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Gene Cloning and DNA Analysis: An Introduction, Sixth Edition ...

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102<br />

Figure 6.15<br />

(a) A typical cosmid<br />

(b) <strong>Cloning</strong> with pJB8<br />

amp R<br />

BamHI<br />

Circular<br />

pJB8<br />

cos<br />

BamHI BamHI BamHI<br />

ori<br />

BamHI<br />

Restrict with<br />

BamHI<br />

cos amp New<br />

<strong>DNA</strong><br />

cos<br />

R<br />

Recombinant<br />

cosmid <strong>DNA</strong><br />

amp R<br />

λ particles<br />

pJB8<br />

5.4 kb<br />

In vitro package<br />

cos<br />

Infect E. coli<br />

BamHI BamHI<br />

cos<br />

Linear pJB8<br />

Ligate<br />

A typical cosmid <strong>and</strong> the way it is used to clone long fragments of <strong>DNA</strong>.<br />

Part I The Basic Principles of <strong>Gene</strong> <strong>Cloning</strong> <strong>and</strong> <strong>DNA</strong> <strong><strong>An</strong>alysis</strong><br />

λ <strong>DNA</strong><br />

amp R<br />

Catenane<br />

BamHI BamHI<br />

New <strong>DNA</strong><br />

Colonies containing circular<br />

recombinant pJB8 molecules<br />

Ampicillin medium<br />

A cloning experiment with a cosmid is carried out as follows (Figure 6.15b). The<br />

cosmid is opened at its unique restriction site <strong>and</strong> new <strong>DNA</strong> fragments inserted. These<br />

fragments are usually produced by partial digestion with a restriction endonuclease, as<br />

total digestion almost invariably results in fragments that are too small to be cloned with<br />

a cosmid. Ligation is carried out so that catenanes are formed. Providing the inserted<br />

<strong>DNA</strong> is the right size, in vitro packaging cleaves the cos sites <strong>and</strong> places the recombinant<br />

cosmids in mature phage particles. These e phage are then used to infect an E. coli<br />

culture, though of course plaques are not formed. Instead, infected cells are plated onto<br />

a selective medium <strong>and</strong> antibiotic-resistant colonies are grown. All colonies are recombinants,<br />

as non-recombinant linear cosmids are too small to be packaged into e heads.<br />

6.4 8 <strong>and</strong> other high-capacity vectors enable<br />

genomic libraries to be constructed<br />

The main use of all e-based vectors is to clone <strong>DNA</strong> fragments that are too long to be<br />

h<strong>and</strong>led by plasmid or M13 vectors. A replacement vector, such as eEMBL4, can carry<br />

up to 20 kb of new <strong>DNA</strong>, <strong>and</strong> some cosmids can manage fragments up to 40 kb. This

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