Spliceosome: a large and complex enzymeplugin-autotooltip__default plugin-autotooltip_bigEnzyme: a macromolecule, usually a protein (but sometimes an RNA), that functions as a catalyst of some kind of biochemical reaction. located in the nucleusplugin-autotooltip__default plugin-autotooltip_bigNucleus: in eukaryotes, the membrane-bound organelle in cells that contains the chromosomes. of eukaryoticplugin-autotooltip__default plugin-autotooltip_bigeukaryote: organism whose cells have membrane bound organelles, including the nucleus. cells that catalyzes intronplugin-autotooltip__default plugin-autotooltip_bigIntron: sequences in a pre-mRNA that sit between exons and are removed by splicing. Formally speaking, introns are defined as being part of pre-mRNA, but the DNA sequence that codes for introns can also be informally described as introns. splicingplugin-autotooltip__default plugin-autotooltip_bigIntron splicing: the act of remove introns and joining exons from pre-mRNA to form an mRNA. This occurs in the nucleus of eukaryotic cells and is catalyzed by an enzyme called the spliceosome..