The length of the different phases of the cell cycle can be measured by performing a pulse-chase experiment with radioactive thymidine and waiting to see how long it takes for radioactivity to appear in mitotic chromosomes. After the radioactivity first arrives in mitotic chromosomes, it is observed that the amount of radioactivity in these chromosomes increases steadily over several hours, reaches a plateau, and then declines. What cell-cycle phase is measured by the time between the first and last appearance of radioactive thymidine in condensed, mitotic chromosomes