During what phase of the cell cycle is DNA replicated?
There are two main phases of the cell cycle in eukaryotes: Mitosis and interphase. Since the time a cell spends in interphase takes approximately 90% of the entire cell cycle, it is helpful to further break interphase down into three sub-phases: G1 (first period of growth), S (synthesis), and G2 (second period of growth).
DNA is replicated during the S phase of the cell cycle. After passing the G1 checkpoint, which evaluates whether a cell has the size, nutrients, and DNA integrity to continue with replication, the cell becomes committed to division.
Upon entering the S phase, the enzyme helicase will "unzip" the double helix structure, breaking apart the bonds between adenine and thymine and between cytosine and guanine. The structure which resembled a ladder will now look more like a Y as the prongs of the replication fork become templates for the new strands of DNA.
A leading strand and a lagging strand will undergo different processes as new DNA is created using the "unzipped" DNA on each side. When the process is complete, an enzyme called DNA ligase will seal up the new sequence of DNA into two separate double strands of DNA. Each new strand of DNA winds back up into the recognizable double helix structure and prepares to enter the G2 phase of cellular division.