Several models of DNA replication have been proposed throughout the history of science (Figure 1). The conservative model proposed that brand new copies of double-stranded DNA are synthesized without altering the original double-stranded DNA molecules. Another model, the dispersive model, proposed that the original DNA molecules are broken up into several pieces which then get dispersed throughout the newly produced strands of DNA.

Today, however, neither of these models is accepted as correct. Instead, a scientific consensus has formed around a third model of DNA replication, known as the semi-conservative model. As shown in Figure 1, the semi-conservative model proposes that the original double-stranded DNA molecule is separated into two single strands, which then serve as the template for the synthesis of two new strands. These new strands are called complementary strands because their nucleotide base pairs are “complementary” to the original template strand. Thus, adenine and cytosine on one strand will always be paired with a thymine and guanine respectively on the other strand. The result of semi-conservative replication is two daughter DNA molecules with a nucleotide sequence identical to that of the parent molecule and which each have one strand from the parent molecule and one newly synthesized stand. The fact that one strand is conserved from the parent molecule but not both is why this model of DNA replication is labelled “semi-conservative”.