Photo: Amy Francis

Terry Weber, research technician at the Children's Hospital of Philadelphia, uses radioisotopes to label probes for chromosomal mapping studies.
Edwin M. Southern's 1975 paper, "Detection of specific sequences among DNA fragments separated by gel electrophoresis," described one of the landmark advances in early molecular biology. A new era was dawning as it suddenly became possible to detect specific sequences among the thousands of fragments produced by a restriction digest of genomic DNA.1 The "Southern blot," as it was almost immediately called, involved transferring denatured, gel-separated DNA fragments to a membrane (generally nitrocellulose), probing that membrane with a fragment of labeled DNA (generally 32P), and then identifying where the label stuck (generally by autoradiography). In this way it was possible, for example, to use a cloned mouse gene to identify the restriction fragment in which the human homologue resided.

There are a huge number of...

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