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OpenStem
@openstem · Joined Jul 2026
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Quiz5 questions
A UV-induced thymine dimer, which distorts the DNA helix over several bases, is primarily repaired by:
ABase excision repairBNucleotide excision repairCMismatch repairDDirect reversal by MGMT
Biology · L4 · DNA Repair Mechanisms Quiz
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Biology · L4 · DNA Repair Mechanisms QuizQuiz5 questions
A sigmoidal (rather than hyperbolic) velocity-vs-substrate curve is characteristic of an enzyme that shows:
AA single active site with no communication between subunitsBPositive cooperativity among multiple subunits, where binding at one site increases affinity at the othersCPure competitive inhibition onlyDNo regulation of any kind
Biology · L4 · Enzyme Regulation and Allostery Quiz
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Biology · L4 · Enzyme Regulation and Allostery QuizQuiz5 questions
The oxygen-hemoglobin dissociation curve is sigmoidal rather than hyperbolic because:
AHemoglobin has only one oxygen-binding siteBOxygen binding at one subunit increases the oxygen affinity of the remaining subunits (positive cooperativity)CHemoglobin binds carbon dioxide instead of oxygenDOxygen binding decreases the affinity of remaining subunits
Biology · L4 · Hematology and Blood Physiology Quiz
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Biology · L4 · Hematology and Blood Physiology QuizQuiz5 questions
Butyrate, produced by bacterial fermentation of dietary fiber, is primarily important to host health because it:
AIs toxic to colonocytes and triggers their turnoverBServes as the primary energy source for colonocytes and supports epithelial barrier integrityCIs absorbed unchanged into the bloodstream without any local effectDDirectly encodes new host genes
Biology · L4 · Human Microbiome Quiz
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Biology · L4 · Human Microbiome QuizQuiz5 questions
Photorespiration occurs when rubisco binds which molecule instead of CO2, and what is a consequence of this reaction?
AWater; it produces excess NADPHBOxygen; it produces 2-phosphoglycolate and releases previously fixed carbon as CO2CNitrogen; it fixes additional nitrogen into amino acidsDMalate; it stores carbon for later use
Biology · L4 · Photosynthesis (C3, C4, CAM) Quiz
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Biology · L4 · Photosynthesis (C3, C4, CAM) QuizQuiz5 questions
Traditional 'Reptilia,' which excludes birds despite birds descending from the reptilian common ancestor, is best described as:
AMonophyleticBParaphyleticCPolyphyleticDAn outgroup
Biology · L4 · Phylogenetics and Cladistics Quiz
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Biology · L4 · Phylogenetics and Cladistics QuizQuiz5 questions
According to the acid growth hypothesis, auxin promotes cell elongation by:
ADirectly increasing turgor pressure inside the cellBActivating H+-ATPases that acidify the cell wall, activating expansins that loosen the wallCIncreasing chlorophyll synthesisDBlocking water uptake by the cell
Biology · L4 · Plant Hormones and Tropisms Quiz
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Biology · L4 · Plant Hormones and Tropisms QuizQuiz5 questions
Why must GnRH be released in a pulsatile pattern to sustain normal FSH/LH secretion?
AContinuous GnRH exposure causes receptor downregulation and desensitization, paradoxically suppressing gonadotropin releaseBContinuous GnRH exposure always increases FSH/LH release furtherCPulsatile release has no functional significanceDGnRH cannot cross into the bloodstream unless released in pulses
Biology · L4 · Reproductive Physiology Quiz
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Biology · L4 · Reproductive Physiology QuizQuiz5 questions
A small group of birds is blown to a remote island and founds a new, genetically distinct population through drift acting on a small founding group. This is an example of:
ASympatric speciation via disruptive selectionBPeripatric speciation via a founder effectCParapatric speciation across a hybrid zoneDReinforcement of prezygotic isolation
Biology · L4 · Speciation Mechanisms Quiz
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Biology · L4 · Speciation Mechanisms QuizQuiz5 questions
Countercurrent heat exchange in a limb primarily functions to:
AIncrease heat loss from the extremity to speed coolingBTransfer heat from warm arterial blood to cool venous blood, retaining it in the coreCEqualize blood pressure between arteries and veinsDGenerate heat directly through friction between opposing blood flows
Biology · L4 · Thermoregulation and Osmoregulation Quiz
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Biology · L4 · Thermoregulation and Osmoregulation QuizQuiz5 questions
An enhancer can activate a gene even when located far away and on either side of the promoter primarily because:
AIt diffuses through the nucleoplasm to reach the promoterBDNA looping brings the enhancer-bound activators into physical contact with the promoter-bound machinery, often via MediatorCEnhancers directly encode a protein that travels to the promoterDEnhancers must always be located immediately upstream of the TATA box
Biology · L4 · Eukaryotic Transcriptional Regulation Quiz
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Biology · L4 · Eukaryotic Transcriptional Regulation QuizQuiz5 questions
A negative-sense single-stranded RNA virus must package which enzyme within its virion to establish infection, and why?
