---
title: "Homologous Recombination and Double-Strand Break Repair"
description: "A break across both strands is repaired by invading a matching sequence and copying from it, which is also how crossovers form in meiosis. The same mechanism is what makes targeted gene replacement po"
canonical: https://lightmysky.com/learn/science/homologous-recombination-and-double-strand-break-repair-mt_yWlwqAsXEf
source: https://lightmysky.com/learn/science/homologous-recombination-and-double-strand-break-repair-mt_yWlwqAsXEf.md
retrieved: 2026-09-12
---

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# Homologous Recombination and Double-Strand Break Repair

A break across both strands is repaired by invading a matching sequence and copying from it, which is also how crossovers form in meiosis. The same mechanism is what makes targeted gene replacement possible.

Subject: Science · Area: Genetics & Evolution · Ages 19 to 21
Page: https://lightmysky.com/learn/science/homologous-recombination-and-double-strand-break-repair-mt_yWlwqAsXEf

## Ready when they can

- Traces a double-strand break through strand invasion to a repaired duplex.
- Explains why this route needs a homologous partner and the alternative route does not.
- Links the mechanism to the crossovers counted in a linkage map.

## Lesson: Borrow a copy to heal

A double-strand break is the most dangerous lesion because both backbones are severed with no intact template in the same duplex. Homologous recombination repairs it accurately by trimming the ends to single-stranded tails, invading the sister chromatid or homolog, and copying the missing sequence from that partner. Resolving the joint molecule restores two intact duplexes, sometimes with a crossover between them.

Accuracy has a price: dependence on a partner, which is why this route dominates in S and G2 when sisters are present. The alternative, end joining, simply trims and ligates the broken ends with no template at all. It works all cycle long and needs no partner, but small insertions or deletions often remain at the join. Cells choose by context.

Meiosis puts the same machinery to a second use. Homologs pair, exchange segments, and lock together with structures that help them segregate correctly. The same strand invasion that fixes breaks now deliberately shuffles parental alleles onto recombinant chromatids.

**Tip.** Every crossover scored in a linkage map is one of these exchange events caught in the act. One percent recombinants means one crossover per hundred meioses in that interval, on average. Linked genes travel together precisely because crossovers between them are rare.

**Recap.** Copy from a partner for fidelity, ligate directly for speed, and count crossovers on the map.

## Practice

8 questions on this page, each with its working shown.

## Needs first

- [DNA Damage and the Repair Pathways That Fix It](https://lightmysky.com/learn/science/dna-damage-and-the-repair-pathways-that-fix-it-mt_r0qipp0YRR)
- [Linkage, Recombination Frequency and Genetic Maps](https://lightmysky.com/learn/science/linkage-recombination-frequency-and-genetic-maps-mt_xBjZXt3TVn)

## Opens up

- [CRISPR: A Bacterial Defence Turned into a Tool](https://lightmysky.com/learn/science/crispr-a-bacterial-defence-turned-into-a-tool-mt_0joPWRvOLw)
- [Transcription Initiation: Promoters, General Factors and Polymerase Recruitment](https://lightmysky.com/learn/science/transcription-initiation-promoters-general-factors-and-polymerase-recruitment-mt_0Ky-pYgh0Y)
- [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)
- [Antibody Diversity, Memory and What Tolerance Failure Costs](https://lightmysky.com/learn/science/antibody-diversity-memory-and-what-tolerance-failure-costs-mt_5YXKhiOqoA)