DNA is replicated during the S phase (Synthesis phase) of Interphase. During S phase, which follows G 1 phase, all the chromosomes are replicated. Following replication, each chromosome now consists of two sister chromatids . Thus, the amount of DNA in the cell has effectively doubled, even though the ploidy , or chromosome count, of the cell remains at 2 n . The number of chromosomes in the nucleus, which determines the ploidy, remains unchanged from the beginning to the end of the S phase.