AReverse transcriptase, to convert its genome into DNABRNA-dependent RNA polymerase, because the genome cannot be read directly by host ribosomes as mRNACIntegrase, to insert its genome into the host chromosomeDDNA polymerase, to replicate its genome
Biology · L4 · Virology Basics Quiz
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Biology · L4 · Virology Basics QuizQuiz7 questions
The nucleobase below is adenine. What structural feature classifies it as a purine rather than a pyrimidine?
AIt has a single six-membered ring containing two nitrogensBIt has two fused rings — a six-membered ring joined to a five-membered ringCIt carries an amino group, and only purines have amino groupsDIt contains no oxygen, and purines are the bases without oxygen
Biology · L4 · Reading Biomolecules & Saturation Curves — 7 Q
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Biology · L4 · Reading Biomolecules & Saturation Curves — 7 QQuiz3 questions
In the lac operon, which molecule acts as the true inducer that releases the LacI repressor from the operator?
ALactoseBAllolactoseCGlucoseDcAMP
Biology · L5 · Gene Regulation & Epigenetics
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Biology · L5 · Gene Regulation & EpigeneticsQuiz4 questions
In a population at Hardy–Weinberg equilibrium, allele A has frequency p = 0.6. What is the expected frequency of heterozygotes (Aa)?
A0.36B0.48C0.16D0.24
Biology · L5 · Population Genetics
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Biology · L5 · Population GeneticsQuiz5 questions
Which chromatin remodeler family is defined by its activity of exchanging canonical H2A–H2B dimers for the H2A.Z variant?
ASWI/SNFBISWICCHDDINO80/SWR1
Biology · L5 · Advanced Epigenetics and Chromatin Biology
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Biology · L5 · Advanced Epigenetics and Chromatin BiologyQuiz5 questions
A 3 gigabase (Gb) haploid genome is sequenced, producing 90 Gb of mappable, aligned bases from short reads. What is the approximate average sequencing depth (coverage)?
A3xB10xC30xD90x
Biology · L5 · Genomics and Bioinformatics
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Biology · L5 · Genomics and BioinformaticsQuiz5 questions
In X-ray crystallography, what is the 'phase problem', and why does it arise specifically at the detection step of the experiment?
ADetectors cannot resolve reflections at high scattering angles, so high-resolution phases are unmeasurable.BA detector records diffraction intensity (amplitude squared) but not the phase of each scattered wave, so phases must be recovered by an independent method before Fourier synthesis of electron density.CProtein crystals diffract too weakly at all resolutions to give any usable intensities.DX-ray sources are not sufficiently monochromatic to define a single phase per reflection.
Biology · L5 · Structural Biology and Protein Chaperones
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Biology · L5 · Structural Biology and Protein ChaperonesQuiz5 questions
In Knudson's two-hit hypothesis, hereditary retinoblastoma differs from sporadic retinoblastoma primarily because:
AHereditary cases require mutation of only one RB1 allele, while sporadic cases require both.BHereditary cases inherit one mutant RB1 allele in the germline, so only a single somatic "second hit" is needed in a retinal cell, producing earlier, often bilateral tumors.CHereditary cases involve a dominant gain-of-function RB1 mutation, whereas sporadic cases involve RAS activation instead.DHereditary retinoblastoma arises from TP53 mutations rather than RB1 mutations.
Biology · L5 · Cancer Biology
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Biology · L5 · Cancer BiologyQuiz5 questions
A thymocyte in the medulla binds a self-peptide–MHC complex, presented via AIRE-dependent promiscuous gene expression, with high avidity. What is the most likely fate of this cell?
APositive selection into the mature CD4 or CD8 lineageBDeath by neglect from insufficient TCR signalCClonal deletion by negative selectionDDifferentiation into a memory T cell
Biology · L5 · Advanced Immunology: Tolerance & Cytokine Networks
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Biology · L5 · Advanced Immunology: Tolerance & Cytokine NetworksQuiz5 questions
Why is the NMDA receptor described as a molecular coincidence detector in the context of Hebbian synaptic plasticity?
