Chromosomes are doubled in the S phase (synthesis phase) of the cell cycle. During the S phase, the DNA of the chromosomes is replicated, resulting in the formation of identical copies of each chromosome, or sister chromatids. These sister chromatids are joined at a central point called the centromere. After replication is complete, the cell has twice the amount of DNA as it did before replication, and each chromosome is composed of two identical sister chromatids. This process of DNA replication is critical for the cell to be able to divide and produce two identical daughter cells, each with the same genetic information as the parent cell.