CHAPTER 5DNA Replication, Repair, and Recombination

The ability of cells to maintain a high degree of order in a chaotic universe depends on the accurate duplication of vast quantities of genetic information carried in chemical form as DNA. This process, called DNA replication, must occur before a cell can produce two genetically identical daughter cells. Maintaining order also requires the continued surveillance and repair of this genetic information, because DNA inside cells is repeatedly damaged by chemicals and radiation from the environment, as well as by thermal accidents and reactive molecules generated inside the cell. In this chapter, we describe the protein machines that replicate and repair the cell’s DNA. These machines catalyze some of the most rapid and accurate processes that take place within cells, and their mechanisms provide clear illustrations of the elegance and efficiency of cell chemistry.

The short-term survival of a cell depends on preventing harmful changes in its DNA. But the long-term survival of a species requires that these same DNA sequences be changeable over many generations to permit evolutionary adaptation to changing circumstances. We shall see that, despite the great efforts that cells make to protect their DNA, occasional changes in DNA sequences are unavoidable. These changes produce the genetic variation that is required for natural selection to drive the evolution of organisms.

We begin this chapter with a brief discussion of the changes that occur in DNA as it is passed down from generation to generation. Next, we discuss the mechanisms—DNA replication and DNA repair—that are responsible for minimizing these changes. Finally, we consider some of the most intriguing pathways that alter DNA sequences—those of DNA recombination. These pathways include the movement within chromosomes of special DNA sequences called transposable elements.