AIt only opens when calcium and sodium ions bind to it simultaneouslyBIt requires both presynaptic glutamate binding and postsynaptic depolarization to relieve its voltage-dependent magnesium blockCIt detects coincident release of glutamate and GABA from the same terminalDIt requires simultaneous ligand binding at both AMPA and NMDA receptor sites
Biology · L5 · Advanced Neuroscience
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Biology · L5 · Advanced NeuroscienceQuiz5 questions
A Hox gene positioned at the 3' end of its cluster is, relative to a gene at the 5' end of the same cluster, characteristically:
AExpressed later in development and in a more posterior domainBExpressed earlier in development and in a more anterior domainCExpressed at the same time and in an overlapping domain, since cluster position carries no regulatory informationDTranscribed only in extraembryonic tissue regardless of cluster position
Biology · L5 · Evolutionary Developmental Biology Quiz
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Biology · L5 · Evolutionary Developmental Biology QuizQuiz5 questions
Under strict neutral theory, why is the rate of molecular substitution expected to be independent of effective population size (Ne)?
ABecause natural selection acts equally strongly regardless of NeBBecause the higher number of new neutral mutations arising in a larger population (2Neμ) is exactly offset by each mutation's lower probability of fixation (1/2Ne)CBecause mutation rate μ itself scales inversely with NeDBecause genetic drift is negligible in populations of any size
Biology · L5 · Molecular Evolution and Phylogenetics Quiz
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Biology · L5 · Molecular Evolution and Phylogenetics QuizQuiz5 questions
In synthetic biology, what does it mean for an engineered genetic circuit component to be orthogonal to its host?
AIt operates using distinct transcriptional or translational machinery so that it does not cross-react with the host cell's endogenous regulatory networkBIt is physically inserted into the host genome at a 90-degree angle relative to the origin of replicationCIt is derived entirely from genes native to the host organismDIt requires no promoter because it is constitutively active at a fixed rate
Biology · L5 · Synthetic Biology Engineering Quiz
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Biology · L5 · Synthetic Biology Engineering QuizQuiz5 questions
Which combination of transcription factors constitutes the original Yamanaka reprogramming cocktail described in 2006?
AOct4, Sox2, Klf4, and c-MycBOct4, Sox2, Nanog, and Lin28COct4, Klf4, c-Myc, and NanogDSox2, Nanog, Lin28, and c-Myc
Biology · L5 · Stem Cells and Regenerative Biology
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Biology · L5 · Stem Cells and Regenerative BiologyQuiz5 questions
Why do most cultured somatic cells stop dividing after a finite number of divisions (the Hayflick limit)?
AThey run out of available nutrients in any culture medium regardless of replenishment.BProgressive telomere shortening, due to the end-replication problem and low telomerase activity in most somatic cells, eventually triggers a persistent DNA damage response that enforces permanent cell-cycle arrest.CMitochondrial DNA is completely degraded after a fixed number of divisions.Dp53 is permanently inactivated after several rounds of division.
Biology · L5 · Aging Biology and Cellular Senescence
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Biology · L5 · Aging Biology and Cellular SenescenceQuiz5 questions
How does mTORC1 activity under nutrient-rich conditions suppress autophagy initiation at the molecular level?
AmTORC1 directly degrades LC3-II on the autophagosomal membrane.BmTORC1 phosphorylates ULK1 at an inhibitory site, suppressing ULK1 complex activity and hence autophagosome nucleation.CmTORC1 has no direct interaction with any component of the core autophagy machinery.DmTORC1 directly ubiquitinates ATG7 for proteasomal degradation.
Biology · L5 · Autophagy and Cell Death Pathways
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Biology · L5 · Autophagy and Cell Death PathwaysQuiz5 questions
Which algorithm should be used to find the single best-aligning conserved domain shared between two proteins that are otherwise unrelated across most of their length?
ANeedleman-Wunsch global alignmentBSmith-Waterman local alignmentCNeighbor-joiningDMaximum parsimony
Biology · L5 · Bioinformatic Sequence Analysis
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Biology · L5 · Bioinformatic Sequence AnalysisQuiz5 questions
What distinguishes magnetic tweezers from optical tweezers in terms of a unique manipulation capability?
AMagnetic tweezers can apply torque by rotating the magnetic field, enabling direct study of DNA supercoiling and torsional motor protein mechanics, a capability optical tweezers do not straightforwardly provide.BOnly optical tweezers can apply any force to a tethered molecule at all.CMagnetic tweezers cannot hold a constant force over any timescale.DMagnetic tweezers require no bead of any kind to function.
Biology · L5 · Biophysics and Single-Molecule Methods
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Biology · L5 · Biophysics and Single-Molecule MethodsQuiz5 questions
What key experimental step distinguishes Hi-C from the earlier, locus-specific 3C technique?
AHi-C requires no crosslinking of chromatin at all.BHi-C biotinylates ligation junctions and sequences the resulting library genome-wide, rather than requiring locus-specific PCR primers designed for a single candidate pair of loci.CHi-C can only be performed on isolated, purified DNA with no cells involved.DHi-C uses a different crosslinking chemistry that does not involve formaldehyde.
Biology · L5 · Chromatin and 3D Genome Organization
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Biology · L5 · Chromatin and 3D Genome Organization