formed daughter strands. Indeed, in a classic example of British—and scientific—understatement, Watson and Crick wrote in their landmark Nature paper that, ''It has not escaped our notice that the specific pairings (of bases in DNA) we have postulated immediately suggests a possible copying mechanism for the genetic material."

This discovery was the culmination of decades of work by scores of researchers. Yet it marked the beginning, not the end, of a new era of biological research. It triggered a desire to know more about the bricks, mortar, and embellishments that filled out the basic architecture of DNA described by Watson and Crick and the precise building mechanisms used to construct it. Specific issues that were once beyond the scope of experiments—such as when does a cell know when to duplicate its genetic material and how does it do it?—could now be tackled. Moreover, broad problems—such as the way DNA duplication fits into the total cell cycle, the ongoing process of cell growth and division—could be addressed (see Box beginning on p. 100).

Over the past 40 years, the incentives for pursuing such research have been many and diverse. On one level there is the sheer intellectual excitement of getting to know the ways of life on an intimate basis. On another, such studies offer the opportunity to use new information about the structure and behavior of DNA for useful purposes: for example, improved understanding of DNA replication provides a means for controlling the unrestrained proliferation of DNA found in cancer cells (see Box beginning on p. 106).

And, over the years, experimental approaches to the study of genetic material have been equally diverse. Some researchers look at the simple genomes of viruses, which are often considered to be in a biological netherworld since they can't live on their own, while others look at the slightly larger, circular genomes of bacteria. The genetic material of yeasts, which are the simplest organisms to have DNA in organized chromosomes (and are used to make bread and beer), are the focus of still other labs. Finally, some researchers accept the challenge of looking at cells taken from advanced organisms, such as rats, frogs, plants, and man.

As with the study of most complex phenomena, the early results from studies of DNA replication and gene expression in these various organisms were murky and incomplete. It was not always clear that the piecemeal results could be synthesized into a coherent picture of what

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