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carried out by using restriction enzyme mapping. Although this is not an unambiguous

method of ordering clones it is useful when also applying statistical probabilities of the

overlap between clones. In order to link the contigs techniques such asin situhybrid-

isation may be used or a probe generated from one end of a contig in order to screen a

different disconnected contig. This method of probe production and identification

is termedwalking, and has been used successfully in the production of physical maps

Cosmid/ YAC library Ordering of clones

by contigs

Overlapping

sequences

Fig. 6.47Physical mapping using continuous overlapping cloned fragments (contigs). In order to assign the

position of cloned DNA fragments resulting from the construction of a library in a YAC or cosmid vector,

overlaps are detected between the clone fragments. These are created because of the use of partial digestion

conditions when the libraries are constructed.

Table 6.6Current selected genome-sequencing projects

Organism Genome size (Mb)

Bacteria Escherichia coli 4.6

Yeast Saccharomyces cerevisiae 14

Roundworm Caenorhabditis elegans 100

Fruit fly Drosophila melanogaster 165

Puffer fish Fugu rubripes rubripes 400

Mouse Mus musculus 3000

255 6.9 Analysing whole genomes