Historical background

The ability to hybridize DNA strands to one another derives from the complementary nature of DNA so elegantly demonstrated in Watson and Crick's model of the structure of DNA. The earliest applications, carried out in solution, concerned the renaturation (annealing) of DNA that had been made single stranded by denaturation, and demonstrated the complex nature of genomic DNA by defining components that renatured at different rates. Similar studies involving the formation of DNA/RNA hybrids demonstrated tissue- and cell-specific differences in RNA expression. Hybridization later evolved to become more the process which we recognize today, with the development of the concepts of unlabeled target sequences (usually a heterogeneous mixture of sequences) and a labeled probe capable of detecting specific sequences within the target. A further development was the immobilization of the target sequences on an inert matrix - initially nitrocellulose powder, and later nitrocellulose membrane.

The major breakthrough in hybridization came with the demonstration by Southern in 1975 that DNA fragments separated by gel electrophoresis could be easily transferred to a sheet of nitrocellulose membrane. This allowed characterization of DNA sequences not only by their hybridization behavior but also by the simultaneous determination of their sizes. The basic concept of blotting was later adapted to many other situations where spatial identification of a specific target was required. Blotting of bacterial colonies or phage plaques allows identification of specific clones in gene libraries and specific RNAs or proteins may be identified by blotting procedures known as northern and western blotting respectively.

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