A hairpin loop from a pre-mRNA. Highlighted are the nucleobases (green) and the ribose-phosphate backbone (blue). This is a single strand of RNA that folds back upon itself.
Three-dimensional representation of the 50S ribosomal subunit. Ribosomal RNA is in ochre, proteins in blue. The active site is a small segment of rRNA, indicated in red.
Watson-Crick base pairs in a siRNA (hydrogen atoms are not shown)
Structure of a fragment of an RNA, showing a guanosyl subunit.
Secondary structure of a telomerase RNA.
Structure of a hammerhead ribozyme, a ribozyme that cuts RNA
Uridine to pseudouridine is a common RNA modification.
Double-stranded RNA
Robert W. Holley, left, poses with his research team.

Polymeric molecule essential in various biological roles in coding, decoding, regulation and expression of genes.


500 related topics



Nucleotides are organic molecules consisting of a nucleoside and a phosphate.

This nucleotide contains the five-carbon sugar deoxyribose (at center), a nucleobase called adenine (upper right), and one phosphate group (left). The deoxyribose sugar joined only to the nitrogenous base forms a <u title="Nucleotide">Deoxyribonucleoside called deoxyadenosine, whereas the whole structure along with the phosphate group is a <u title="Deoxyadenosine monophosphate" href="deoxyadenosine monophosphate">nucleotide, a constituent of DNA with the name deoxyadenosine monophosphate.
Showing the arrangement of nucleotides within the structure of nucleic acids: At lower left, a monophosphate nucleotide; its nitrogenous base represents one side of a base-pair. At the upper right, four nucleotides form two base-pairs: thymine and adenine (connected by double hydrogen bonds) and guanine and cytosine (connected by triple hydrogen bonds). The individual nucleotide monomers are chain-joined at their sugar and phosphate molecules, forming two 'backbones' (a double helix) of nucleic acid, shown at upper left.
Structural elements of three nucleo tides —where one-, two- or three-phosphates are attached to the nucleo side (in yellow, blue, green) at center: 1st, the nucleotide termed as a nucleoside mono phosphate is formed by adding a phosphate (in red); 2nd, adding a second phosphate forms a nucleoside di phosphate; 3rd, adding a third phosphate results in a nucleoside tri phosphate. + The nitrogenous base (nucleobase) is indicated by "Base" and "glycosidic bond" (sugar bond). All five primary, or canonical, bases—the purines and pyrimidines—are sketched at right (in blue).
The synthesis of UMP. The color scheme is as follows: enzymes, <span style="color: rgb(219,155,36);">coenzymes, <span style="color: rgb(151,149,45);">substrate names , <span style="color: rgb(128,0,0);">inorganic molecules
The synthesis of IMP. The color scheme is as follows: enzymes, <span style="color: rgb(219,155,36);">coenzymes, <span style="color: rgb(151,149,45);">substrate names , <span style="color: rgb(227,13,196);">metal ions , <span style="color: rgb(128,0,0);">inorganic molecules

They serve as monomeric units of the nucleic acid polymers – deoxyribonucleic acid (DNA) and ribonucleic acid (RNA), both of which are essential biomolecules within all life-forms on Earth.


Chemical structure of uridine

Uracil (symbol U or Ura) is one of the four nucleobases in the nucleic acid RNA that are represented by the letters A, G, C and U. The others are adenine (A), cytosine (C), and guanine (G).


Traube purine synthesis

Guanine (symbol G or Gua) is one of the four main nucleobases found in the nucleic acids DNA and RNA, the others being adenine, cytosine, and thymine (uracil in RNA).


Nucleobases, also known as nitrogenous bases or often simply bases, are nitrogen-containing biological compounds that form nucleosides, which, in turn, are components of nucleotides, with all of these monomers constituting the basic building blocks of nucleic acids.

Base pairing: Two base pairs are produced by four nucleotide monomers, nucleobases are in blue. Guanine (G) is paired with cytosine (C) via three hydrogen bonds, in red. Adenine (A) is paired with uracil (U) via two hydrogen bonds, in red.
Purine nucleobases are fused-ring molecules.
Pyrimidine nucleobases are simple ring molecules.
Chemical structure of DNA, showing four nucleobase pairs produced by eight nucleotides: adenine (A) is joined to thymine (T), and guanine (G) is joined to cytosine (C). + This structure also shows the directionality of each of the two phosphate-deoxyribose backbones, or strands. The 5' to 3' (read "5 prime to 3 prime") directions are: down the strand on the left, and up the strand on the right. The strands twist around each other to form a double helix structure.

