Recombination

Recombination

  • Definition: Recombination is the process by which DNA molecules are broken and rejoined to create new genetic combinations.

  • Importance: Essential for several biological processes including:

    • Genetic diversity

    • Accurate chromosome segregation

    • DNA repair

    • Genome engineering

  • Central Theme: The controlled breakage and repair of DNA is a unifying concept across various recombination processes.

Types of Genetic Recombination

1. Homologous Recombination

  • Characteristics:

    • Requires extensive sequence similarity between DNA molecules.

    • Exhibits high fidelity during the exchange process.

2. Site-Specific Recombination

  • Characteristics:

    • Occurs at defined DNA sequences without the need for extensive homology.

3. Transposition

  • Characteristics:

    • Involves mobile genetic elements that can move within and between genomes.

Homologous Recombination: Overview

  • Process: The exchange of DNA strands occurs between homologous DNA molecules.

  • Key Occasions:

    • Meiosis

    • DNA repair

    • Genetic engineering

  • Requirements: Precise alignment of homologous sequences is vital for successful recombination.

Molecular Basis of Homologous Recombination

1. Double-Strand Break Formation

  • In bacteria, recombination often begins at a double-strand break.

  • RecBCD binds to DNA ends and unwinds the duplex.

When RecBCD encounters a chi site, it cleaves one strand and generates a 3′ single-stranded tail

  • In eukaryotes, Spo11 creates programmed double-strand breaks during meiosis

2. Strand Invasion

  • RecA (bacteria) binds the 3′ single-stranded DNA.

  • The RecA-coated strand invades a homologous duplex forming a triple helix.

  • This creates a Holliday junction

In yeast and eukaryotes:

Rad51 (RecA homolog) performs the same function

  • Dmc1 assists specifically in meiosis.

3. Holliday Junction Formation & Branch Migration

  • A four-stranded DNA structure forms.

  • RuvA and RuvB promote branch migration

4. Resolution

  • Resolvases (e.g., RuvC in E. coli) cleave the junction.

  • Resolution can produce:

    • Patch (non-crossover) recombinants

    • True crossover recombinants

The Holliday Junction Model

Structure

  • Four-stranded DNA cross structure.

  • Contains heteroduplex regions.

  • Can isomerize between conformations

Significance

  • Central intermediate of homologous recombination.

  • Allows exchange of genetic material.

Outcomes of Resolution

  1. Non-crossover (patch recombinant) – only a short heteroduplex patch remains.

  2. Crossover (true recombinant) – full exchange of chromosome arms

Rearrangement and Resolution of a Holliday Junction

  • Patch Recombinant: A type of outcome resulting from the rearrangement and resolution(only a short heteroduplex patch remains).

  • True Recombinant: Another outcome showcasing the results of the recombination process( full exchange of chromosome arms)

Proteins Involved in Homologous Recombination

RecA (Bacteria)

  • Binds single-stranded DNA.

  • Promotes strand invasion and homology searching.

  • Forms nucleoprotein filaments

RecBCD

  • Binds double-strand breaks.

  • Unwinds DNA.

  • Recognizes chi sites.

  • Generates 3′ ssDNA tails

RuvA/RuvB

  • Promote branch migration

RuvC

  • Resolvase that cleaves Holliday junctions

Rad51 (Eukaryotes)

  • Functional homolog of RecA.

  • Binds ssDNA and mediates strand invasion

Dmc1

  • Meiosis-specific recombinase

Homologous Recombination as DNA Repair

  • Function: Major pathway for double-strand break repair, utilizing an intact homologous sequence as a template.

  • Accuracy: High accuracy compared to non-homologous end joining (NHEJ).

  • Significance: Critical for maintaining genome stability.

Site-Specific Recombination: Overview

  • Process: Recombination occurs at short, specific DNA sequences.

  • Catalysis: Catalyzed by site-specific recombinases.

  • Characteristics:

    • Does not require extensive sequence homology.

    • Involves binding to specific DNA sites, introducing controlled DNA breaks, and exchanging subsequently rejoining DNA strands.

    • The overall reaction is reversible and tightly regulated.

Biological Roles of Site-Specific Recombination

  • Functions:

    • Integration and excision of bacteriophage DNA.

    • Resolution of chromosome dimers.

    • DNA inversions that control gene expression.

  • Applications: Widely used as molecular tools, such as the Cre-lox system

Feature

Homologous Recombination

Site-Specific Recombination

Sequence requirement

Requires long homologous sequences

Requires short specific recognition sites

Mechanism

Strand invasion → Holliday junction → resolution

Recombinase binds specific sites → staggered cuts → rejoining

Proteins

RecA/Rad51, RecBCD, Ruv proteins

Integrases (e.g., lambda Int), excisionase

Biological role

DNA repair, genetic diversity (meiosis)

Viral integration, genetic regulation

Integration of Lambda DNA—Detail of Crossover

  • Lambda contains attachment site attP.

  • E. coli chromosome contains attB.

  • Lambda integrase (Int protein) recognizes core region.

  • Staggered cuts are made in both DNAs.

  • A Holliday junction intermediate forms.

  • Resolution inserts lambda DNA into chromosome.

  • Excision requires both Int and Xis proteins

  • A single crossover is sufficient because lambda DNA circularizes before integration.

Targeted Gene Modification by Homologous Recombination

  • Process: Involves the introduction of a designed DNA construct that is flanked by homologous sequences surrounding a selectable marker.

  • Outcome: The endogenous gene is either replaced or disrupted based on homologous recombination.

  • Example:

    • Yeast ORFan Project:

    • Strategy: replace target gene with selectable marker flanked by homologous DNA sequences to ensure precise integration.

Applying Homologous Recombination to Targeted Gene Knockouts (ORFan Strategy)

Principle:

  • A DNA construct is designed containing:

    • A selectable marker (e.g., antibiotic resistance gene)

    • Flanking regions homologous to the target gene.

Process:

  1. Construct enters bacterial cell.

  2. Homologous recombination occurs between construct and chromosomal gene.

  3. Target gene is replaced with marker via double crossover.

  4. Selection identifies successful recombinants.

This relies on:

  • Homology-directed strand invasion.

  • Holliday junction formation.

  • Resolution producing replacement rather than insertion.

Thus, the ORFan knockout strategy exploits the same molecular machinery described for bacterial homologous recombination

Homologous Recombination Connection to CRISPR Gene Editing

  • Mechanism: CRISPR introduces a targeted double-strand break.

  • Repair Mechanisms: The cell may repair this break using:

    • Non-homologous end joining (error-prone).

    • Homologous recombination (HDR) when a template is provided.

  • Homology-Directed Repair (HDR):

    • Utilizes supplied donor DNA as a template, enabling precise gene edits and is mechanistically akin to natural homologous recombination.

Learning Objectives

  • After attending the course and completing the readings, students will be able to:

    • Define terms: recombination, crossover, heteroduplex, chi site, resolvase, and site-specific recombination.

    • Describe molecular steps of homologous recombination, including double-strand break formation, strand invasion, and resolution of recombination intermediates.

    • Explain the structure and significance of the Holliday junction while predicting possible crossover and non-crossover outcomes.

    • Identify key proteins involved in homologous recombination (e.g., RecA/Rad51) and detail their roles in DNA strand exchange.

    • Compare homologous recombination and site-specific recombination based on sequence requirements, mechanisms, and biological functions.

    • Describe the integration mechanism of lambda phage into the E. coli chromosome.

    • Apply principles of homologous recombination to elucidate how targeted gene knockouts are generated, particularly in the context of the ORFan project strategy.