Z-DNA • A form of DNA in which the molecule spirals to the left rather than to the right.

Zooplankton (zoe´ o plang ton) [Gr. zoon: animal + planktos: wandering] • The animal portion of the plankton.

Zoospore (zoe´ o spore) [Gr. zoon: animal + spora: seed] • In algae and fungi, any swimming spore. May be diploid or haploid.

Zygote (zye´ gote) [Gr. zygotos: yoked] • The cell created by the union of two gametes, in which the gamete nuclei are also fused. The earliest stage of the diploid generation.

Zymogen • An inactive precursor of a digestive enzyme secreted into the lumen of the gut, where a protease cleaves it to form the active enzyme.