S Phase

S phase, the synthesis phase, is the stage of interphase during which the cell's DNA is actually replicated; cells caught in S phase therefore show an intermediate DNA content, somewhere between the 2C level of G1 and the 4C level reached by the end of S. Alongside DNA replication, the cell continues to grow, and it synthesises the histone proteins that package the newly made DNA, so that each replicated chromosome ends up correctly wrapped in chromatin rather than as bare DNA. The centrioles, which will later organise the mitotic spindle, also duplicate in the cytoplasm during S phase, so that by the time the cell enters mitosis it already has two pairs ready to migrate to opposite poles. A classic worked example illustrates how S-phase length is measured experimentally: if an asynchronously dividing culture of animal cells is briefly labelled with radioactive 3H-thymidine (which is incorporated only into cells actively replicating DNA) for 10 minutes, and autoradiography then shows 50% of cells are labelled, the length of the S period can be estimated as the fraction of cells caught in DNA replication multiplied by th …