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In Chapter 4 we introduced DNA and its role in gene expression. Then in Chapter 13 we presented evidence that DNA is the carrier of genetic information and described how DNA is replicated prior to cell division. Here we will focus on the evidence that proteins are the major products of gene expression, and we will describe how a gene is expressed as protein.

Scientists had a molecular understanding of phenotypes before it was known that DNA was the genetic material; they had studied the chemical differences between individuals carrying wild-type and mutant alleles in organisms as diverse as humans and bread molds and knew that the major phenotypic differences resulted from differences in specific proteins. But just how different proteins arise in different individuals was not understood.