An erythroblast is a RBC precursor that is a large cell with few organelles. In the cytoplasm, one may distinguish numerous polysomes and mitochondria. The DNA synthesis phase (S phase) is easily revealed by the method of autoradiography. Though the track records are rather bizarre (considering the distribution of reduced silver grains), it is evident that the radioactive precursor (³Н-thymidine) is incorporated in the areas where decondensed chromatin is present. Errors of enzyme functioning during the DNA synthesis (DNA polymerases) are rapidly fixed before the production finishes; as a rule, this is provided by faulty base excision and repair. Depending on conditions and cell types, S phase can last for a different period of time. Its average duration is about 8 hours for a population of rapidly proliferating cells. By the end of S phase, the DNA amount is doubled in the nucleus compared to interphase (in somatic cells nuclei 4n and 2n, respectively), i.e. the nuclei become polyploid. In the nucleus, one may see an active nucleolus.