Editing Internal transcribed spacer (section)

== Organization ==
[[File:Eucaryot rdna.png|thumb|right|Organization of the eukaryotic nuclear ribosomal DNA tandem repeats]]

In [[bacteria]] and [[archaea]], the ITS occurs in one to several copies, as do the flanking 16S and [[23S ribosomal RNA|23S]] genes. When there are multiple copies, these do not occur adjacent to one another. Rather, they occur in discrete locations in the circular chromosome. It is not uncommon in bacteria to carry [[tRNA]] genes in the ITS.<ref>{{cite journal |last1=Takada |first1=Hiraku |last2=Shimada |first2=Tomohiro |last3=Dey |first3=Debashish |last4=Quyyum |first4=M. Zuhaib |last5=Nakano |first5=Masahiro |last6=Ishiguro |first6=Akira |last7=Yoshida |first7=Hideji |last8=Yamamoto |first8=Kaneyoshi |last9=Sen |first9=Ranjan |last10=Ishihama |first10=Akira |title=Differential Regulation of rRNA and tRNA Transcription from the rRNA-tRNA Composite Operon in Escherichia coli |journal=PLOS ONE |date=22 December 2016 |volume=11 |issue=12 |pages=e0163057 |doi=10.1371/journal.pone.0163057|pmid=28005933 |pmc=5179076 |bibcode=2016PLoSO..1163057T |doi-access=free }}</ref><ref>{{cite journal |last1=Stewart |first1=Frank J. |last2=Cavanaugh |first2=Colleen M. |title=Intragenomic Variation and Evolution of the Internal Transcribed Spacer of the rRNA Operon in Bacteria |journal=Journal of Molecular Evolution |date=July 2007 |volume=65 |issue=1 |pages=44–67 |doi=10.1007/s00239-006-0235-3|pmid=17568983 |bibcode=2007JMolE..65...44S |s2cid=13536182 |citeseerx=10.1.1.456.2659 }}</ref>

In eukaryotes, genes encoding ribosomal RNA and spacers occur in [[tandem repeat]]s that are thousands of copies long, each separated by regions of non-transcribed DNA termed ''[[intergenic spacer]]'' (IGS) or ''non-transcribed spacer'' (NTS).

Each eukaryotic ribosomal cluster contains the 5' [[external transcribed spacer]] (5' ETS), the [[18S rRNA]] gene, the ITS1, the [[5.8S rRNA]] gene, the ITS2, the [[28S rRNA|26S or 28S rRNA]] gene, and finally the 3' ETS.<ref name="Bena1998">{{Cite journal|last1=Bena|first1=Gilles|last2=Jubier|first2=Marie-France|last3=Olivieri|first3=Isabelle|last4=Lejeune|first4=Bernard|year=1998|title=Ribosomal External and Internal Transcribed Spacers: Combined Use in the Phylogenetic Analysis of ''Medicago'' (Leguminosae)|journal=Journal of Molecular Evolution|volume=46|issue=3|pages=299–306|doi=10.1007/PL00006306|pmid=9502673|issn=0022-2844|bibcode=1998JMolE..46..299B|s2cid=38838013}}</ref>

During rRNA maturation, ETS and ITS pieces are excised. As non-functional by-products of this maturation, they are rapidly degraded.<ref>{{Cite journal|last1=Michot|first1=Bernard|last2=Bachellerie|first2=Jean-Pierre|last3=Raynal|first3=Francoise|date=1983-05-25|title=Structure of mouse rRNA precursors. Complete sequence and potential folding of the spacer regions between 18S and 28S rRNA|journal=Nucleic Acids Research|volume=11|issue=10|pages=3375–3391|doi=10.1093/nar/11.10.3375|pmid=6304630|pmc=325970|issn=0305-1048}}</ref>