Pronucleus: In a zygoteplugin-autotooltip__default plugin-autotooltip_bigZygote: a cell formed by the fertilization of an oocyte by a sperm., the nucleiplugin-autotooltip__default plugin-autotooltip_bigNucleus: in eukaryotes, the membrane-bound organelle in cells that contains the chromosomes. of the sperm and oocyte that have not yet fused to form a diploidplugin-autotooltip__default plugin-autotooltip_bigDiploid: a term that describes a cell or organism that has two copies of similar genetic information, usually obtaining one copy from a male parent and the other copy from a female parent. nucleusplugin-autotooltip__default plugin-autotooltip_bigNucleus: in eukaryotes, the membrane-bound organelle in cells that contains the chromosomes..