DNA: Structure and Replication

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James Watson (an American microbial geneticist) and Francis Crick (an English physicist) solved the structure of DNA in 1953. Their model of the structure of DNA was revolutionary. It proposed a definition for the gene in chemical terms and, in doing so, paved the way for an understanding of gene action and heredity at the molecular level. A measure of the importance of their discovery is that the double-helical structure has become a cultural icon that is seen more and more frequently in paintings, in sculptures, and even in playgrounds (Figure 7-1).

The story begins in the first half of the twentieth century, when the results of several experiments led scientists to conclude that DNA is the genetic material, not some other biological molecule such as a carbohydrate, protein, or lipid. DNA is a simple molecule made up of only four different building blocks (the four nucleotide bases). It was thus necessary to understand how this very simple molecule could be the blueprint for the incredible diversity of organisms on Earth.

The model of the double helix proposed by Watson and Crick was built upon the results of scientists before them. They relied on earlier discoveries of the chemical composition of DNA and the ratios of its bases. In addition, X-ray diffraction pictures of DNA revealed to the trained eye that DNA is a helix of precise dimensions. Watson and Crick concluded that DNA is a double helix composed of two strands of linked nucleotide bases that wind around each other.

The proposed structure of the hereditary material immediately suggested how it could serve as a blueprint and how this blueprint could be passed down through the generations. First, the information for making an organism is encoded in the sequence of the nucleotide bases composing the two DNA strands of the helix. Second, because of the rules of base complementarity discovered by Watson and Crick, the sequence of one strand dictates the sequence of the other strand. In this way, the genetic information in the DNA sequence can be passed down from one generation to the next by having each of the separated strands of DNA serve as a template for producing new copies of the molecule.

In this chapter, we focus on DNA, its structure, and the production of DNA copies in a process called replication. Precisely how DNA is replicated is still an active area of research more than 50 years after the discovery of the double helix. Our current understanding of the mechanism of replication gives a central role to a protein machine, called the replisome. This complex of proteins coordinates the numerous reactions that are necessary for the rapid and accurate replication of DNA.