Not so long ago, researchers had somewhat limited choices for locating and following a particular piece of DNA. A probe could be labeled using radioactivity, by kinasing an end or nick-translating the whole piece. A fragment of interest could be visualized (along with all other DNA and RNA species in the preparation) using ethidium bromide. With sufficient skill and patience an investigator could obtain from these rather crude techniques fairly impressive information, such as the precise 5' end of an RNA transcript or the presence of a previously characterized mutation.

Today, however, the questions are more demanding. In addition to determining whether a particular piece of DNA is present, researchers usually want to know the precise sequence of the target and how many copies are present. Frequently this precise identity and quantity question is asked of an enormous number of sequences at once in an array format, or "on the...

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