Rucete ✏ AP Biology In a Nutshell
15. Regulation and Mutations — Practice Questions 3
This chapter explores how genetic information is regulated and modified through operon models, eukaryotic transcriptional control, epigenetic changes, and both vertical and horizontal mutation mechanisms.
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(Multiple Choice — Click to Reveal Answer)
1. Which of the following best describes the relationship between a repressor and an operator in a repressible operon?
(A) The repressor binds directly to the promoter to block RNA polymerase
(B) The repressor needs a corepressor to bind to the operator
(C) The repressor always remains bound to the operator
(D) The repressor causes histone deacetylation near the operator
Answer
(B) — In repressible operons, the repressor protein only becomes active in the presence of a corepressor, allowing it to bind to the operator and inhibit transcription.
2. Which of the following would most likely result from a mutation that inactivates the CAP-binding site of the lac operon?
(A) Reduced transcription even when glucose is low and lactose is present
(B) Transcription only occurs in the presence of both glucose and lactose
(C) The repressor protein cannot bind to the operator
(D) Constitutive expression of lac operon genes
Answer
(A) — Without CAP binding, RNA polymerase has reduced affinity for the promoter even when lactose is available, leading to decreased transcription.
3. Which mutation is most likely to increase gene expression in eukaryotic cells?
(A) Insertion of a stop codon in the first exon
(B) Methylation of promoter CpG islands
(C) Acetylation of histone tails near the promoter
(D) Deletion of the TATA box
Answer
(C) — Histone acetylation loosens chromatin, increasing accessibility to transcription machinery and enhancing gene expression.
4. A cell expresses a regulatory protein that binds to DNA and enhances RNA polymerase recruitment. This protein is most likely a:
(A) Repressor
(B) Transposase
(C) Histone
(D) Transcription activator
Answer
(D) — Activator proteins increase the efficiency of transcription by binding to enhancer elements and facilitating RNA polymerase recruitment.
5. Which of the following mechanisms introduces genetic variation through viral infection in bacteria?
(A) Conjugation
(B) Transformation
(C) Transduction
(D) Transposition
Answer
(C) — Transduction is the process by which bacteriophages transfer genetic material between bacterial cells, introducing variation.
6. What is the role of the TATA box in a eukaryotic gene promoter?
(A) Termination of transcription
(B) Binding site for RNA polymerase and transcription factors
(C) Ribosome assembly site
(D) Location of mRNA splicing
Answer
(B) — The TATA box is a core promoter element that helps position RNA polymerase and general transcription factors to initiate transcription.
7. Which molecule can bind to mRNA and block its translation without degrading it?
(A) snRNA
(B) tRNA
(C) miRNA
(D) DNA polymerase
Answer
(C) — MicroRNAs (miRNAs) can bind to complementary mRNA sequences and inhibit translation without necessarily degrading the transcript.
8. What is a common effect of DNA methylation near a gene’s promoter region?
(A) Increased gene expression
(B) Enhanced translation
(C) Repressed gene transcription
(D) Stabilization of the mRNA transcript
Answer
(C) — DNA methylation near promoter regions is commonly associated with gene silencing in eukaryotic cells.
9. What type of mutation replaces one nucleotide with another but does not change the resulting amino acid?
(A) Nonsense mutation
(B) Missense mutation
(C) Silent mutation
(D) Insertion mutation
Answer
(C) — Silent mutations alter the DNA sequence without changing the encoded amino acid due to redundancy in the genetic code.
10. Which of the following correctly matches a regulatory mechanism with its function?
(A) Operator – site where ribosomes bind
(B) Enhancer – increases transcription in eukaryotes
(C) Corepressor – activates the lac operon
(D) miRNA – edits DNA during replication
Answer
(B) — Enhancers are noncoding DNA elements that bind activator proteins to increase transcription in eukaryotic genes.
11. Which structure is directly involved in regulating operon expression in prokaryotes?
(A) Nucleosome
(B) Operator
(C) Spliceosome
(D) Ribosome
Answer
(B) — The operator is a DNA region in prokaryotic operons that interacts with repressor proteins to regulate transcription.
12. Which type of gene mutation is most likely to have no effect on the protein product?
(A) Frameshift
(B) Nonsense
(C) Silent
(D) Inversion
Answer
(C) — Silent mutations don’t change the amino acid sequence and usually don’t affect protein function.
13. What type of mutation results from the addition of a single base into the DNA sequence of a gene?
(A) Missense mutation
(B) Nonsense mutation
(C) Frameshift mutation
(D) Substitution mutation
Answer
(C) — Inserting a base shifts the reading frame, often altering all downstream codons (frameshift mutation).
14. What is the function of general transcription factors in eukaryotic transcription?
(A) Degrade introns
(B) Help RNA polymerase bind to the promoter
(C) Stabilize ribosomes
(D) Remove methyl groups from DNA
Answer
(B) — General transcription factors assist RNA polymerase in recognizing and binding to promoter sequences.
15. Which type of regulatory molecule prevents transcription by binding directly to the operator in a prokaryotic operon?
