Cell cycle consists of events that ultimately result in generation of two daughter cells from a single cell. These events include Interphase, mitotic phase and cytokinesis. The interphase is divided into three parts: G1, S and G2. It is during the S-phase (or synthesis phase), that replication takes place. During the preceding G1 phase (or growth phase), the cell starts gaining mass and grows in size, builds organelles and protein. In the S-phase, DNA replication takes place and the cell ends up with exactly double the amount of chromosomes, with each chromosome consisting of two chromatids. This is a relatively fast step, especially compared to the G1 phase and proceeds at a fast pace to protect the exposed base pairs.
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Replication of DNA happens in the S phase of the cell cycle. It is an abbreviation for "synthesis" which was discovered when radioactive phosphate was found to be incorporated in the time frame of cell division. There were two gaps between mitosis and synthesis, Gap 1 (G1) and Gap 2 (G2).
It happens during the G1 phase