8.1. DNA Replication

When a cell divides, it is important that each daughter cell receives an identical copy of the DNA. This is accomplished by the process of DNA replication. The replication of DNA occurs during the synthesis (S) phase of the cell cycle, before the cell divides by either mitosis or meiosis.

Discovery of DNA structure as a double helix provided the clue as to how DNA is copied. Recall the specific complementary pairing between nucleotide nitrogenous bases: adenine nucleotides pair with thymine nucleotides (A-T), and cytosine with guanine (C-G). For example, if one strand of DNA has a nucleotide sequence of AGTCATGA the other strand will have the complementary sequence TCAGTACT (Figure 8.1.1.).

Because of the complementarity of the two strands, having one strand means that it is possible to recreate the other strand. This model for replication suggests that the two strands of the double helix separate during replication, and each strand serves as a template from which the new complementary strand is copied (Figure 8.1.2.).

Because each of the two strands that make up the double helix serves as a template the process of creating a new DNA molecule is called semiconservative replication: the new double helix will contain one ‘old’ or parent strand that served as a template and one newly formed ‘complementary strand (nitrogenous base sequences  complementary to the parent strand). As a result of semiconservative replication, the newly formed DNA has an identical sequence of nucleotide bases and are divided equally into two daughter cells

DNA replication is an enzyme-catalyzed process. We will focus on two of the several enzymes involved. The two DNA strands must first separate. Helicase accomplishes this by breaking the hydrogen bonds between the complementary nitrogenous bases. New nucleotides must be added to the template (old strand) according to the rule of complementary pairing: A to T and G to C, DNA polymerase adds the new nucleotides at the 3′ end of the new strand (Figure 8.1.3.).