Z disc (Z line) Platelike region of a muscle sarcomere to which the plus ends
of actin filaments are attached. Seen as a dark transverse line in micrographs.

zinc finger DNA-binding structural motif present in many gene regulatory
proteins. Composed of a loop of polypeptide chain held in a hairpin bend
bound to a zinc atom.

zona pellucida Glycoprotein layer on the surface of the unfertilized egg. It is
often a barrier to fertilization across species.

zygote Diploid cell produced by fusion of a male and female gamete. A
fertilized egg.

zygotene Second stage of division I of meiosis, in which the synaptonemal
complex begins to form between the two sets of sister chromatids in each
bivalent chromosome.