Abstract
Hydroxyurea (10 mM) blocks the exponential growth of populations of Tetrahymena pyriformis (GL-I) by arresting progress through the cell cycle once the cells enter S phase. Fluoromicroscopic examination of euchrysine-stained exponential growth phase cells exposed to HU for a minimum of 90 min show a radical morphological change in the macronucleus. This phenomenon, termed the ‘halo-effect’, is characterized by the formation of apparently membrane-associated aggregates surrounding a constricted chromatin mass. Electron microscopic examination reveals a condensation of chromatin granules away from the surrounding network of membrane-associated nucleoli. Halo-induction by HU is S phase specific. Upon removal of the HU block, the halo remains until the first recovery division. The physiological significance of the halo effect is discussed.
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