(A) Inducer
(B) Activator
(C) Repressor
(D) tRNA
Answer
(C) — Repressors bind to the operator region and block RNA polymerase from transcribing the operon.
16. Which mechanism leads to different mRNA transcripts from the same pre-mRNA in eukaryotic cells?
(A) Histone methylation
(B) Alternative splicing
(C) Operon switching
(D) RNA interference
Answer
(B) — Alternative splicing produces different combinations of exons, generating diverse proteins from one gene.
17. Which type of RNA interference can result in degradation of mRNA transcripts in the cytoplasm?
(A) tRNA
(B) siRNA
(C) snRNA
(D) rRNA
Answer
(B) — Small interfering RNAs (siRNAs) are involved in gene silencing by promoting degradation of specific mRNA targets.
18. A mutation causes an early stop codon in a gene. What is the most likely result?
(A) Silent mutation
(B) Extended translation
(C) A truncated protein
(D) Enhanced transcription
Answer
(C) — A premature stop codon (nonsense mutation) leads to early termination of translation, creating a shortened protein.
19. Which of the following is true about repressible operons?
(A) They are normally inactive and require an inducer
(B) They are typically on and can be turned off by a corepressor
(C) They are only found in eukaryotes
(D) They code only for enzymes involved in DNA replication
Answer
(B) — Repressible operons, like the trp operon, are usually expressed and are shut off when a corepressor binds to the repressor protein.
20. Which of the following changes is most likely to cause gene silencing?
(A) Histone acetylation
(B) DNA demethylation
(C) DNA methylation
(D) Exon duplication
Answer
(C) — DNA methylation of promoter regions typically leads to chromatin condensation and transcriptional silencing.
21. What is the function of the promoter region in gene transcription?
(A) Site where tRNA binds to the ribosome
(B) Site for RNA splicing
(C) Binding site for RNA polymerase and initiation factors
(D) Sequence that terminates translation
Answer
(C) — The promoter is where RNA polymerase and associated factors bind to initiate transcription.
22. A mutation in the coding sequence of a gene changes a polar amino acid to a nonpolar one. What type of mutation is this?
(A) Silent
(B) Missense
(C) Nonsense
(D) Frameshift
Answer
(B) — Missense mutations alter the amino acid sequence, and the effect depends on how different the new amino acid is.
23. What type of mutation does not change the protein product despite changing the DNA sequence?
(A) Silent
(B) Missense
(C) Frameshift
(D) Nonsense
Answer
(A) — Silent mutations do not alter the protein because the new codon still codes for the same amino acid.
24. What is one reason why eukaryotic gene regulation is more complex than in prokaryotes?
(A) Eukaryotes lack introns
(B) Prokaryotes have multiple chromosomes
(C) Eukaryotic transcription involves chromatin structure and multiple regulatory sequences
(D) Eukaryotes use operons
Answer
(C) — Eukaryotic regulation involves chromatin remodeling, enhancers, silencers, and transcription factors, making it more complex than in prokaryotes.
25. Which type of mutation is least likely to change the phenotype?
(A) Nonsense mutation
(B) Missense mutation
(C) Silent mutation
(D) Frameshift mutation
Answer
(C) — Silent mutations do not alter the amino acid sequence and are least likely to impact the phenotype.
26. A mutation disables the repressor protein in the trp operon. What would be the likely result in a bacterial cell?
(A) Transcription would occur only in the absence of tryptophan
(B) The operon would be permanently turned off
(C) Transcription would proceed even when tryptophan is abundant
(D) The ribosome would degrade the mRNA
Answer
(C) — Without a functional repressor, the operon cannot respond to tryptophan levels and remains active regardless of environmental conditions.
27. Which of the following would most likely result from a deletion of the enhancer region in a eukaryotic gene?
(A) Translation is enhanced
(B) Transcription decreases
(C) Splicing fails to occur
(D) DNA replication is halted
Answer
(B) — Enhancers increase transcription rates by binding activators; their deletion reduces transcription efficiency.
28. A base substitution mutation changes an amino acid in an enzyme’s active site. What is the most likely effect on the enzyme?
(A) Increased stability of the gene
(B) Loss of catalytic function due to altered protein shape
(C) Faster mRNA transcription
(D) Faster translation by the ribosome
Answer
(B) — Changing an amino acid in the active site can disrupt substrate binding and catalysis, rendering the enzyme ineffective.
29. A mutation introduces a new start codon upstream of the normal one. What is a possible consequence?
(A) No protein is produced
(B) A longer protein is translated, possibly with incorrect function
(C) The ribosome cannot bind
(D) RNA polymerase terminates transcription prematurely
Answer
(B) — A new upstream start codon could result in a longer protein that may not fold or function properly.
30. Which of the following best describes how miRNAs regulate gene expression?
(A) By binding to DNA and blocking transcription
(B) By modifying histones and relaxing chromatin
(C) By degrading target mRNA or blocking its translation
(D) By attaching to RNA polymerase
Answer
(C) — MicroRNAs (miRNAs) bind to complementary sequences on mRNA molecules, leading to their degradation or inhibition of translation.
