Rucete ✏ AP Biology In a Nutshell
15. Regulation and Mutations — Practice Questions
This chapter introduces how gene expression is regulated in prokaryotes and eukaryotes, the role of mutations in genetic variation, and the mechanisms by which changes in DNA impact phenotype.
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(Multiple Choice — Click to Reveal Answer)
1. In a repressible operon, which of the following events would most likely occur when the operon’s end product is abundant in the environment?
(A) Structural genes are transcribed more frequently
(B) The repressor protein dissociates from the operator
(C) The repressor protein binds the corepressor and attaches to the operator
(D) RNA polymerase is more active at the promoter
Answer
(C) — In repressible operons, the presence of the corepressor (such as an amino acid) activates the repressor to bind the operator and block transcription.
2. Which of the following best describes an inducible operon system?
(A) Repressor protein is always bound to the operator
(B) The operon functions to synthesize essential amino acids
(C) The presence of an inducer removes the repressor and enables transcription
(D) RNA polymerase requires a corepressor to bind to the promoter
Answer
(C) — In inducible operons, an inducer molecule binds to the repressor, preventing it from blocking the operator, thereby initiating transcription.
3. Which of the following genetic elements in a bacterial operon is considered a noncoding regulatory sequence?
(A) Structural gene
(B) Ribosome binding site
(C) Operator
(D) Coding sequence for the repressor protein
Answer
(C) — The operator is a noncoding region where the repressor protein binds to control transcription.
4. What feature distinguishes repressible operons from inducible operons?
(A) Repressible operons lack a promoter region
(B) Repressible operons require an inducer to activate gene expression
(C) The repressor protein in repressible operons must bind a corepressor to function
(D) RNA polymerase binds directly to the operator in repressible operons
Answer
(C) — Repressible operons require the end product to act as a corepressor that activates the repressor protein, which then binds to the operator to shut down transcription.
5. Which environmental condition would result in the highest expression level of the lac operon in E. coli?
(A) High glucose, high lactose
(B) Low glucose, low lactose
(C) High glucose, no lactose
(D) Low glucose, high lactose
Answer
(D) — High lactose (inducer present) and low glucose (increases cAMP-CAP activation) lead to the highest expression of the lac operon.
6. What is the primary function of the promoter region in gene expression?
(A) It codes for a repressor protein
(B) It binds RNA polymerase to initiate transcription
(C) It enhances ribosomal activity
(D) It splices introns from mRNA
Answer
(B) — The promoter is the DNA sequence where RNA polymerase binds to begin transcription.
7. Which of the following best describes a missense mutation?
(A) A mutation that causes a premature stop codon
(B) A mutation that does not alter the amino acid sequence
(C) A substitution that changes one amino acid to another
(D) A mutation that deletes the entire gene
Answer
(C) — Missense mutations change one amino acid in the polypeptide sequence by substituting a single base.
8. Which type of mutation is least likely to affect the function of a protein?
(A) Nonsense mutation
(B) Missense mutation
(C) Silent mutation
(D) Frameshift mutation
Answer
(C) — Silent mutations change a codon but do not change the resulting amino acid due to the redundancy of the genetic code.
9. Which molecule is responsible for regulating gene expression in eukaryotic cells by binding to enhancers?
(A) Ribosomes
(B) Transcription factors
(C) tRNA
(D) Repressor protein
Answer
(B) — Transcription factors are proteins that bind to enhancers or promoters to regulate transcription.
10. What is the role of a repressor protein in prokaryotic gene regulation?
(A) It enhances transcription by binding to the promoter
(B) It blocks RNA polymerase from transcribing the genes
(C) It splices pre-mRNA
(D) It recruits ribosomes to the mRNA
Answer
(B) — Repressor proteins bind to the operator to block transcription by RNA polymerase in operon systems.
11. Which type of mutation shifts the reading frame of a gene?
(A) Silent
(B) Missense
(C) Frameshift
(D) Nonsense
Answer
(C) — Frameshift mutations result from insertions or deletions that alter the grouping of codons.
12. Which of the following is an example of negative gene regulation?
(A) An activator protein enhances RNA polymerase activity
(B) A transcription factor opens up chromatin
(C) A repressor binds to the operator to block transcription
(D) RNA splicing removes introns
Answer
(C) — Negative regulation involves repressors inhibiting transcription by blocking RNA polymerase.
13. What determines whether a mutation will impact the phenotype of an organism?
(A) The length of the gene
(B) Whether it occurs in introns
(C) Whether it changes the amino acid sequence or protein function
(D) The total number of mutations in the genome
Answer
(C) — Only mutations that alter gene products (e.g., protein shape or function) affect phenotype.
14. Which of the following is TRUE about the lac operon?
(A) It is repressible and always active
(B) It is inducible and only active in the presence of lactose
(C) It is found only in eukaryotes
(D) It does not respond to glucose concentration
Answer
(B) — The lac operon is an inducible operon activated when lactose is present and glucose is low.
