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18. A researcher is studying DNA replication in human cells. He identifies an enzyme that synthesizes short RNA sequences on the lagging strand to initiate DNA synthesis. Which of the following enzymes is he most likely studying?
A) DNA polymerase I
B) DNA polymerase III
C) Primase
D) Ligase
E) Helicase
Answer: C) Primase
Explanation: Primase synthesizes short RNA primers to provide a 3’-OH group for DNA polymerase to initiate synthesis.
17. A mutation results in the loss of nucleotide excision repair (NER) activity. This mutation is most likely associated with which of the following conditions?
A) Xeroderma pigmentosum
B) Lynch syndrome
C) Ataxia-telangiectasia
D) Bloom syndrome
E) Fanconi anemia
Answer: A) Xeroderma pigmentosum
Explanation: XP is caused by defective nucleotide excision repair, leading to increased sensitivity to UV light.
16. A scientist is investigating post-transcriptional modifications of mRNA. Which of the following processes occurs in the nucleus before the mRNA is exported to the cytoplasm?
A) Addition of the poly-A tail
B) Charging of tRNA
C) Translation initiation
D) Splicing of ribosomal RNA
E) Ribosome assembly
Answer: A) Addition of the poly-A tail
Explanation: The poly-A tail, 5′ capping, and splicing are all post-transcriptional modifications that occur in the nucleus before mRNA export.
15. A researcher studying prokaryotic gene expression introduces lactose to a culture of E. coli. She observes increased expression of genes involved in lactose metabolism. This is primarily due to:
A) Activation of a sigma factor
B) Binding of the lac repressor to the operator
C) Removal of the repressor from the operator site
D) Binding of a corepressor to the operator
E) Methylation of the promoter region
Answer: C) Removal of the repressor from the operator site
Explanation: In the lac operon, lactose binds the repressor, causing it to detach from the operator, allowing transcription.
14. A mutation affects a gene coding for a zinc finger protein involved in the regulation of transcription. Which of the following best describes the likely function of this mutated protein?
A) Helicase unwinding DNA
B) Enhancer binding to DNA
C) Transcription factor binding to DNA
D) DNA polymerase proofreading
E) Spliceosome component
Answer: C) Transcription factor binding to DNA
Explanation: Zinc finger proteins are DNA-binding domains commonly found in transcription factors.
13. A child presents with severe immunodeficiency, facial abnormalities, and photosensitivity. Laboratory analysis shows a defect in non-homologous end joining. Which of the following disorders is most consistent with this presentation?
A) Ataxia-telangiectasia
B) Fanconi anemia
C) Lynch syndrome
D) Bloom syndrome
E) Severe combined immunodeficiency (SCID, ATM type)
Answer: A) Ataxia-telangiectasia
Explanation: Ataxia-telangiectasia is caused by mutations in the ATM gene, affecting DNA double-strand break repair via non-homologous end joining.
12. A researcher investigates gene silencing through DNA methylation in human cells. Which of the following is the most likely outcome of increased methylation in a gene’s promoter region?
A) Enhanced gene transcription
B) Decreased mRNA degradation
C) Increased mRNA splicing
D) Inhibition of gene expression
E) Increased DNA replication
Answer: D) Inhibition of gene expression
Explanation: DNA methylation of CpG islands in promoter regions leads to transcriptional silencing.
11. A mutation affects the spliceosome machinery in a eukaryotic cell. This would most directly impair which of the following processes?
A) Addition of the 5’ cap
B) Polyadenylation
C) Removal of introns
D) Export of mRNA from the nucleus
E) Ribosome binding to mRNA
Answer: C) Removal of introns
Explanation: The spliceosome removes introns from pre-mRNA during processing.
10. A pharmacologist studies an antibiotic that binds the 30S ribosomal subunit of prokaryotes and prevents the attachment of aminoacyl-tRNA. Which of the following antibiotics is most likely being studied?
A) Chloramphenicol
B) Tetracycline
C) Erythromycin
D) Rifampin
E) Streptomycin
Answer: B) Tetracycline
Explanation: Tetracyclines bind to the 30S subunit and prevent attachment of aminoacyl-tRNA to the A site, inhibiting protein synthesis.
9. A mutation in a gene leads to loss of function of a protein that normally halts the cell cycle in the presence of DNA damage. This mutation increases the risk for various cancers. Which of the following genes is most likely mutated?
