17-1 or Purves 9.4. The two stages of I and M corresponds to the two states of DNA described above. This was not obvious when M first discovered, because two parts of a doubled chromosome can stick together and look like one structure. There is a period after DNA is made but before division which is called G-2 or gap 2. Lengths of stages .
Fork grows too slowly for one origin , even with bidirectional replication.
So need large number of origins per chromosome . Telomerase allows regrowth of ends. See text or previous article from NY Times for more details on telomerase and cell immortality. E. Chromosomes, Chromatids and Centromeres. Chromosomes after S contain two double-stranded DNA molecules.