RNA polymerases catalyse the DNA dependent polymerisation of RNA. Prokaryotes contain a single RNA polymerase compared to three in eukaryotes (not including mitochondrial and chloroplast polymerases). This domain, domain 3, represents the pore domain. The 3' end of RNA is positioned close to this domain. The pore delimited by this domain is thought to act as a channel through which nucleotides enter the active site and/or where the 3' end of the RNA may be extruded during back-tracking [<cite idref="PUB00002975"/>, <cite idref="PUB00008731"/>].