Answer each before reading the worked answer. Questions 1–6 go with Tuesday, 7–12 with Wednesday, 13–18 with Thursday.
1 A tomato is engineered so that an enzyme causing softening is reduced. Sequencing shows the enzyme's gene is intact and unaltered. What technique was most likely used, and how does it work?
RNAi. An introduced sequence produces RNA matching the enzyme's mRNA; the cell's machinery degrades or blocks that mRNA, so less enzyme protein is made. The gene itself is untouched, which is exactly what the sequencing shows.
2 A wild potato gene for blight resistance is moved into a cultivated potato. Transgenic or cisgenic, and why does the distinction matter?
Cisgenic: the gene comes from a sexually compatible relative, so conventional breeding could in principle achieve the same result. The distinction is about gene origin, and it figures in how different countries regulate the product.
3 Two fields are planted with the same engineered seed. Field A yields 20% more than the control; field B shows no difference. Explain without assuming anyone made an error.
Phenotype depends on genotype and environment. Conditions in field B may not have triggered the advantage (for example, a drought-tolerance trait in a wet season), or other limiting factors masked it. If the trait is polygenic, performance is especially context-dependent. One field's result is evidence for that field's conditions.
4 Why was Gurdon's frog result surprising, and what did it establish?
It was widely assumed that specialization involved permanently losing or inactivating unneeded genetic information. A nucleus from a specialized intestinal cell produced a complete tadpole, establishing that a differentiated cell keeps the full genome in usable form. Specialization is a change in gene regulation, not gene content.
5 Rank by potency and justify: a zygote, a skin-derived iPSC, a blood stem cell, a mature neuron.
Zygote (totipotent: embryo plus placenta) > iPSC (pluripotent: all body cell types, not a whole organism alone) > blood stem cell (multipotent: blood lineages only) > neuron (differentiated: itself). Reprogramming moves a cell up this ranking; differentiation moves it down.
6 A lab makes iPSC-derived dopamine neurons from a Parkinson's patient and they survive in culture. A press release says the lab has "cured Parkinson's in a dish." What is wrong with that?
Making the right cell type is one step. Unproven: whether the cells mature fully, survive transplantation, avoid tumor formation, escape immune rejection, integrate into existing circuits, and release dopamine in a regulated way that restores movement. Cells in a dish also lack the brain's environment and the disease's progression.
7 Order these and say why each is needed: differentiate, biopsy, expand, harvest, establish a cell line.
Biopsy (get cells) → establish a cell line (isolate and maintain the useful cells, mainly satellite cells) → expand (proliferate to get enough material) → differentiate (stop dividing, fuse into muscle fibers) → harvest. Expansion without differentiation gives cells but not muscle; differentiating too early gives too little material.
8 A bioreactor run is contaminated with bacteria. Why is that more serious in a cell-culture facility than in an animal?
The animal has an immune system, and the medium is not freely accessible. Cultured animal cells have no immune defense, and the medium is an ideal growth substrate for bacteria and fungi, which divide far faster than animal cells and will outcompete them. Sterility is the only protection, so one breach ruins the batch.
9 A company produces excellent cultured chicken in a 2-liter vessel and claims it is ready for a 20,000-liter plant. What should you ask?
Whether oxygen can be delivered throughout a vessel that large, how mixing will be done without shear-damaging the cells, how waste metabolites will be removed, how sterility will be maintained at that volume, what the medium and growth factors cost at scale, and whether the product is structured or just a cell mass. Small-scale success does not scale linearly; several of these problems get worse with volume.
10 Why can't you make a thick steak simply by letting a culture grow longer?
No vasculature. Oxygen and nutrients diffuse only a fraction of a millimeter, so the interior starves and waste accumulates while the surface stays healthy. You also need scaffolds to align cells into organized fibers; without structure you get a mass, not a cut of meat.
11 You want to study whether human interneurons migrate normally in a genetic disorder. Would you use 2D culture, an animal model, an organoid, or an assembloid? Defend it.
An assembloid. Migration between regions is the process in question, so you need separately generated regions fused together with human patient-derived cells. 2D culture has no spatial organization; an animal model may not reproduce human-specific features; a single organoid has only one region, so there is nowhere to migrate from.
12 A brain organoid from a patient shows reduced neuronal activity compared with a control organoid. List what this supports and three limits.
Supports: cells carrying the patient's genome show a measurable difference in neural activity under these conditions, consistent with a cellular contribution to the condition. Limits: organoids are variable, so you need many of them and a matched control line; they are immature and lack vascularization and full cell diversity, so the difference may not reflect mature tissue; there is no behavior or body, so it says nothing about symptoms in a person.
13 Explain why a BCI can help with spinal cord injury but not with motor cortex damage.
A BCI records intention signals from the motor cortex and routes them to a device, bypassing the broken link. In spinal cord injury, the cortex is intact and still generating those signals. If the motor cortex itself is destroyed, there is no signal to record, so there is nothing for the BCI to decode.
14 A participant's cursor control degraded, then recovered past baseline after software changes. What does that show, and what does it not?
It shows that decoding quality, not only electrode count, limits performance, and that lost signal can be partly compensated in software. It does not show that the hardware is stable, that the biological problem was fixed, or that the nervous system changed. Implant stability remains an open question precisely because the threads retracted.
15 Why do noninvasive BCIs give poorer control than intracortical ones?
The skull and scalp attenuate and blur electrical signals, so scalp recordings average over large populations and lose the fine detail. Intracortical electrodes sit next to the neurons and capture individual or small-group firing, which supports faster, finer decoding. The cost is surgery and the long-term risks of an implant.
16 Walk through producing human insulin in bacteria, naming each tool.
Cut the insulin gene and a plasmid with the same restriction enzyme, leaving matching ends. Join them with DNA ligase to make recombinant DNA. Introduce the plasmid into a host bacterium. The bacterium replicates the plasmid and transcribes and translates the gene, producing human insulin, which is then purified. This works because the genetic code is shared across organisms.
17 Researchers engineer a bacterium to tolerate higher salt. A news story calls this "dangerous gain-of-function research." Evaluate.
It is genetic engineering, and salt tolerance is a gain of function, which is a measured change in activity. But gain of function does not mean increased danger. Danger in a pathogen means greater transmissibility or virulence; salt tolerance is neither, and the organism may not be a pathogen at all. Establishing any claim about changed function also requires comparison with the unmodified strain, direct measurement and controls.
18 A lab works with a bacterium that causes serious lung disease and spreads by inhalation. Which BSL, and what changes if they switch from reading slides to procedures that generate aerosols?
BSL-3, because the agent causes serious disease by an inhalation route: controlled access, inward directional airflow, non-recirculated exhaust, respiratory protection, and all work in containment devices. Containment is chosen from agent hazard, exposure route and activity together, so aerosol-generating procedures raise the requirements even for the same organism, pushing work into biosafety cabinets or higher containment.
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