OpenStem

@openstem · Joined Jul 2026

7420 public items8 groups

Quiz5 questions

Why does a cytosine base editor avoid the indels and translocations associated with standard Cas9 nuclease editing?

AIt uses a guide RNA with no PAM requirement at all.BIt uses a Cas9 nickase rather than a double-strand-cutting nuclease, so only one DNA strand is nicked while the deaminase-edited base is copied by the cell's own mismatch repair, avoiding a double-strand break intermediate.CIt does not require any Cas protein at all.DIt works only on RNA, never on genomic DNA.

Biology

Biology · L5 · Advanced CRISPR Genome Editing

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Quiz5 questions

What is the key mechanistic difference between N-linked and O-linked glycan biosynthesis?

AN-linked glycosylation transfers a pre-assembled 14-sugar precursor en bloc from dolichol phosphate onto asparagine within a defined sequon, while O-linked glycosylation builds glycans sugar-by-sugar directly on serine/threonine in the Golgi with no single universal precursor.BO-linked glycosylation always occurs before N-linked glycosylation on the same protein.CN-linked and O-linked glycosylation use an identical dolichol-phosphate precursor and consensus sequence.DOnly O-linked glycosylation occurs in the endoplasmic reticulum.

Biology

Biology · L5 · Glycobiology

@openstem

Quiz5 questions

What is a principal advantage of shotgun metagenomic sequencing over 16S rRNA amplicon sequencing for characterizing a microbial community?

AIt is substantially cheaper per sample than 16S amplicon sequencing.BIt captures viruses, archaea, and eukaryotic microbes alongside bacteria and enables strain-level taxonomic resolution plus direct functional gene annotation, rather than being restricted to bacterial/archaeal 16S sequence.CIt works even when only trace amounts of degraded DNA are available.DIt requires no bioinformatic analysis after sequencing.

Biology

Biology · L5 · Metagenomics and Microbial Ecology

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Quiz5 questions

According to the chemiosmotic hypothesis, what directly powers ATP synthesis by ATP synthase?

ADirect transfer of a high-energy phosphate intermediate from Complex IV to ATP synthase.BThe electrochemical proton gradient (proton-motive force) generated across the inner mitochondrial membrane by electron-transport-chain proton pumping, dissipated as protons flow back through ATP synthase.CDirect enzymatic transfer of electrons from NADH to ADP.DHeat released by the electron transport chain.

Biology

Biology · L5 · Advanced Mitochondrial Bioenergetics

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Quiz5 questions

In the amyloid cascade hypothesis, which enzyme complex performs the second, rate-limiting cleavage of APP that releases amyloid-beta42, the more aggregation-prone species?

AAlpha-secretaseBGamma-secretase (containing presenilin as the catalytic subunit)CGSK-3betaDTDP-43

Biology

Biology · L5 · Mechanisms of Neurodegeneration

@openstem

Quiz5 questions

Incomplete lineage sorting is expected to be most prevalent, causing the greatest gene tree/species tree discordance, under which combination of conditions?

AWidely spaced speciation events and small ancestral effective population size.BClosely spaced speciation events in time combined with large ancestral effective population size.CILS frequency is completely independent of speciation timing or population size.DOnly when hybridization has occurred between the species in question.

Biology

Biology · L5 · Phylogenomics and Comparative Genomics

@openstem

Quiz5 questions

How does auxin binding to TIR1 trigger transcriptional activation of auxin-responsive genes?

AAuxin directly binds ARF transcription factors to activate them without involving any other proteins.BAuxin acts as a molecular glue increasing TIR1's affinity for Aux/IAA repressor proteins, triggering their polyubiquitination and proteasomal degradation, which releases ARF transcription factors to activate transcription.CAuxin inhibits the SCF-TIR1/AFB E3 ubiquitin ligase complex entirely.DAuxin permanently prevents any degradation of Aux/IAA proteins.

Biology

Biology · L5 · Advanced Plant Molecular Biology

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Quiz5 questions

A significantly negative Tajima's D at a genomic locus is most consistent with which evolutionary scenario?

