventing the repressor from binding. What is the most likely outcome?
(A) The operon will be permanently repressed
(B) The operon will be overexpressed regardless of end product concentration
(C) Transcription will only occur in the presence of a corepressor
(D) RNA polymerase will stop at the promoter
Answer
(B) — Without repressor binding to the operator, RNA polymerase can transcribe continuously regardless of feedback regulation.
27. A mutation causes the histone acetyltransferase enzyme to become nonfunctional. What is the most likely consequence for gene expression?
(A) Global increase in transcription
(B) Increased mRNA stability
(C) Chromatin remains tightly packed and transcription decreases
(D) Frameshift mutations increase
Answer
(C) — Acetylation loosens chromatin; without it, chromatin stays condensed and transcription decreases.
28. Why can the same mutation in a gene have different phenotypic effects in different individuals?
(A) DNA replication varies between people
(B) The ribosome decodes codons differently in each person
(C) Genetic background and environmental factors modulate expression
(D) Each individual has a different set of codons
Answer
(C) — The expression of mutations can be influenced by other genes and environmental conditions, leading to variable phenotypes.
29. What would be the effect of a mutation that leads to constitutive expression of the trp operon in bacteria?
(A) Tryptophan would not be synthesized at all
(B) The trp repressor would be permanently bound to the operator
(C) The cell would continuously make tryptophan even when it is abundant
(D) RNA polymerase would only bind if tryptophan is absent
Answer
(C) — If the repressor cannot bind due to mutation, the operon will be expressed regardless of tryptophan levels, wasting energy.
30. Which of the following mechanisms allows a gene to be expressed at different levels in different cell types in a multicellular organism?
(A) Different codon usage
(B) Use of unique start codons
(C) Combinations of cell-type-specific transcription factors
(D) Multiple ribosomes on the same mRNA
Answer
(C) — Cell-type-specific transcription factors regulate gene expression in response to developmental and environmental signals.
31. A chromosomal mutation leads to the relocation of an enhancer far from its original gene. What is a likely consequence?
(A) The gene becomes polycistronic
(B) The gene is still transcribed at the same rate
(C) The gene may lose proper regulation and be underexpressed
(D) RNA polymerase skips transcription initiation
Answer
(C) — Enhancer relocation can prevent it from interacting with the promoter, reducing transcription efficiency.
32. What explains how a single nucleotide substitution can cause both a missense mutation and a disease phenotype like sickle cell anemia?
(A) The substitution disrupts the chromatin loop
(B) It replaces an amino acid, changing protein shape and function
(C) It alters tRNA synthesis
(D) It activates a pseudogene
Answer
(B) — A single base change leads to one amino acid substitution that dramatically alters hemoglobin’s structure and function.
33. A eukaryotic cell lacks functional transcription factors for a specific gene. What is the likely outcome?
(A) The gene will be translated but not transcribed
(B) The gene will be methylated
(C) RNA polymerase cannot bind the promoter and transcription will not occur
(D) Introns will be retained
Answer
(C) — Transcription factors are essential for RNA polymerase binding; without them, transcription cannot initiate.
34. What would happen if a mutation occurred in a splice site that prevented proper removal of an intron?
(A) A longer protein would be synthesized
(B) Transcription would not initiate
(C) A nonfunctional protein may be produced due to frameshift or early stop codon
(D) Ribosomes would fail to bind the mRNA
Answer
(C) — Improper splicing can introduce frameshifts or premature stops, leading to malfunctioning proteins.
35. A mutation causes excessive histone methylation of a tumor suppressor gene promoter. What is the most likely effect?
(A) The gene is overexpressed, leading to cell cycle arrest
(B) Transcription is silenced, increasing cancer risk
(C) Translation is enhanced, protecting the cell
(D) The protein undergoes enhanced folding
Answer
(B) — Excessive methylation silences gene expression, and if a tumor suppressor is silenced, uncontrolled cell division can occur.
36. Explain why a mutation in a regulatory gene can have broader effects than a mutation in a structural gene.
Answer
Regulatory genes control the expression of many other genes; a mutation here can disrupt entire pathways, whereas a structural gene mutation typically affects a single protein product.
37. What is the role of DNA methyltransferase enzymes in gene regulation?
Answer
They add methyl groups to cytosine bases in DNA, often leading to gene silencing by preventing transcription factor binding or promoting chromatin compaction.
38. How might a mutation that eliminates a stop codon affect the resulting protein?
Answer
The ribosome would continue translating beyond the intended endpoint, potentially creating a longer, nonfunctional or unstable protein.
39. Describe one way that histone modification can affect gene expression without altering the DNA sequence.
Answer
Histone acetylation loosens chromatin, making DNA more accessible for transcription, while methylation can either activate or repress genes depending on the location and context.
40. Why might a mutation in an enhancer region lead to reduced gene expression even if the coding sequence is unchanged?
Answer
Enhancers increase transcription efficiency; a mutation could disrupt activator binding, reducing transcription despite a normal coding region.
41. What is a potential evolutionary advantage of having redundant codons in the genetic code?
Answer
Redundancy allows some mutations to be silent, reducing the likelihood of harmful effects and increasing genetic robustness.
42. A researcher discovers that a certain gene is expressed only in the liver. How might gene regulation explain this tissue-specific expression?
Answer
Liver cells express specific transcription factors or activators that bind to regulatory regions of the gene, promoting its expression only in that tissue.
43. Why are frameshift mutations often more damaging than point mutations?
Answer
Frameshifts alter the entire reading frame, changing every downstream codon and usually producing a completely dysfunctional protein.
44. Describe how microRNAs (miRNAs) regulate gene expression post-transcriptionally.
Answer
miRNAs bind to complementary sequences in mRNA, leading to mRNA degradation or inhibition of translation, thereby decreasing protein production.
45. What would be the result of a loss-of-function mutation in a gene encoding a tumor suppressor protein?
Answer
The cell may lose regulation over the cell cycle or apoptosis, increasing the risk of uncontrolled cell division and cancer.
46. Explain how chromatin remodeling affects access to gene promoters.
Answer
Chromatin remodeling rearranges nucleosomes, making promoter regions more or less accessible to transcription factors and RNA polymerase, thus regulating gene expression.
47. What is one mechanism by which a point mutation could affect mRNA splicing?
Answer
A point mutation at a splice site could prevent correct removal of an intron, leading to inclusion of intronic sequences or exon skipping in the mature mRNA.
48. How can an epigenetic modification be inherited without changing the DNA sequence?
Answer
Epigenetic marks like DNA methylation and histone modifications can be passed through cell division and affect gene expression in daughter cells without altering nucleotide sequences.
49. What type of mutation is least likely to be harmful: missense, nonsense, frameshift, or silent? Explain why.
Answer
Silent mutations are least harmful because they do not alter the amino acid sequence due to redundancy in the genetic code.
50. Describe how a mutation in a transcription factor gene could result in developmental abnormalities.
Answer
Transcription factors regulate genes essential for development; a mutation may disrupt timing, location, or levels of gene expression, leading to abnormal tissue or organ formation.