What is the major event that takes place during the S phase?

The major event that takes place during the S phase is DNA replication. DNA replication occurs in the nucleus as that is where DNA is stored. Each chromosome becomes duplicated and thus, is composed of two sister chromatids. The sister chromatids remain interconnected until mitosis when they separate into two daughter cells. It is important to note that centriole duplication is another major event that occurs during the S phase. Centriole duplication is characterized by the formation of procentrioles at the proximal end of each parental centriole. The centriole forms spindle fibers during cell division in animal cells which direct the movement of chromosomes in the cell.