The task of the S phase is to precisely replicate all of the DNA exactly one time. At this point in the cycle, the chromosomes still each consist of one chromatid. Once again, various proteins and enzymes are used to guarantee a high rate of duplication and a low error rate. The duration of the S phase is relatively constant, requiring 7-8 hours in nearly all human tissues. It begins with doubling of the centrioles, followed by the synthesis of , and finally doubling of the chromatids. Each chromosome then consists of two identical sister chromatids. Afterward the cell checks to see if the chromatid duplication was free of error.