ALong-term balancing selection maintaining multiple alleles at intermediate frequency.BA recent hard selective sweep or population expansion, both of which produce an excess of low-frequency variants relative to neutral expectation.CComplete absence of any mutation at the locus.DA locus under no selective constraint whatsoever, evolving at a perfectly neutral rate.

Biology

Biology · L5 · Population Genomics and Selection

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Quiz5 questions

Anfinsen's ribonuclease A refolding experiment provided direct evidence for which conclusion?

AProtein folding always requires a dedicated chaperone protein to reach the native state.BThe amino acid sequence alone contains sufficient information to specify a protein's correct native fold, since denatured, reduced ribonuclease A spontaneously refolded to its correct disulfide pairing and recovered activity.CDisulfide bonds have no influence on ribonuclease A's three-dimensional structure.DProteins fold via exhaustive random sampling of every possible conformation.

Biology

Biology · L5 · Protein Folding and Molecular Chaperones

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Quiz5 questions

A target-decoy search reports 200 decoy PSMs and 20,000 target PSMs above a given score threshold. What is the estimated FDR at that threshold?

A0.1%B1%C10%D20%

Biology

Biology · L5 · Proteomics and Mass Spectrometry

@openstem

Quiz5 questions

What is the primary goal of the decellularization process when generating a tissue engineering scaffold from donor tissue?

ATo increase the immunogenicity of the donor tissue for stronger immune activation.BTo remove donor cellular and nuclear material (and its immunogenic antigens) while preserving the native three-dimensional extracellular matrix architecture and its embedded structural and biochemical cues.CTo completely dissolve the extracellular matrix, leaving only isolated donor cells for later use.DTo convert the donor tissue into a synthetic polymer scaffold.

Biology

Biology · L5 · Regenerative Medicine and Tissue Engineering

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Quiz5 questions

In the first transesterification reaction of splicing, what nucleophile attacks the 5' splice site phosphodiester bond?

AThe 3'-hydroxyl of the terminal exon 2 nucleotide.BThe 2'-hydroxyl of the branch-point adenosine within the intron.CA free hydroxide ion from solvent water.DThe 5'-phosphate of the U1 snRNA.

Biology

Biology · L5 · RNA Biology, Splicing, and Noncoding RNA

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Quiz5 questions

What dynamical behavior does a coherent type-1 feed-forward loop with AND logic characteristically produce in response to a brief, transient activating pulse?

AIt amplifies the pulse into a much larger, sustained response.BIt filters out the brief pulse (does not activate the target gene) while still responding to a persistent signal, acting as a sign-sensitive delay element.CIt converts the pulse into sustained oscillations.DIt has no effect on the dynamics of the target gene.

Biology

Biology · L5 · Systems Biology and Network Modeling

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Quiz5 questions

M2-polarized tumor-associated macrophages contribute to tumor progression primarily by:

AProducing high levels of IL-12 and TNF-alpha to boost cytotoxic T cell activity.BSecreting anti-inflammatory and pro-angiogenic factors (IL-10, TGF-beta, VEGF) that suppress cytotoxic T cell activity and promote angiogenesis and invasion.CDirectly killing tumor cells through phagocytosis at a high rate.DPreventing any vascularization of the tumor.

Biology

Biology · L5 · Tumor Microenvironment and Metastasis

@openstem

Quiz5 questions

What chemical feature allows RIG-I to distinguish many viral RNA replication intermediates from host cytoplasmic mRNA?

AThe presence of an uncapped 5' triphosphate or diphosphate group, absent from 5'-capped host mRNA.BThe presence of a poly-A tail, which only viral RNA has.CRIG-I cannot distinguish viral RNA from host RNA under any circumstances.DThe presence of thymine bases instead of uracil.

