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Unit 6 of AP Biology covers gene expression and regulation, one of the most crucial topics for the AP exam. This guide provides classified questions with step-by-step answers and explanations to help you ace the test.
Key Topics Covered in Unit 6:
- DNA structure and replication
- Gene expression and regulation
- Mutations and their effects
- Biotechnology applications
- Viruses and genetic engineering
Classified Questions & Answers
Below are categorized questions based on the main subtopics of Unit 6, along with expert explanations.
1. DNA Replication
Question: What enzyme is responsible for unwinding the DNA double helix during replication?
Answer: Helicase
Explanation: Helicase breaks the hydrogen bonds between base pairs, separating the DNA strands to allow replication to occur.
2. Gene Expression and Regulation
Question: How does an operon function in prokaryotic gene regulation?
Answer: An operon is a group of genes regulated together. In the lac operon, the presence of lactose inactivates the repressor, allowing transcription of enzymes that break down lactose.
Explanation: Operons like the lac operon in bacteria allow for efficient gene regulation in response to environmental changes.
3. Mutations and Their Effects
Question: What is the difference between a point mutation and a frameshift mutation?
Answer: A point mutation changes a single nucleotide, while a frameshift mutation adds or deletes a nucleotide, altering the reading frame.
Explanation: Frameshift mutations are generally more harmful because they affect every codon after the mutation, leading to a completely different protein.
4. Biotechnology Applications
Question: What is the purpose of polymerase chain reaction (PCR)?
Answer: PCR amplifies DNA, creating millions of copies of a specific sequence.
Explanation: PCR is widely used in forensic science, genetic testing, and research to analyze DNA sequences efficiently.
5. Viruses and Genetic Engineering
Question: How do viruses use the lytic and lysogenic cycles to replicate?
Answer: In the lytic cycle, the virus takes over the host cell and replicates rapidly, leading to cell lysis. In the lysogenic cycle, the viral DNA integrates into the host genome and replicates with it, remaining dormant until triggered.
Explanation: Some viruses, like bacteriophages, switch between these cycles depending on environmental conditions.
More questions
1. Lac Operon Regulation
Question:
In the lac operon, when glucose is present and lactose is absent, which statement is true?
A) The repressor is inactive, and the operon is transcribed.
B) The repressor is active, and the operon is not transcribed.
C) CAP is bound to cAMP, increasing transcription.
D) The repressor is bound to the operator, preventing transcription.
Answer: D
Explanation:
When lactose is absent, the repressor binds to the operator, blocking transcription. Glucose availability affects CAP binding (low cAMP when glucose is high), but the repressor’s activity depends on lactose presence.
2. DNA Methylation
Question:
DNA methylation typically results in:
A) Increased transcription due to chromatin remodeling.
B) Decreased transcription due to a more condensed chromatin structure.
C) Activation of enhancer regions.
D) Recruitment of RNA polymerase.
Answer: B
Explanation:
Methylation adds methyl groups to DNA, compacting chromatin (heterochromatin) and reducing accessibility for transcription machinery.
3. RNA Processing
Question:
Which modifications are added to eukaryotic mRNA during processing?
A) 5′ methyl cap
B) Poly-A tail at the 3′ end
C) Introns
D) Both A and B
Answer: D
Explanation:
The 5′ cap and poly-A tail are added post-transcription. Introns are removed during splicing.
4. CRISPR-Cas9 Mechanism
Question:
What is the role of guide RNA (gRNA) in CRISPR-Cas9?
A) It cuts DNA at specific sites.
B) It binds to the target DNA sequence to guide Cas9.
C) It repairs DNA after cutting.
D) It methylates DNA.
Answer: B
Explanation:
gRNA directs Cas9 to the specific DNA sequence for cleavage.
5. Prokaryotic vs. Eukaryotic Regulation
Question:
A key difference between prokaryotic and eukaryotic gene regulation is:
A) Prokaryotes use operons; eukaryotes regulate genes individually.
B) Eukaryotes use transcription factors; prokaryotes do not.
C) Prokaryotic mRNA is polycistronic; eukaryotic mRNA is monocistronic.
D) Both A and C.
Answer: D
Explanation:
Prokaryotes use operons (polycistronic mRNA), while eukaryotes regulate genes individually (monocistronic mRNA).
6. Mutation in Regulatory Gene
Question:
A mutation causing constitutive expression of a metabolic pathway likely affects a:
A) Repressor protein.
B) Enhancer sequence.
C) Promoter sequence.
D) Ribosomal binding site.
Answer: A
Explanation:
A defective repressor cannot bind the operator, leading to constant transcription.
7. Gel Electrophoresis
Question:
Gel electrophoresis separates DNA fragments based on:
A) Charge
B) Size
C) Nucleotide sequence
D) Both A and B
Answer: B
Explanation:
DNA fragments separate by size (smaller fragments migrate faster); charge is uniform (negative).
8. Poly-A Tail Function
Question:
The poly-A tail in mRNA primarily:
A) Facilitates ribosome binding.
B) Protects mRNA from exonuclease degradation.
C) Signals transcription termination.
D) Aids DNA replication.
Answer: B
Explanation:
The poly-A tail stabilizes mRNA by preventing exonuclease attack.
9. RNA Splicing
Question:
Which removes introns from pre-mRNA?
A) Ribosomes
B) DNA polymerase
C) Spliceosomes
D) Helicase
Answer: C
Explanation:
Spliceosomes (snRNPs) excise introns and ligate exons.
10. DNA Methyltransferase Inhibition
Question:
Inhibiting DNA methyltransferase would likely:
A) Increase chromatin condensation.
B) Decrease gene expression.
C) Increase gene expression.
D) Have no effect.
Answer: C
Explanation:
Reduced methylation decreases gene silencing, enhancing expression.
Bonus: RNA Interference (RNAi)
Question:
RNAi functions by:
A) Enhancing translation.
B) Degrading mRNA via siRNA.
C) Methylating DNA.
D) Repairing DNA mutations.
Answer: B
Explanation:
siRNA or miRNA guide RNA-induced silencing complexes (RISC) to degrade target mRNA.
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