--- title: "Genome Engineering: Cut Site, Repair Choice and Editing Outcome" description: "The nuclease only makes a break; the cell decides the edit. This stop works out how guide design, the repair pathway available in that cell type and the donor template together determine which allele " canonical: https://lightmysky.com/learn/science/genome-engineering-cut-site-repair-choice-and-editing-outcome-mt_3DqvkO4wdE source: https://lightmysky.com/learn/science/genome-engineering-cut-site-repair-choice-and-editing-outcome-mt_3DqvkO4wdE.md retrieved: 2026-09-12 --- > **Agent view.** This is the Markdown twin of the page, for tools and assistants. > When to use this site, and the call that answers each job: https://lightmysky.com/agent-instructions.md > API description (OpenAPI 3.1): https://lightmysky.com/openapi.json · Authentication: https://lightmysky.com/auth.md > Pricing: https://lightmysky.com/pricing.md · Catalog: https://lightmysky.com/llms.txt · Full catalog: https://lightmysky.com/llms-full.txt > Every machine-readable file on this domain: https://lightmysky.com/.well-known/ai-catalog.json > Ask for Markdown with `Accept: text/markdown`, a `.md` address, or `?mode=agent`. # Genome Engineering: Cut Site, Repair Choice and Editing Outcome The nuclease only makes a break; the cell decides the edit. This stop works out how guide design, the repair pathway available in that cell type and the donor template together determine which allele you end up with. Subject: Science · Area: Genetics & Evolution · Ages 22 to 24 Page: https://lightmysky.com/learn/science/genome-engineering-cut-site-repair-choice-and-editing-outcome-mt_3DqvkO4wdE ## Ready when they can - Design a guide for a target site and predict the indel spectrum end joining will produce - Explain why homology-directed repair rates depend on cell cycle stage and on the donor design - Assess off-target risk from guide sequence and from the sensitivity of the assay used to measure it ## Lesson: The cutter proposes, the cell decides The nuclease only makes a break; the cell decides the edit. A guide pairs tightly with its target beside the required motif, and the cutter breaks both strands there. Quick end joining then glues the ends, usually leaving small insertions and deletions clustered at the break that shift the reading frame. A glowing marker only proves delivery, not correct editing, so design starts with a guide that grips its target and grips nowhere else. **Example.** Suppose you need a clean single-letter fix in dividing cells. End joining would mostly break the gene, so you supply a donor template with long matching arms and the change centred. Template repair runs best in S and G2, when a sister copy is present, so the same guide gives knockouts in resting cells and knock-ins where template repair runs. That is why one guide can knock out in one cell type and cleanly fix in another. Off-target risk starts with the guide sequence: any lookalike address can draw fire, above all when mismatches sit far from the seed end, and long cutter exposure raises the odds. Delivery decides how many cells even see the cutter. Sensitive sequencing of both the target and the likeliest lookalike sites turns a hopeful edit into a proven one, because a weak assay misses rare edits and fakes safety. **Tip.** To judge any editing claim, check three things in order: the cut site from guide pairing, the repair pathway open in that cell type, and the assay sensitivity behind the safety claim. Easy disruption versus hard correction is the central trade-off. Confirm the edit landed well and behaves, and check the lookalikes with the most sensitive test available. **Recap.** Guides aim the break, repair writes the ending, and sensitive sequencing proves what really happened. ## Practice 8 questions on this page, each with its working shown. ## Needs first - [CRISPR: A Bacterial Defence Turned into a Tool](https://lightmysky.com/learn/science/crispr-a-bacterial-defence-turned-into-a-tool-mt_0joPWRvOLw) - [Mapping Regulatory Elements: Accessibility, Occupancy and Contact](https://lightmysky.com/learn/science/mapping-regulatory-elements-accessibility-occupancy-and-contact-mt_W2YKGBADJ-) - [Homologous Recombination and Double-Strand Break Repair](https://lightmysky.com/learn/science/homologous-recombination-and-double-strand-break-repair-mt_yWlwqAsXEf) ## Opens up - [Pooled Genetic Screens: Library Design, Selection and Reading a Hit List](https://lightmysky.com/learn/science/pooled-genetic-screens-library-design-selection-and-reading-a-hit-list-mt_0n-a7GfLOL) - [Base and Prime Editing: Changing a Letter Without Breaking Both Strands](https://lightmysky.com/learn/science/base-and-prime-editing-changing-a-letter-without-breaking-both-strands-mt_roM4eWoWVn) - [Ethics of Emerging Biotechnology: Editing and Biosecurity](https://lightmysky.com/learn/science/ethics-of-emerging-biotechnology-editing-and-biosecurity-mt_U5OY2wvimB)