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One of the overarching themes of biology discussed in Chapter 1 is that the functional unit of life is the cell, a theme that rests in turn on the fundamental concept that all cells come from preexisting cells. In Chapter 11, we considered the mechanics of cell division and how it is regulated. Prokaryotic cells multiply by binary fission, whereas eukaryotic cells multiply by mitosis and cytokinesis. These processes ensure that cellular reproduction results in daughter cells that are like the parental cell. In other words, in cell division, like begets like. The molecular basis for the resemblance between parental cells and daughter cells is that a double-stranded DNA molecule in the parental cell duplicates and gives rise to two double-stranded daughter DNA molecules that are identical to each other except for rare mutations that may have taken place.

The process of duplicating a DNA molecule is called DNA replication, and it occurs in virtually the same way in all organisms, reflecting its evolution very early in life’s history. The process is conceptually simple—the parental strands separate and new complementary partner strands are made—but the molecular details are more complicated. Once scientists understood the molecular mechanisms of DNA replication, they could devise improved methods for manipulating and studying DNA. An understanding of DNA replication is therefore fundamental not only to understanding how cells and organisms produce offspring like themselves, but also to understanding some of the key experimental techniques in modern biology.