(^180) Pingoud, Alves, and Geiger
Table 12
Main Suppliers of Restriction Enzymes
Amersham Buchler (Braunschweig, Germany)
Applied Genetechnology Systems (AGS, Heidelberg, Germany)
Appligene (Illkirch, France)
Biozym Diagnostik (Hameln, Germany)
Boehringer (Mannheim, Germany)
GIBCO-BRL (Gaithersburg, MD)
ICN Biomedicals (Cleveland, OH)
International Biotechnologies, Inc. (IBI, New Haven, CT)
Janssen Biochimica (Beerse, Belgium)
New England Biolabs (Beverly, MA)
New Brunswick Scientific Company (Hatfield, UK)
Paesel & Lorei (Frankfurt/Main, Germany)
Pharmacia-LKB (Uppsala, Sweden)
Promega (Madison, WI)
Sigma (St. Louis, MO)
Stehelin (Basel, Switzerland)
Stratagene (La Jolla, CA)
United States Biochemical Corporation (USB, Cleveland, OH)
by loading the gel slice on top of another gel for the analysis in the
second dimension. For small DNAs, such as plasmid DNAs, there is
no difficulty in finding a suitable set of enzymes for restriction map-
ping. For longer DNAs, one has to find rare cutting enzymes to obtain
well-resolved restriction fragments. This puts the upper limit for
restriction mapping in the range of the length of small prokaryotic
genomes (186,187).
Restriction mapping may be used to obtain an estimate of the num-
ber of copies of genes present in a genome. Digestion of the total genomic
DNA by a restriction enzyme that has no site within the gene (more
precisely the DNA region complementary to the probe) to be analyzed
followed by electrophoresis, transfer to nitrocellulose, and hybridiza-
tion to a specific nucleic acid probe identifies a number of bands that
correspond to the minimum number of genes within this genome.
4.2. Restriction Fragment Length Polymorphisms
The pattern of DNA restriction fragments is characteristic for a
given DNA. Point mutations or rearrangements of the DNA lead to a
change in the length of one or more restriction fragments. With a specific
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