Choosing how to perturb a gene and at what scale: why an sirna library screen sets a representation and delivery requirement before any biology, how crispr gene knockout compares on completeness, timescale and off target profile and what a crispr control construct has to be, what a dna transfection reagent and an sirna transfection reagent each have to achieve in the cells you actually use, how gene delivery systems differ in persistence and immune exposure, where a gfp plasmid is the control that calibrates delivery, what cumate and other inducible systems buy in timing control, how a maxi prep plasmid and large scale plasmid purification feed a screen that consumes DNA faster than expected, and where enzymes such as 3c protease, pngase, mnase, rnase r and saci sit as the tools that prepare the samples rather than perturb the cells

Designing a perturbation experiment

1. Choose knockdown or knockout from the biology. Essential genes and dose response questions suit partial, reversible silencing. Clean loss of function suits cutting. Neither is generally better and the choice should be stated with a reason.
2. Solve delivery before scaling. In a screen, delivery efficiency multiplies through every well. Establish it with a fluorescent control and a known positive perturbation before committing library scale material.
3. Size the screen on representation. Library scale work sets a minimum number of cells per reagent that has to be maintained at every step. Calculate it backwards from library size, because a screen below representation is noise.
4. Use an inducible system where timing matters. Where a perturbation is lethal or where the timing of onset is the question, an inducible system separates the perturbation from the selection. It adds a construct and answers a question a constitutive system cannot.
5. Budget the plasmid. Screens consume more DNA than plans allow for. Large scale preparation, or a preparation service, belongs in the plan rather than discovered mid screen.
6. Keep sample preparation enzymes separate in the design. Proteases, glycosidases and nucleases used to prepare samples are analytical tools. Recording them alongside perturbation reagents confuses two different parts of the experiment.

Arithmetic before biology

Library size, cells per reagent, transduction efficiency, selection survival and sequencing depth multiply into a required culture scale that is usually larger than the first plan. That calculation is the screen design.

Do it on paper before ordering anything. Screens fail on representation far more often than on reagent quality.

Controls make a null result usable

A screen with no positive control cannot distinguish a biological negative from a technical failure, and at screen scale a technical failure is expensive.

Include known positive and non targeting reagents distributed across every plate, and check them before analysing anything else.

Common questions

Silencing or cutting for a screen?

Silencing for essential genes, partial loss and fast, reversible perturbation. Cutting for clean loss of function and for a selection based readout over longer timescales. Both are used and the choice follows the readout.

What makes a screen fail?

Representation, almost always: too few cells per reagent at transduction, selection, sorting or sequencing. The remedy is arithmetic done before the screen rather than analysis afterwards.

Why include a fluorescent delivery control?

Because it separates a failed perturbation from a failed delivery. Without it, a null screen result cannot be interpreted at all.

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