15. What kind of mutation is most likely to produce a nonfunctional protein?
(A) Silent
(B) Nonsense
(C) Point mutation
(D) Synonymous
Answer
(B) — Nonsense mutations introduce a premature stop codon, truncating the protein and often rendering it nonfunctional.
16. What is the function of a corepressor in a repressible operon system?
(A) It blocks RNA polymerase from binding to the promoter
(B) It binds to the repressor to activate it
(C) It serves as the operator
(D) It increases translation directly
Answer
(B) — Corepressors bind to the repressor protein to activate it, allowing it to bind to the operator and inhibit transcription.
17. Which of the following statements best describes an enhancer element in eukaryotic cells?
(A) It is a protein that binds to mRNA
(B) It is a DNA sequence that increases transcription when bound by activators
(C) It blocks the function of repressors
(D) It is found only in prokaryotic operons
Answer
(B) — Enhancers are regulatory DNA elements that increase transcription when bound by specific transcription factors.
18. What kind of regulation allows multiple genes to be turned on or off together in prokaryotic cells?
(A) RNA interference
(B) Alternative splicing
(C) Operon systems
(D) Methylation
Answer
(C) — Operons are clusters of genes under the control of a single promoter, allowing coordinated gene regulation.
19. Why are frameshift mutations often more damaging than point mutations?
(A) They affect only one base pair
(B) They often cause silent mutations
(C) They shift the entire reading frame, altering all downstream amino acids
(D) They are easily corrected by DNA polymerase
Answer
(C) — Frameshift mutations alter the grouping of codons, usually changing every amino acid after the mutation.
20. What is a common result of mutations in the promoter region of a gene?
(A) The protein product is longer
(B) RNA polymerase cannot bind effectively, reducing transcription
(C) The gene becomes dominant
(D) The introns are skipped during splicing
Answer
(B) — Mutations in the promoter can interfere with RNA polymerase binding, reducing or eliminating transcription.
21. What is the typical effect of DNA methylation on gene expression in eukaryotes?
(A) Increased translation
(B) Activation of repressor proteins
(C) Gene silencing
(D) DNA replication inhibition
Answer
(C) — DNA methylation typically inhibits gene expression by condensing chromatin or blocking transcription factor binding.
22. Which statement about mutations is CORRECT?
(A) All mutations are harmful
(B) Only point mutations change DNA sequences
(C) Mutations can be beneficial, neutral, or harmful
(D) Mutations occur only during translation
Answer
(C) — Mutations vary in effect: some may confer advantages, others cause diseases, and many have no effect at all.
23. What term describes a permanent change in the DNA sequence of a gene?
(A) Gene expression
(B) Mutation
(C) Translation
(D) Duplication
Answer
(B) — A mutation is any permanent change in the DNA sequence of an organism’s genome.
24. Which factor most directly affects the binding of RNA polymerase in eukaryotic transcription?
(A) Ribosome density
(B) Repressor proteins
(C) General transcription factors and promoter sequences
(D) Introns and exons
Answer
(C) — In eukaryotes, RNA polymerase requires general transcription factors to bind to promoter regions like the TATA box.
25. Which mutation type involves the replacement of one nucleotide with another?
(A) Insertion
(B) Deletion
(C) Substitution
(D) Translocation
Answer
(C) — Substitution mutations change a single nucleotide base to another.
26. A mutation changes the operator sequence of a prokaryotic operon so that the repressor cannot bind. What is the likely effect?
(A) The operon will be permanently repressed
(B) Transcription will occur continuously, even without the inducer
(C) RNA polymerase will stop binding to the promoter
(D) The structural genes will be deleted
Answer
(B) — If the repressor cannot bind to the operator, RNA polymerase is free to transcribe the genes at all times.
27. A nonsense mutation occurs in the middle of a gene. What is the likely outcome for the protein product?
(A) A single amino acid substitution
(B) A longer-than-normal protein
(C) A truncated, likely nonfunctional protein
(D) No effect due to redundancy
Answer
(C) — Nonsense mutations introduce premature stop codons, resulting in shortened, nonfunctional proteins.
28. Which of the following best explains how a single nucleotide change can lead to sickle cell disease?
(A) It creates a frameshift mutation in the hemoglobin gene
(B) It deletes a codon involved in iron binding
(C) It replaces a hydrophilic amino acid with a hydrophobic one
(D) It removes the poly-A tail from the mRNA
Answer
(C) — In sickle cell disease, a single base substitution changes glutamic acid to valine, altering the hemoglobin structure.
29. Which of the following regulatory mechanisms is unique to eukaryotic gene expression?
(A) Operator-repressor binding
(B) Activator proteins binding enhancers far from the promoter
(C) Ribosome binding directly to the mRNA without processing
(D) Polycistronic transcription of gene clusters
Answer
(B) — Eukaryotes utilize enhancers and activators located far upstream or downstream of the promoter region.