A) RAS
B) BCL-2
C) MYC
D) TP53
E) HER2
Answer: D) TP53
Explanation: TP53 is a tumor suppressor gene that encodes p53, which halts the cell cycle at the G1/S checkpoint in response to DNA damage. Loss of function leads to uncontrolled proliferation.
8. A researcher discovers a gene that promotes unregulated cell growth when overexpressed. The gene product is part of a signal transduction pathway. This gene is most likely an example of which of the following?
A) Tumor suppressor gene
B) Oncogene
C) DNA repair gene
D) Housekeeping gene
E) Telomerase gene
Answer: B) Oncogene
Explanation: Oncogenes are mutated or overexpressed versions of proto-oncogenes that promote cell growth and proliferation.
7. A scientist uses a method to amplify a specific DNA sequence from a small tissue biopsy. The method requires primers, heat-stable DNA polymerase, and cycles of heating and cooling. Which of the following techniques is being used?
A) Northern blot
B) Southern blot
C) Polymerase chain reaction (PCR)
D) Western blot
E) ELISA
Answer: C) Polymerase chain reaction (PCR)
Explanation: PCR amplifies specific DNA sequences using primers and Taq polymerase through repeated heating/cooling cycles.
6. A researcher is studying programmed cell death. She identifies mitochondrial release of cytochrome c as a key event in the intrinsic pathway. This process is most directly regulated by which of the following protein families?
A) Caspases
B) BCL-2 family
C) Integrins
D) Cyclins
E) Heat shock proteins
Answer: B) BCL-2 family
Explanation: BCL-2 family proteins regulate mitochondrial membrane permeability and cytochrome c release, thus controlling intrinsic apoptosis.
5. A mutation in a G-protein coupled receptor causes increased cAMP production and overactivation of protein kinase A (PKA). This signaling cascade is most likely mediated through which of the following G protein subunits?
A) Gq
B) Gs
C) Gi
D) Go
E) Gt
Answer: B) Gs
Explanation: Gs activates adenylate cyclase, increasing cAMP, which activates PKA. This is common in hormone signaling pathways (e.g., TSH, ACTH).
4. A scientist wants to detect and compare the expression of a specific protein in tumor vs. normal cells. Which of the following lab techniques is most appropriate for this purpose?
A) Northern blot
B) Southern blot
C) Western blot
D) PCR
E) FISH
Answer: C) Western blot
Explanation: Western blot detects proteins using antibodies, allowing analysis of protein expression levels.
3. A 6-month-old infant presents with lethargy, vomiting, and hepatomegaly after prolonged fasting. Labs show hypoglycemia and elevated dicarboxylic acids. A deficiency in which of the following enzymes is most likely responsible?
A) Glucose-6-phosphatase
B) Carnitine acyltransferase I
C) Medium-chain acyl-CoA dehydrogenase (MCAD)
D) Pyruvate dehydrogenase
E) Galactose-1-phosphate uridyltransferase
Answer: C) Medium-chain acyl-CoA dehydrogenase (MCAD)
Explanation: MCAD deficiency impairs β-oxidation, causing hypoketotic hypoglycemia and accumulation of medium-chain fatty acids during fasting.
2. A patient with colon cancer is found to have a mutation in a tumor suppressor gene responsible for DNA mismatch repair. Family history reveals multiple relatives with early-onset colorectal and endometrial cancer. What is the most likely diagnosis?
A) Familial adenomatous polyposis
B) Peutz-Jeghers syndrome
C) Lynch syndrome (HNPCC)
D) Li-Fraumeni syndrome
E) Cowden syndrome
Answer: C) Lynch syndrome (HNPCC)
Explanation: Lynch syndrome is due to defective DNA mismatch repair genes (e.g., MLH1, MSH2), predisposing to colorectal and endometrial cancers.
1. A 30-year-old woman develops muscle cramps and dark urine after intense exercise. Muscle biopsy shows increased glycogen content with normal structure. Which of the following enzymes is most likely deficient?
A) Glucose-6-phosphatase
B) Glycogen phosphorylase (muscle isoform)
C) Debranching enzyme
D) Branching enzyme
E) Phosphofructokinase-1
Answer: B) Glycogen phosphorylase (muscle isoform)
Explanation: This is McArdle disease (type V glycogen storage disease)—due to a muscle glycogen phosphorylase deficiency, leading to impaired glycogenolysis during exercise.
Simon Downes, MD, PhD 