--- title: "Base and Prime Editing: Changing DNA Without a Planned Double-Strand Break" description: "Conventional cytosine and adenine base editors link a deaminase to a DNA-targeting protein and act within an editing window. Prime editors copy a template carried on a guide RNA. Compare their intende" canonical: https://lightmysky.com/learn/science/base-and-prime-editing-changing-dna-without-a-planned-double-strand-break-mt_roM4eWoWVn source: https://lightmysky.com/learn/science/base-and-prime-editing-changing-dna-without-a-planned-double-strand-break-mt_roM4eWoWVn.md retrieved: 2026-10-19 --- > **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`. # Base and Prime Editing: Changing DNA Without a Planned Double-Strand Break Conventional cytosine and adenine base editors link a deaminase to a DNA-targeting protein and act within an editing window. Prime editors copy a template carried on a guide RNA. Compare their intended changes, bystander edits, off-target risks and repair-dependent outcomes. Subject: Science · Area: Genetics & Evolution · Ages 22 to 24 Page: https://lightmysky.com/learn/science/base-and-prime-editing-changing-dna-without-a-planned-double-strand-break-mt_roM4eWoWVn ## Ready when they can - State the intended substitutions of conventional cytosine and adenine base editors and the changes prime editing can specify - Explain the editing window and why bystander edits can appear beside the intended change - Distinguish on-target unwanted outcomes from off-target changes and explain why editor comparisons need matched conditions and assays ## Lesson: Editing without a planned double-strand break Earlier editing lessons showed that repair affects outcomes. Conventional base editors link a deaminase to a DNA-targeting Cas protein, often a nickase that cuts one strand. A cytosine base editor, or CBE, aims for C to T. An adenine base editor, or ABE, aims for A to G. These changes describe the edited strand; the opposite-strand descriptions are G to A and T to C. Other base-editor systems can have different menus. An editing window contains positions where the chosen editor can act. If a target C and another C both lie inside it, either may change. An unwanted neighbouring change is a bystander edit. A different guide may exclude that neighbour, but its required nearby DNA motif and window placement must permit this. Some targets have no suitable alternative guide. Conversion is possible, not guaranteed. *(drawing: All four positions are inside this illustrative CBE window. Both C bases may change.)* A Cas9 prime editor uses a nickase and reverse transcriptase, which copies RNA into DNA. The guide's targeting sequence pairs with the target DNA strand. The nickase cuts the other, non-target strand. Reverse transcriptase extends it using the extended guide's template. Cells can retain or reject this edited DNA. Prime editing can specify all 12 single-base substitutions and small insertions or deletions, with variable efficiency. Some versions also nick the other strand. Classify location and outcome separately. A neighbouring base change at the intended locus is an on-target bystander. An indel there is on-target; whether it is wanted depends on the requested edit. A change at another genomic site is off-target. Some deaminase editors can also affect RNA. Prime editors and nucleases can have unwanted outcomes. Compare the same cells with suitable assays; no editor class has a universal precision or speed ranking. Cellular processing can retain or reverse editing intermediates. In some prime-editing designs, an opposite-strand nick can favour the intended sequence but also increase unwanted outcomes. Endpoint sequencing alone cannot tell whether an undetected edit never formed or formed and was lost. Failed delivery, low editor activity and repair are possible causes. An edited fraction may also fall below the assay's detection limit. Match the requested change to the editor and target site. Conventional CBE and ABE menus exclude C to A, a transversion; prime editing is a candidate for that change. A nuclease cut alone specifies no replacement letter and can yield varied repair products. A nuclease plus a donor can support precise editing, but that is a different design. Avoiding a planned double-strand break does not make indel risk zero. **Recap.** Match the change to the editor and target site. Check bystanders, off-target changes and repair outcomes instead of assuming one editor is always most precise. ## Practice 8 questions on this page, each with its working shown. ## Needs first - [Genome Engineering: Cut Site, Repair Choice and Editing Outcome](https://lightmysky.com/learn/science/genome-engineering-cut-site-repair-choice-and-editing-outcome-mt_3DqvkO4wdE) ## 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)