A membrane-bounded organelle of eukaryotic cells in which chromosomes are housed and replicated. In most cells, the nucleus contains all of the cell's chromosomes except the organellar chromosomes, and is the site of RNA synthesis and processing. In some species, or in specialized cell types, RNA metabolism or DNA replication may be absent [goid 5634] [evidence IEA]; That part of the nuclear content other than the chromosomes or the nucleolus [goid 5654] [pmid 9790902] [evidence EXP]; That part of the nuclear content other than the chromosomes or the nucleolus [goid 5654] [pmid 8946909] [evidence EXP]; That part of the nuclear content other than the chromosomes or the nucleolus [goid 5654] [pmid 1939271] [evidence EXP]; That part of the nuclear content other than the chromosomes or the nucleolus [goid 5654] [pmid 12676794] [evidence EXP]; That part of the nuclear content other than the chromosomes or the nucleolus [goid 5654] [pmid 2449431] [evidence EXP]; That part of the nuclear content other than the chromosomes or the nucleolus [goid 5654] [pmid 9512541] [evidence EXP]; That part of the nuclear content other than the chromosomes or the nucleolus [goid 5654] [pmid 9409616] [evidence EXP]; RNA polymerase II, one of three nuclear DNA-directed RNA polymerases found in all eukaryotes, is a multisubunit complex; typically it produces mRNAs, snoRNAs, and some of the snRNAs. Two large subunits comprise the most conserved portion including the catalytic site and share similarity with other eukaryotic and bacterial multisubunit RNA polymerases. The largest subunit of RNA polymerase II contains an essential carboxyl-terminal domain (CTD) composed of a variable number of heptapeptide repeats (YSPTSPS). The remainder of the complex is composed of smaller subunits (generally ten or more), some of which are also found in RNA polymerases I and III. Although the core is competent to mediate ribonucleic acid synthesis, it requires additional factors to select the appropriate template [goid 5665] [pmid 8797801] [evidence TAS]