Biochemical studies can tell us much about an individual protein, but they cannot prove that it is required for cell division or any other cell process. The importance of a protein is demonstrated most firmly if a mutation that prevents its synthesis or makes it nonfunctional adversely affects the process under study.

In a classical genetics approach, scientists isolate and characterize mutants that lack the ability to do something a normal organism can do. Often large genetic “screens” are done to look for many different mutant individuals (e.g., fruit flies, yeast cells) that are unable to complete a certain process, such as cell division or muscle formation. Mutations are usually produced by treatment with a mutagen, a chemical or physical agent that promotes mutations in a largely random fashion. But how can we isolate and maintain mutant organisms or cells that are defective in some process, such as cell division or protein secretion, that is essential for survival?

One way is to isolate organisms with a temperature-sensitive mutation. These mutants are able to grow at the permissive temperature, but not at another, usually higher temperature, the nonpermissive temperature. Normal cells can grow at either temperature. In most cases, a temperature-sensitive mutant produces an altered protein that works at the permissive temperature but unfolds and is nonfunctional at the nonpermissive temperature. Screens for temperature-sensitive mutations are most readily done with haploid organisms such as yeasts because they have only one copy of each gene, and thus a mutation in it will immediately have a consequence.

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By analyzing the effects of numerous different temperature-sensitive mutations that altered the division of haploid yeast cells, geneticists discovered most of the genes necessary for cell division without knowing anything, initially, about which proteins they encode or how these proteins participate in the process. In general, the great power of genetics is to reveal the existence and relevance of all proteins required for a particular cell function without prior knowledge of their biochemical identity or molecular function. These “mutation-defined” genes can be isolated and replicated (cloned) with recombinant DNA techniques discussed in Chapter 6. With the isolated genes in hand, the encoded proteins can be produced in a test tube or in engineered bacteria or cultured cells. In this way, biochemists can investigate whether the genes necessary for cell division encode proteins that associate with other proteins or DNA or catalyze particular chemical reactions during cell division (see Chapter 19).

Most of these yeast cell cycle genes are found in human cells as well, and the encoded proteins have similar amino acid sequences. Proteins from different organisms, but with similar amino acid sequences, are said to be homologous; such proteins may have the same or similar functions. Remarkably, it has been shown that a human cell cycle protein, when expressed in a mutant yeast defective in the homologous yeast protein, is able to “rescue the defect” of the mutant yeast (that is, to allow the cell to grow normally), thus demonstrating the protein’s ability to function in a very different type of eukaryotic cell. This experimental result, which garnered a Nobel Prize for Paul Nurse, was especially notable because the common ancestor of present-day yeasts, plants, and humans is thought to have lived over a billion years ago. Clearly the eukaryotic cell cycle and many of the genes and proteins that catalyze and regulate it evolved early in biological evolution and have remained quite constant over a very long period of evolutionary time. Subsequent studies showed that mutations in many yeast cell cycle proteins that allow uncontrolled cell growth also frequently occur in human cancers (see Chapter 24), again attesting to the important conserved functions of these proteins in all eukaryotes.