DNA Replication, PCR and Repair
A researcher accidentally uses primers that bind to multiple regions of the genome during PCR. What is the most likely outcome?
A) No product is made
B) Only one gene amplifies correctly
C) Multiple DNA fragments of different sizes appear
D) The reaction stops after one cycle
C) Multiple DNA fragments of different sizes appear
Telomerase maintains chromosome stability by:
A) Cutting off damaged DNA ends
B) Adding repetitive DNA to chromosome tips
C) Removing RNA primers at chromosome ends
D) Repairing thymine dimers
B) Adding repetitive DNA to chromosome tips
What is the primary purpose of the 5′ cap and 3′ poly-A tail in eukaryotic mRNA?
A) Catalyze peptide bonds
B) Protect mRNA and assist ribosome binding
C) Regulate DNA replication
D) Act as tRNA binding sites
B) Protect mRNA and assist ribosome binding
Which interaction is responsible for stabilizing tertiary structure by connecting distant cysteine residues?
A) Hydrogen bond
B) Disulfide bridge
C) Ionic interaction
D) Peptide bond
B) Disulfide bridge
DNA methylation typically has which effect on gene transcription?
A) Activates transcription
B) Silences transcription
C) Speeds RNA polymerase binding
D) Induces mutations
B) Silences transcription
A scientist mutates E. coli DNA polymerase so it cannot proofread. After several generations, the bacteria show increased mutation rates. Which enzyme function is directly lost?
A) 3′ → 5′ exonuclease activity
B) 5′ → 3′ exonuclease activity
C) Helicase unwinding
D) RNA primer removal
A) 3′ → 5′ exonuclease activity
A scientist observes fibroblast cultures with rapidly shortening telomeres. What will eventually happen to these cells?
A) They will become cancerous
B) They will stop dividing and enter senescence
C) They will activate telomerase and become stem cells
D) They will lose DNA proofreading ability
B) They will stop dividing and enter senescence
A scientist compares two genes: one intronless and one with multiple introns. The second gene can produce multiple proteins, but the first cannot. Why?
A) The intronless gene lacks codons
B) Introns enable alternative splicing
C) Introns add poly-A tails
D) Introns block RNA degradation
B) Introns enable alternative splicing
A substitution of a hydrophobic amino acid with a polar one in the protein core most likely results in:
A) Improved folding efficiency
B) Aggregation or instability
C) Enhanced hydrophobic core packing
D) No change in stability
B) Aggregation or instability
A histone acetyltransferase (HAT) is overactive in a cell. Predict the effect on gene expression.
A) Transcription increases due to loosened chromatin
B) Transcription decreases due to condensed chromatin
C) mRNA stability decreases
D) Promoter regions become methylated
A) Transcription increases due to loosened chromatin
During PCR optimization, a student forgets to add magnesium ions (Mg²⁺) to the reaction. Which specific step of PCR will fail, and why?
A) Denaturation — strands won’t separate
B) Primer binding — H-bonds can’t form
C) Extension — polymerase requires Mg²⁺ as a cofactor
D) Cooling — primers degrade
C) Extension — polymerase requires Mg²⁺ as a cofactor
Why do germline and stem cells, but not most somatic cells, express telomerase?
A) They divide more frequently and must maintain genome integrity
B) They lack mitochondria for replication energy
C) They don’t experience oxidative stress
D) Their chromosomes are circular
A) They divide more frequently and must maintain genome integrity
A eukaryotic gene is inserted into E. coli, but no functional protein forms. What explains this?
A) Prokaryotes lack RNA polymerase
B) Prokaryotes cannot remove introns from pre-mRNA
C) The bacterial ribosomes are too small
D) The genetic code differs between species
B) Prokaryotes cannot remove introns from pre-mRNA
Why does a single glutamic acid → valine mutation in hemoglobin lead to sickle-cell disease?
