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Recombination

Cutting and covalent joining of DNA sequences.

Inter-molecular recombination Intra-molecular recombination

Between two different DNA molecules Between two regions of a single DNA molecule

Recombination may occur between linear and circular ds DNAtemplates or between two circular ds DNA templates.

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Intermolecular recombination

Donor dsDNA : Red Recipient dsDNA : Black Recombinant DNA : Red and Black line.X : Reciprocal cross-over between the two dsDNA molecules .half-X : Non- reciprocal cross-over between the two dsDNA molecules .

Case 1 ::

(A): Single, reciprocal cross-over between two linear dsDNA

(B): Single, non- reciprocal cross-over between two linear dsDNA

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Case 2 ::

(C): Double, reciprocal cross-over between two linear dsDNA

(D): Double, non- reciprocal cross-over between two linear dsDNA

Case 3 ::

(E): Single reciprocal cross-over between linear dsDNA & circular dsDNA

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Case 4 ::

(F): Double, reciprocal cross-over Single between linear dsDNA & circular dsDNA

(G): Double, Non- reciprocal cross-over Single between linear dsDNA & circulardsDNA

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Intramolecular recombination

Recombination between two regions on a single dsDNA molecule.

Looping out of the intermediate region.Products :: A shorter dsDNA molecule and a separate circular dsDNA molecule.Reverse reaction can also occur - resulting in the integration of a circular dsDNAmolecule into another dsDNA molecule

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Types of recombination

Site-specific recombination

Homologous recombination

Occurs between DNA sequences that arehomologous -- that is, the two DNAsequences are nearly identical.

Non-Homologous recombination

Random insertion into or replacement of DNA sequences of target sequence.

DNA strand exchange takes place

between segments possessing onlya limited degree of sequencehomology.

Occurs at specific DNA sequences. Specific sequences are recognized

by enzymes and catalyze recombination with a

specific recipient DNA.

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Homologous Recombination from a Molecular Perspective

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Site-specific Recombination

DNA strand exchange takes place between segments possessing only a limited degree of sequence homology.

Occurs at specific DNA sequences (recombination sites). Specific sequences are recognized by enzymes (recomb inases), and catalyze recombination with a specific recipient DNA.

Integrase in :: Recombinase

attP site in & attB site on E.coli ::Specific sites of recombination.

Integrase cleaves DNA at both the the specificsequences & insert the phage DNA into thebacterial genome between gal & bio operons.

Two hybrid sites are formed as a result:- attL &attR

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Cre-Lox recombination

Cre protein :: RecombinaseloxP site :: Specific site of recombination

The result of recombination depends on the orientation of the loxP sites.

(A) For two lox sites on the same chromosome arm, inverted loxP sites will cause an inversion of theintervening DNA.

(A) If loxP sites are on different chromosomes translocation occurs.

(A) Direct repeat of loxP sites on same chromosome will cause a deletion event of intervening DNA.

When cells that have loxP sites in their genome express Cre, a recombination event can occurbetween the loxP sites. The double stranded DNA is cut at both loxP sites by the Cre protein. Thestrands are then rejoined with DNA ligase.