The construction of overlapping fragments of cloned DNA (contigs) is most efficient if end-pieces of the inserts can be rapidly isolated from the cloning vector. This may be accomplished by various procedures that are based on specific features of the vector. In the case of cosmids three methods are currently in use:i) the preparation of end-specific RNA probes generated via specific promoter sequences of the cloning vector (1,2);ii) PCR with one non-specific and one specific primer (3), and iii) inverted PCR(4).
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