Hydroxyurea: use in determining the duration of G2 in Tetrahymena

Observation of division of individual cells in microdrops, plus autoradiographic studies using tritiated thymidine and standard cell cycle analysis techniques, reveal that hydroxyurea (10 mM) reversibly arrests the normal progression of exponentially growing Tetrahymena pyriformis through the initial 92% of S-phase while not affecting cells in the terminal 8% and in G2 and division. Thus the fraction of the population of cells that is in G2 can be approximately determined by the fraction of the population able to divide in the presence of hydroxyurea. This fraction can be related to the approximate duration of G2 by calculations which compensate for the age gradient.