18S ribosomal RNA (abbreviated 18S rRNA) is a part of the

ribosomal RNA Ribosomal ribonucleic acid (rRNA) is a type of non-coding RNA which is the primary component of ribosomes, essential to all cells. rRNA is a ribozyme which carries out protein synthesis in ribosomes. Ribosomal RNA is transcribed from ribosomal ...

eukaryote The eukaryotes ( ) constitute the Domain (biology), domain of Eukaryota or Eukarya, organisms whose Cell (biology), cells have a membrane-bound cell nucleus, nucleus. All animals, plants, Fungus, fungi, seaweeds, and many unicellular organisms ...

s. It is a component of the

Eukaryotic small ribosomal subunit (40S) The eukaryotic small ribosomal subunit (40S) is the smaller subunit of the 80S, eukaryotic 80S ribosomes, with the other major component being the eukaryotic large ribosomal subunit (60S), large ribosomal subunit (60S). The "40S" and "60S" names o ...

and the

cytosol The cytosol, also known as cytoplasmic matrix or groundplasm, is one of the liquids found inside cells ( intracellular fluid (ICF)). It is separated into compartments by membranes. For example, the mitochondrial matrix separates the mitochondri ...

ic homologue of both the 12S rRNA in

mitochondria A mitochondrion () is an organelle found in the cells of most eukaryotes, such as animals, plants and fungi. Mitochondria have a double membrane structure and use aerobic respiration to generate adenosine triphosphate (ATP), which is us ...

and the

16S rRNA 16S ribosomal RNA (or 16Svedberg, S rRNA) is the RNA component of the 30S subunit of a prokaryotic ribosome (SSU rRNA). It binds to the Shine-Dalgarno sequence and provides most of the SSU structure. The genes coding for it are referred to as ...

plastid A plastid is a membrane-bound organelle found in the Cell (biology), cells of plants, algae, and some other eukaryotic organisms. Plastids are considered to be intracellular endosymbiotic cyanobacteria. Examples of plastids include chloroplasts ...

s and

prokaryote A prokaryote (; less commonly spelled procaryote) is a unicellular organism, single-celled organism whose cell (biology), cell lacks a cell nucleus, nucleus and other membrane-bound organelles. The word ''prokaryote'' comes from the Ancient Gree ...

s. Similar to the prokaryotic 16S rRNA, the genes of the 18S ribosomal RNA have been widely used for phylogenetic studies and biodiversity screening of eukaryotes.

Research history

Along with the 28S and 5.8S rRNA in eukaryotes, the 18S rRNA was early identified as integral structural element of

ribosome Ribosomes () are molecular machine, macromolecular machines, found within all cell (biology), cells, that perform Translation (biology), biological protein synthesis (messenger RNA translation). Ribosomes link amino acids together in the order s ...

s which were first characterized by their sedimentation properties and named according to measured Svedberg units. Given its ubiquitous presence in eukaryotic life, the evolution of the 18S rRNA was soon proposed as marker for phylogenetic studies to resolve the evolution of

Structure and function

The 18S ribosomal RNA is the structural RNA of the small subunit in the

eukaryotic The eukaryotes ( ) constitute the Domain (biology), domain of Eukaryota or Eukarya, organisms whose Cell (biology), cells have a membrane-bound cell nucleus, nucleus. All animals, plants, Fungus, fungi, seaweeds, and many unicellular organisms ...

cytoplasmic

The genomic sequence of the 18S rRNA is organized in a group with the 28S and 5.8S rRNA, separated and flanked by the ITS-1, ITS-2 and ETS spacer regions. These regions of ribosomal DNA (rDNA) are present with several hundred copies in the active genome, clustered in nucleolus organizer regions (NORs). In ribosome biogenesis, these genes are transcribed together by the RNA polymerase I and are processed in the

nucleolus The nucleolus (; : nucleoli ) is the largest structure in the cell nucleus, nucleus of eukaryote, eukaryotic cell (biology), cells. It is best known as the site of ribosome biogenesis. The nucleolus also participates in the formation of signa ...

structure of the nucleus. The length of the 18S rRNA varies considerably in the eukaryotic phylogenetic tree, corresponding to a range of 16S-19S in Svedberg units, with the average length commonly given as around 2000

nucleotide Nucleotides are Organic compound, organic molecules composed of a nitrogenous base, a pentose sugar and a phosphate. They serve as monomeric units of the nucleic acid polymers – deoxyribonucleic acid (DNA) and ribonucleic acid (RNA), both o ...

s. The 18S rRNA of

human Humans (''Homo sapiens'') or modern humans are the most common and widespread species of primate, and the last surviving species of the genus ''Homo''. They are Hominidae, great apes characterized by their Prehistory of nakedness and clothing ...

s has a length of 1869 nucleotides.Page ''Homo sapiens RNA, 18S ribosomal N5 (RNA18SN5), ribosomal RNA'' on

Uses

The universal presence of the 18S rRNA in

eukaryotes The eukaryotes ( ) constitute the domain of Eukaryota or Eukarya, organisms whose cells have a membrane-bound nucleus. All animals, plants, fungi, seaweeds, and many unicellular organisms are eukaryotes. They constitute a major group of ...

and generally high degree of conservation in evolution allow the construction of universal primers for DNA amplification by

polymerase chain reaction The polymerase chain reaction (PCR) is a method widely used to make millions to billions of copies of a specific DNA sample rapidly, allowing scientists to amplify a very small sample of DNA (or a part of it) sufficiently to enable detailed st ...