Biology

Biology · L5 · Advanced Virus-Host Interactions

@openstem

Note~256 words · 1 min

Fur, Feathers, Scales, and Skin Every animal has a covering on the outside of its body. This covering is not just for looks. It has an important job to do, like keeping the animal warm, dry, safe, or hidden. Why Coverings Matter Animals liv

Biology

Biology · L1 · Animal Coverings: Fur, Feathers, Scales, and Skin

@openstem

Note~298 words · 1 min

How Animals Move Animals need to move to find food, escape danger, and explore the world around them. Different animals move in different ways, using different body parts. Legs for Walking, Running, and Jumping Many animals use legs to get

Biology

Biology · L1 · How Animals Move

@openstem

Note~61 words · 1 min

What Plants Need Plants are living things. To grow big and strong, a plant needs a few simple things. Take any one away, and the plant cannot grow well.

Biology

Biology · L1 · What Plants Need to Grow

@openstem

Note~207 words · 1 min

How Plants Grow Plants are living things, just like animals. They grow, they need food, and they make new plants. But plants grow in their own special way. The Parts of a Plant Most plants have four main parts, and each part has its own job

Biology

Biology · L1 · How Plants Grow

@openstem

Note~240 words · 1 min

Sorting Animals Into Groups There are so many animals in the world! To make sense of them, scientists sort animals into groups. Animals in the same group share things in common, like the way their skin looks or how their babies are born. Si

Biology

Sorting Animals Into Groups

@openstem

Note~172 words · 1 min

What Is a Habitat? A habitat is a home for a plant or animal. It gives them food, water, and shelter, which means a safe place to stay. Kinds of Habitats Why Habitats Matter Every animal needs the right habitat to survive. If a habitat chan

Biology

Habitats: Homes for Animals

@openstem

Note~333 words · 2 min

Day and Night Animals Some animals are awake in the day. Some animals are awake at night. Each animal has its own special time to be active. Diurnal Animals Animals that are awake during the day are called diurnal. Squirrels, most birds, an

Biology

Biology · L1 · Day and Night Animals

@openstem

Note~284 words · 1 min

Animal Teeth: Shapes Show What They Eat Teeth are tools. Just like a fork and a knife do different jobs, different tooth shapes do different jobs. If you look closely at an animal's teeth, you can often guess what it likes to eat. Flat Teet

Biology

Animal Teeth: Shapes Show What They Eat

@openstem

Note~258 words · 1 min

Inside a Seed and How Seeds Travel A seed looks small and simple, but it is carrying everything a new plant needs to get started. Seeds also have clever ways of traveling far from the plant that made them. What's Inside a Seed? If you could

Biology

Biology · L1 · Inside a Seed and How Seeds Travel

@openstem

Note~278 words · 1 min

Caring for Pets and Farm Animals Many people keep animals as pets or on farms. These animals cannot take care of themselves, so they depend on people to look after them. What Every Animal Needs Whether an animal is a pet in a home or a farm

Biology

Caring for Pets and Farm Animals

@openstem

Note~273 words · 1 min

Your Amazing Body Your body is like a busy team. Every part has a job to do, and all the parts work together to keep you moving, breathing, and growing. Bones and Muscles Work Together Your Heart and Lungs Keep You Going Your Skin Covers It

Biology

Your Amazing Body

@openstem

Note~121 words · 1 min

Staying Healthy Every Day Your body works hard every day. Simple habits help it stay strong and happy. Eating healthy food gives your body the fuel it needs. Drinking water keeps you cool and helps you think. Sleep gives your body time to r

Biology

Biology · L1 · Staying Healthy Every Day

@openstem

Note~211 words · 1 min

Baby Animals and Growing Up Baby animals have special names. They need care from their parents to grow up big and strong. Baby Animal Names Growing and Changing Some baby animals look like tiny copies of their parents. A puppy looks like a

Biology

Biology · L1 · Baby Animals and Growing Up

@openstem

Note~180 words · 1 min

Food Chains in Nature A food chain shows how living things get their energy. Energy moves from one living thing to the next. How a Food Chain Works Most food chains start with the Sun. Plants use sunlight to make their own food. Then animal

Biology

Biology · L1 · Food Chains in Nature

@openstem

Note~229 words · 1 min

How Seasons Change Plants and Animals Every year, the weather moves through four seasons: spring, summer, autumn, and winter. As the weather changes, plants and animals change too, so they can stay safe, find food, and stay warm. The Four S

Biology

How Seasons Change Plants and Animals

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