We then review the collection of techniques that fall under the umbrella of reverse genetics, in which one begins with a gene or gene sequence and attempts to determine its function. These studies start with a genetic screen for isolating mutants of interest, and then proceed toward identification of the gene or genes responsible for the observed phenotype. We begin with the classical genetic approach to studying genes and gene function. In this section, we describe several different approaches to determining a gene's function, whether one starts from a DNA sequence or from an organism with an interesting phenotype. Determining which cellular processes have been disrupted or compromised in such mutants will then frequently provide a window to a gene's biological role. But to tackle directly the problem of how a gene functions in a cell or organism, the most effective approach involves studying mutants that either lack the gene or express an altered version of it. One approach, discussed earlier in the chapter, is to search databases for well-characterized proteins that have similar amino acid sequences to the protein encoded by a new gene, and from there employ some of the methods described in the previous section to explore the gene's function further.
Here the challenge is to translate sequence into function. Today, with numerous genome projects adding tens of thousands of nucleotide sequences to the public databases each day, the exploration of gene function often begins with a DNA sequence.
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