. The possible applications mirror molecular methods involving

prokaryotes A prokaryote (; less commonly spelled procaryote) is a single-celled organism whose cell lacks a nucleus and other membrane-bound organelles. The word ''prokaryote'' comes from the Ancient Greek (), meaning 'before', and (), meaning 'nut' ...

Biodiversity screening

Primers binding in highly conserved regions of the 18S rRNA are an important marker for

biodiversity Biodiversity is the variability of life, life on Earth. It can be measured on various levels. There is for example genetic variability, species diversity, ecosystem diversity and Phylogenetics, phylogenetic diversity. Diversity is not distribut ...

screening, allowing the amplification of unspecified or random targets from environmental samples as well as uncharacterized specimens from collections for

DNA sequencing DNA sequencing is the process of determining the nucleic acid sequence – the order of nucleotides in DNA. It includes any method or technology that is used to determine the order of the four bases: adenine, thymine, cytosine, and guanine. The ...

. Subsequent

sequence alignment In bioinformatics, a sequence alignment is a way of arranging the sequences of DNA, RNA, or protein to identify regions of similarity that may be a consequence of functional, structural biology, structural, or evolutionary relationships between ...

covering the less strictly conserved segments then allows the assignment of a sample to biologic

clades In biology, a clade (), also known as a monophyletic group or natural group, is a group of organisms that is composed of a common ancestor and all of its descendants. Clades are the fundamental unit of cladistics, a modern approach to taxonomy ...

. In the case of 18S rRNA, retrieval of DNA is improved by the abundance of repeating sequences of the rDNA within eukaryotic cells, promoting the sensitivity of the analysis.

Phylogenetics

Multiple properties of the genomic sequence of the 18S rRNA have established it as an important marker gene for large-scale phylogenetic analysis and the reconstruction of the metazoan

tree of life The tree of life is a fundamental archetype in many of the world's mythology, mythological, religion, religious, and philosophy, philosophical traditions. It is closely related to the concept of the sacred tree.Giovino, Mariana (2007). ''The ...

. The integral role in formation and function of the

is a key cause for its omnipresence in eukaryotic life. Meanwhile, the gene maintains a high degree of conservation under a persistent selective pressure in all living beings, highlighting its potential for comparison between distantly related clades. Early studies utilizing the 18S rRNA sequence constructed the first large-scale

phylogenetic tree A phylogenetic tree or phylogeny is a graphical representation which shows the evolutionary history between a set of species or taxa during a specific time.Felsenstein J. (2004). ''Inferring Phylogenies'' Sinauer Associates: Sunderland, MA. In ...

s of the metazoa. Evidence from further studies led to the creation of several important

clade In biology, a clade (), also known as a Monophyly, monophyletic group or natural group, is a group of organisms that is composed of a common ancestor and all of its descendants. Clades are the fundamental unit of cladistics, a modern approach t ...

s, such as the

Ecdysozoa Ecdysozoa () is a group of protostome animals, including Arthropoda (insects, chelicerates (including arachnids), crustaceans, and myriapods), Nematoda, and several smaller phylum (biology), phyla. The grouping of these animal phyla into a single ...

and Lophotrochozoa. During the latter part of the 2000s, and with increased numbers of taxa included into molecular phylogenies, however, two problems became apparent. First, there are prevailing sequencing impediments in representatives of certain taxa, such as the mollusk classes

Solenogastres The Solenogastres (less often referred to as Neomeniomorpha), common name, commonly known as solenogasters, are a class of small, worm-like, shell-less Mollusca, molluscs (Aplacophora), the other class being the Caudofoveata (Chaetodermomorpha).M ...

and Tryblidia, selected

bivalve Bivalvia () or bivalves, in previous centuries referred to as the Lamellibranchiata and Pelecypoda, is a class (biology), class of aquatic animal, aquatic molluscs (marine and freshwater) that have laterally compressed soft bodies enclosed b ...

taxa, and the enigmatic crustacean class

Remipedia Remipedia is a class of blind crustaceans, closely related to hexapods, found in coastal aquifers which contain saline groundwater, with populations identified in almost every ocean basin so far explored, including in Australia, the Caribbean Se ...

. Failure to obtain 18S sequences of single taxa is considered a common phenomenon but is rarely ever reported. Secondly, in contrast to initially high hopes, 18S cannot resolve nodes at all taxonomic levels and its efficacy varies considerably among clades. This has been discussed as an effect of rapid ancient radiation within short periods. Multigene analyses are currently thought to give more reliable results for tracing deep branching events in Metazoa but 18S still is extensively used in phylogenetic analyses.

References

This article incorporates CC-By-2.0 text from the reference. {{DEFAULTSORT:18s Ribosomal Rna Ribosomal RNA