31. What type of gene regulation is demonstrated by the presence of both positive and negative control in the lac operon system?
(A) Operon silencing
(B) Dual regulation by inducers and activators
(C) Transcription bypass
(D) Constitutive expression
Answer
(B) — The lac operon is regulated negatively by a repressor and positively by the CAP-cAMP complex when glucose is scarce.
32. A researcher discovers a mutation in a gene’s 3′ untranslated region (UTR). What effect might this mutation have?
(A) It prevents mRNA capping
(B) It could affect mRNA stability and translation regulation
(C) It removes all introns
(D) It disables the ribosome’s A site
Answer
(B) — The 3′ UTR often contains regulatory sequences that influence mRNA degradation and translation efficiency.
33. Which outcome is most likely if a gene's promoter region undergoes heavy methylation?
(A) Enhanced mRNA splicing
(B) Reduced transcription
(C) Increased ribosome activity
(D) Faster translation rate
Answer
(B) — DNA methylation of promoter regions is associated with chromatin condensation and transcriptional silencing.
34. A mutation occurs in the gene encoding a transcription factor, rendering it unable to bind DNA. What is the most likely consequence for target genes?
(A) Increased translation
(B) Delayed mRNA splicing
(C) Reduced or absent transcription of those genes
(D) Loss of ribosome binding sites
Answer
(C) — Transcription factors must bind DNA to initiate transcription; without this interaction, gene expression is compromised.
35. Which of the following scenarios best illustrates a conditional mutation?
(A) A mutation causes a disease only when an individual consumes a specific nutrient
(B) A mutation deletes an exon
(C) A mutation in the promoter eliminates gene expression
(D) A mutation causes a frameshift in the coding region
Answer
(A) — Conditional mutations manifest their effects only under specific environmental or physiological conditions.
36. Explain how the lac operon is regulated by both lactose and glucose availability in E. coli.
Answer
The lac operon is activated when lactose is present (inactivating the repressor) and glucose is scarce (activating cAMP-CAP complex that enhances transcription).
37. Describe how histone modifications can influence transcription without altering the DNA sequence.
Answer
Modifications like acetylation loosen chromatin structure, increasing access to DNA, while methylation can either activate or repress transcription depending on context.
38. What is the effect of a nonsense mutation early in the coding region of a gene?
Answer
An early nonsense mutation introduces a premature stop codon, leading to a truncated and likely nonfunctional protein.
39. A mutation occurs in a splice site of a pre-mRNA. Predict the consequence of this mutation.
Answer
The spliceosome may fail to recognize the intron boundary, potentially leading to retention of introns or skipping of exons, producing an altered or nonfunctional protein.
40. How can DNA methylation patterns contribute to tissue-specific gene expression?
Answer
Genes not needed in a particular tissue are often methylated, silencing their expression, while active genes remain unmethylated and accessible to transcription machinery.
41. A gene that is normally silent becomes active after histone acetylation. Explain why this change occurred.
Answer
Acetylation of histone tails reduces their positive charge, weakening DNA-histone interactions and making DNA more accessible for transcription.
42. What type of mutation would most likely affect the binding of transcription factors to a gene?
Answer
A mutation in the gene's promoter or enhancer regions could disrupt transcription factor binding, reducing or eliminating transcription.
43. How can a mutation in the operator sequence of an operon lead to constitutive gene expression?
Answer
If the operator can no longer bind the repressor protein, RNA polymerase will not be blocked, and the operon will be transcribed continuously.
44. Why is a frameshift mutation typically more harmful than a point mutation?
Answer
Frameshift mutations alter the reading frame, changing all downstream amino acids and usually resulting in a nonfunctional protein.
45. How can mutations in regulatory genes influence the expression of multiple downstream genes?
Answer
Regulatory genes often encode transcription factors; mutations can prevent them from activating or repressing multiple target genes, disrupting entire pathways.
46. Describe the role of the CAP-cAMP complex in regulating the lac operon.
Answer
When glucose is low, cAMP levels rise and bind CAP, which enhances RNA polymerase binding to the promoter, increasing transcription of the lac operon.
47. How can a mutation in a repressor gene lead to uncontrolled gene expression in prokaryotes?
Answer
If the repressor protein is nonfunctional, it cannot bind the operator, allowing RNA polymerase to transcribe genes without regulation.
48. Explain how alternative splicing increases the diversity of proteins encoded by a single gene.
Answer
Alternative splicing allows different combinations of exons to be included in the final mRNA, resulting in multiple protein isoforms from one gene.
49. How can environmental signals influence gene expression in bacteria?
Answer
Environmental changes (e.g., presence of lactose or absence of glucose) can trigger allosteric changes in regulatory proteins that activate or repress gene transcription.
50. Describe how small noncoding RNAs like siRNA or miRNA can regulate gene expression post-transcriptionally.
Answer
These RNAs bind complementary mRNA sequences and either degrade the mRNA or block its translation, reducing the level of protein produced.