The DNA double helix acts as a template for its own duplication
Because the nucleotide A will successfully pair only with T, and G only with C (Chargaff's rule), each strand of DNA can serve as a template to specify the sequence of nucleotides in its complementary strand by DNA base–pairing. In this way, a double–helical DNA molecule can be copied precisely. Perhaps the most significant aspect of Watson and Crick's discovery of DNA structure was not that it provided scientists with a three–dimensional model of this molecule, but rather that this structure seemed to reveal the way in which DNA was replicated.
All organisms must duplicate their DNA with extraordinary accuracy before each cell division. Scientists have devoted decades of effort to understand how deoxyribonucleic acid (DNA) replicates itself. Replication is the process by which a cell copies its DNA prior to division. In humans, for example, each parent cell must copy its entire six billion base pairs of DNA before undergoing mitosis.
Illustration of DNA replication
During DNA replication, special enzymes move up along the DNA ladder, unzipping the molecule as it moves along. New nucleotides move in to each side of the unzipped ladder. The bases on these nucleotides are very particular about what they connect to. Cytosine (C) will "pair" to guanine (G), and adenine (A) will "pair" to thymine (T). How the bases are arranged in the DNA is what determines the genetic code.
When the enzyme has passed the end of the DNA, two identical molecules of DNA are left behind. Each contains one side of the original DNA and one side made of "new" nucleotides. It is possible that mistakes were made along the way – in other words, that a base pair in one DNA molecule doesn't match the corresponding pair in the other molecule.
The process of DNA replication is semiconservative; that is, the copy of the genome inherited by each daughter cell contains one original and one newly synthesized strand of DNA. After DNA replication is complete, the cell must physically separate the two copies of the genome and divide into two distinct membrane-bound cells through a process known as the cell cycle. DNA replication occurs during a phase of this cycle known as S phase, while the process of segregating chromosomes and splitting the cytoplasm occurs during M phase.