What important event happens in S- Phase of the cell cycle?

DNA replication takes place during the synthesis phase of the cell cycle. The cell also forms a second centrosomeduring this phase.

The synthesis phase is often written as S phase. It occurs after the first growth (G1) phase, and therefore about midway through interphase.

At the start of the S phase, each chromosome has only one DNA molecule, but by the end of the S phase each has two, which, barring copying errors, are genetically identical, i.e. they have identical base sequences.

The doubling of the DNA and the centrosome constitute preparations for a subsequent cell division (mitosis or meiOSis). A cell that is not due to divide will not go through an S phase.