Pooled Genetic Screens: Library Design, Selection and Reading a Hit List
A pooled screen turns a biological question into a counting problem: which guides are enriched or depleted after a selection. This stop covers library coverage, the choice of selection pressure, and how a real hit is told apart from a noisy guide.
What a learner can do afterwards
- Work out the cell number a library of stated size needs for a stated coverage
- Choose a positive or a negative selection design for a stated biological question
- Judge a hit list using agreement between guides targeting the same gene, control guides and effect size
1 · Read
As before, a guide does not guarantee an outcome. A pooled knockout screen mixes cells with guides aimed at disrupting genes. Delivery aims for one guide in most guide-bearing cells. Sample tags before and after selection. For a simple depth adjustment, divide each guide's read count by all guide reads in that sample. Compare these fractions, not raw counts from unequal sequencing depths. Tags identify guides, not confirmed edits. Depletion is inferred; missing cells are not sequenced directly.
Coverage is average guide-bearing cells per guide. Required cells = guides × coverage. For 12,000 guides at 300 cells per guide, this is 3,600,000 cells. If a loss step retains 75%, divide by 0.75: start with 4,800,000 guide-bearing cells to retain 3,600,000. This assumes guide proportions stay unchanged. An adequate average can still hide under-represented guides.
Positive selection seeks enriched guides, such as guides in cells surviving a drug that kills susceptible controls. Negative selection seeks depleted guides, such as knockouts that reduce growth or survival. Compare adjusted final counts with the initial sample. A gene may be required in one cell line or environment but not another.
In this illustration, the final adjusted count is one fifth of the initial count: depletion.
Assess guide effects against controls and across independent repeat cultures. Fold enrichment compares adjusted final and initial counts; 1 means unchanged and 3 means threefold enrichment. Several guides with similar effects support a gene-level signal more than one isolated large effect. Agreement reduces concern about guide-specific artefacts but cannot exclude shared biases. Check guide activity and validate candidates separately before claiming causation.
Non-targeting guides are designed without a genomic target. Their count changes provide a background reference, but do not model every effect of DNA cutting. Known-essential guides are depletion controls when their genes are required in the tested cells and their guides are active. Missing expected depletion calls for checks of editing, timing, conditions and readout before trusting hits.
Plan how many guide-bearing cells you need and account for losses. Compare initial and final guide counts, then assess hits with controls, guide agreement, repeat cultures and separate validation.
Lesson, questions, diagrams and flashcards written for this learning dataset.
Readings and flashcards written for this learning dataset. Linked background video: Applications of DNA technologies, from Khan Academy.
More to read
Follow guides through a mixed population
A pooled knockout screen places many guide assignments in one mixed cell population. Delivery aims for one guide in most guide-bearing cells so that a response can be linked to an assigned perturbation. It does not guarantee a single guide or a successful knockout in every cell.
Take a guide-tag sample before selection and another afterward. Counts adjusted for sequencing depth show changes in relative guide representation. Missing cells are not sampled directly; depletion is inferred from the comparison. Resistance-associated guides may become enriched under a drug, while growth-reducing perturbations may become depleted. Each inference depends on the cells, conditions and actual guide activity.
Calculate coverage at the stage that matters
A coverage target refers to a population at a stated stage. Multiply the guide count by the desired average guide-bearing cells per guide to calculate that population. Cells exposed to delivery but carrying no guide do not contribute to this coverage.
If a later step loses cells, work backward from the population needed afterward. Retaining half of a starting population means that a requirement of four million retained cells needs eight million starting cells. This calculation assumes that guide proportions stay unchanged through the loss. An average is not a minimum: unequal representation can still leave some guides under-covered. Check representation through sampling and selection rather than treating the initial total as a guarantee.
Treat a hit list as evidence for follow-up
Compare each guide's effect with control variation, then ask whether other guides for the same gene show a similar effect. Repeat the experiment in independent cultures. Agreement across guides and repeats supports a gene-level signal more than one unusually large result, but shared biases can remain.
Non-targeting guides provide a background reference. They do not reproduce every effect of DNA cutting. Active guides against genes required in the tested cells can provide expected depletion responses. If those controls remain near baseline, investigate timing, conditions, perturbation activity and the readout before trusting the hit list. Test candidates separately with additional perturbations or measurements before claiming a causal explanation.
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2 · Watch
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