30. In a mutation study, an insertion of three nucleotides is observed within a gene. What is the most likely outcome?
(A) A frameshift mutation that alters every amino acid
(B) A premature stop codon
(C) An addition of one amino acid to the protein sequence
(D) Complete loss of gene function
Answer
(C) — Inserting three bases adds one codon, resulting in the insertion of a single amino acid without a frameshift.
31. A gene is normally silent but becomes active after demethylation of its promoter. What does this suggest about DNA methylation?
(A) Methylation activates histone acetylation
(B) Methylation directly enhances ribosome binding
(C) Methylation suppresses gene expression by altering chromatin structure
(D) Methylation substitutes for enhancer binding
Answer
(C) — DNA methylation typically silences genes by promoting chromatin condensation, making DNA less accessible to transcription factors.
32. What is the primary function of a transcription factor in eukaryotic gene regulation?
(A) It synthesizes mRNA
(B) It replicates DNA
(C) It helps RNA polymerase bind to the promoter
(D) It adds methyl groups to cytosine bases
Answer
(C) — Transcription factors are required for RNA polymerase to recognize and bind promoters in eukaryotes.
33. In which situation would a silent mutation most likely affect the phenotype?
(A) When it occurs in an intron
(B) When it occurs in a regulatory region
(C) When the codon still encodes the same amino acid
(D) When it replaces a codon with a stop codon
Answer
(B) — Even if the protein-coding sequence is unaffected, changes in regulatory elements can alter gene expression levels, affecting phenotype.
34. Which epigenetic change would most likely increase gene expression?
(A) DNA methylation of promoter regions
(B) Histone deacetylation
(C) Histone acetylation
(D) Binding of repressors to enhancers
Answer
(C) — Histone acetylation loosens chromatin, making DNA more accessible for transcription.
35. A protein-coding gene is mutated so that its repressor-binding operator sequence is deleted. What effect is expected?
(A) The gene will be permanently turned off
(B) Transcription will proceed constitutively
(C) The protein will be translated incorrectly
(D) The ribosome will fail to bind mRNA
Answer
(B) — Without an operator, the repressor cannot bind, and transcription cannot be regulated—it continues unregulated.
36. Explain how a frameshift mutation can affect the resulting protein even if it occurs far from the start codon.
Answer
A frameshift changes the reading frame, altering all downstream codons and likely producing a completely different, often nonfunctional protein.
37. What is the role of transcription factors in eukaryotic gene expression?
Answer
Transcription factors bind to specific DNA sequences like enhancers or promoters and help RNA polymerase initiate transcription.
38. Describe the difference between an inducible operon and a repressible operon in prokaryotes.
Answer
Inducible operons are usually off and are activated in the presence of an inducer, while repressible operons are usually on and are deactivated when a corepressor is present.
39. How does DNA methylation regulate gene expression?
Answer
DNA methylation adds methyl groups to cytosine bases, often silencing genes by preventing transcription factor binding or promoting chromatin condensation.
40. Predict the effect of a point mutation that changes a codon to a stop codon near the start of a gene.
Answer
This nonsense mutation would produce a truncated, likely nonfunctional protein, severely affecting cellular function.
41. Why might a silent mutation still impact an organism’s phenotype?
Answer
If it occurs in a regulatory region or affects mRNA stability or splicing, a silent mutation can still influence gene expression levels.
42. How does histone acetylation affect chromatin structure and gene expression?
Answer
Acetylation of histone tails reduces their positive charge, loosening DNA-histone interactions and making genes more accessible for transcription.
43. Describe one way a mutation in a regulatory gene can impact the expression of multiple genes.
Answer
If a transcription factor gene is mutated, it can disrupt the regulation of all target genes that rely on that factor for expression.
44. How can mutations contribute to evolution?
Answer
Beneficial mutations introduce genetic variation, which may provide advantages in certain environments and be passed on through natural selection.
45. A eukaryotic gene is heavily methylated and not expressed. What change could lead to its activation?
Answer
Demethylation of the promoter region could reactivate the gene by making the DNA accessible to transcription machinery.
46. Why are nonsense mutations typically more harmful than missense mutations?
Answer
Nonsense mutations introduce premature stop codons, often producing nonfunctional truncated proteins, while missense mutations may still produce functional proteins.
47. How does the lac operon demonstrate negative feedback regulation?
Answer
When lactose is absent, a repressor binds to the operator to shut off gene expression, conserving energy and resources.
48. What is the consequence of a mutation that deletes an enhancer element in eukaryotic DNA?
Answer
The associated gene may be expressed at lower levels or not at all, since enhancer elements increase transcription efficiency when bound by activators.
49. Describe how small noncoding RNAs (e.g., miRNAs) regulate gene expression in eukaryotic cells.
Answer
miRNAs bind to complementary mRNA sequences and either degrade the mRNA or block its translation, reducing protein production.
50. A mutation changes a regulatory sequence in a gene’s promoter. How might this affect transcription?
Answer
It could reduce or eliminate transcription if RNA polymerase or transcription factors can no longer bind effectively to the promoter.