In map-based sequencing, chromosomes are fragmented into relatively large pieces, around
100000-200000 base pairs (100-200 kb). The fragments are cloned into vectors that can accommodate large DNA sizes. The fragments that can hybridize to the DNA markers in the genetic map are identified.
Then other overlapping fragments are identified by their ability to hybridize to DNA isolated from the ends of the previously identified fragments.
One after another, additional overlapping fragments are identified until a contiguous series of clones (a "contig") is built.