A) It alters the gene’s transcription
B) It changes hydrophobicity, causing aggregation of hemoglobin molecules
C) It disrupts peptide bond formation
D) It destroys iron binding sites
B) It changes hydrophobicity, causing aggregation of hemoglobin molecules
Cancer cells treated with both AZA (a DNMT inhibitor) and TSA (a histone deacetylase inhibitor) show re-expression of silenced genes. Why?
A) DNA becomes more compact
B) Chromatin is opened by decreased methylation and increased acetylation
C) RNA polymerase is degraded
D) Telomeres are lengthened
B) Chromatin is opened by decreased methylation and increased acetylation
A patient with xeroderma pigmentosum (XP) develops skin tumors after mild sun exposure. Which molecular process fails in these cells, and what DNA lesion accumulates?
A) Base excision repair; 8-oxoguanine
B) Nucleotide excision repair; thymine dimers
C) Mismatch repair; G–T pairing
D) Double-strand break repair; chromosomal translocation
B) Nucleotide excision repair; thymine dimers
A cancer cell line is treated with a telomerase inhibitor for several weeks. Early on, growth is unaffected, but months later the culture collapses. Why is this delayed effect observed?
A) Telomerase inhibition causes immediate apoptosis
B) Telomeres shorten gradually with each replication cycle
C) The inhibitor blocks RNA polymerase instead
D) The drug affects only RNA splicing
B) Telomeres shorten gradually with each replication cycle
A microRNA (miRNA) complementary to part of a tumor-suppressor mRNA is overexpressed in cancer cells. What is the effect on gene expression?
A) The mRNA is degraded, silencing the tumor suppressor
B) The mRNA translation increases
C) The miRNA prevents transcription from the promoter
D) The miRNA adds a poly-A tail
A) The mRNA is degraded, silencing the tumor suppressor
A nonsense mutation occurs early in a gene’s coding sequence, while a missense mutation occurs near the end. Which likely has a greater impact, and why?
A) Missense — changes charge at C-terminus
B) Nonsense — truncates the entire downstream region
C) Both equal impact
D) Neither affects protein function
B) Nonsense — truncates the entire downstream region
You observe that a stress hormone activates hundreds of genes simultaneously. Which mechanism best explains this?
A) Shared enhancer sequences that bind the same transcription factor
B) Random activation of promoters
C) Increased DNA methylation
D) Loss of transcription factors
A) Shared enhancer sequences that bind the same transcription factor
You are testing an antibiotic that inhibits bacterial DNA gyrase. After treatment, bacterial chromosomes become tangled and replication halts. Why are human cells unaffected by the same drug?
A) Human topoisomerase uses a different structure and drug cannot bind
B) Human DNA is circular, so it doesn’t supercoil
C) Humans lack Okazaki fragments
D) Eukaryotic polymerase works in both directions
A) Human topoisomerase uses a different structure and drug cannot bind
If a telomerase-activating anti-aging drug became available, which risk would be most significant?
A) Faster telomere shortening
B) Increased mutation rate in mitochondrial DNA
C) Reactivation of telomerase in precancerous cells
D) Acceleration of protein degradation
C) Reactivation of telomerase in precancerous cells
A eukaryotic transcription factor mutation prevents binding to the enhancer region of a developmental gene. What outcome is most likely?
A) The gene’s promoter still initiates strong transcription
B) Transcription decreases despite an intact promoter
C) Translation is upregulated
D) mRNA stability increases
B) Transcription decreases despite an intact promoter
A new drug aims to suppress premature stop codons in cystic fibrosis patients’ CFTR gene. Which molecular process does it target?
A) Ribosomal proofreading
B) Nonsense suppression during translation
C) RNA splicing
D) mRNA degradation
B) Nonsense suppression during translation
Two identical twins share the same genome, but one develops schizophrenia and the other does not. Which biological process best explains this difference?
A) Differential gene mutation
B) Epigenetic modification causing altered gene expression
C) Unequal chromosome segregation
D) Alternative splicing errors
B) Epigenetic modification causing altered gene expression