The ability of nucleobases to form base pairs and to stack one upon another leads directly to long-chain helical structures such as ribonucleic acid (RNA) and deoxyribonucleic acid (DNA).


Base pairing: Two base pairs are produced by four nucleotide monomers, nucleobases are in blue. Guanine (G) is paired with cytosine (C) via three hydrogen bonds, in red. Adenine (A) is paired with uracil (U) via two hydrogen bonds, in red.

Cytosine (symbol C or Cyt) is one of the four nucleobases found in DNA and RNA, along with adenine, guanine, and thymine (uracil in RNA).


All genetic information of an organism.

A label diagram explaining the different parts of a prokaryotic genome
An image of the 46 chromosomes making up the diploid genome of a human male. (The mitochondrial chromosome is not shown.)
Part of DNA sequence - prototypification of complete genome of virus
Composition of the human genome
Log-log plot of the total number of annotated proteins in genomes submitted to GenBank as a function of genome size.

It consists of nucleotide sequences of DNA (or RNA in RNA viruses).

Transfer RNA

The interaction of tRNA and mRNA in protein synthesis.
Secondary cloverleaf structure of tRNAPhe from yeast.
Tertiary structure of tRNA. CCA tail in yellow, Acceptor stem in purple, Variable loop in orange, D arm in red, Anticodon arm in blue with Anticodon in black, T arm in green.
3D animated GIF showing the structure of phenylalanine-tRNA from yeast (PDB ID 1ehz). White lines indicate base pairing by hydrogen bonds. In the orientation shown, the acceptor stem is on top and the anticodon on the bottom
Bulge-helix-bulge motif of a tRNA intron

Transfer RNA (abbreviated tRNA and formerly referred to as sRNA, for soluble RNA ) is an adaptor molecule composed of RNA, typically 76 to 90 nucleotides in length (in eukaryotes), that serves as the physical link between the mRNA and the amino acid sequence of proteins.

Non-coding RNA

Compare untranslated regions.

The roles of non-coding RNAs in the central dogma of molecular biology: Ribonucleoproteins are shown in red, non-coding RNAs in blue.
The cloverleaf structure of Yeast tRNAPhe (inset) and the 3D structure determined by X-ray analysis.
Atomic structure of the 50S Subunit from Haloarcula marismortui. Proteins are shown in blue and the two RNA strands in orange and yellow. The small patch of green in the center of the subunit is the active site.
Electron microscopy images of the yeast spliceosome. Note the bulk of the complex is in fact ncRNA.
The Ro autoantigen protein (white) binds the end of a double-stranded Y RNA (red) and a single stranded RNA (blue). (PDB: 1YVP ).

A non-coding RNA (ncRNA) is an RNA molecule that is not translated into a protein.

RNA interference

Lentiviral delivery of designed shRNAs and the mechanism of RNA interference in mammalian cells.
The dicer protein from Giardia intestinalis, which catalyzes the cleavage of dsRNA to siRNAs. The RNase domains are colored green, the PAZ domain yellow, the platform domain red, and the connector helix blue.
The stem-loop secondary structure of a pre-microRNA from Brassica oleracea.
small RNA Biogenesis: primary miRNAs (pri-miRNAs) are transcribed in the nucleus and fold back onto themselves as hairpins that are then trimmed in the nucleus by a microprocessor complex to form a ~60-70nt hairpin pre-RNA. This pre-miRNA is transported through the nuclear pore complex (NPC) into the cytoplasm, where Dicer further trims it to a ~20nt miRNA duplex (pre-siRNAs also enter the pathway at this step). This duplex is then loaded into Ago to form the “pre-RISC(RNA induced silencing complex)” and the passenger strand is released to form active RISC.
Left: A full-length argonaute protein from the archaea species Pyrococcus furiosus. Right: The PIWI domain of an argonaute protein in complex with double-stranded RNA.
The enzyme dicer trims double stranded RNA, to form small interfering RNA or microRNA. These processed RNAs are incorporated into the RNA-induced silencing complex (RISC), which targets messenger RNA to prevent translation.
Illustration of the major differences between plant and animal gene silencing. Natively expressed microRNA or exogenous small interfering RNA is processed by dicer and integrated into the RISC complex, which mediates gene silencing.
A normal adult Drosophila fly, a common model organism used in RNAi experiments.
Example petunia plants in which genes for pigmentation are silenced by RNAi. The left plant is wild-type; the right plants contain transgenes that induce suppression of both transgene and endogenous gene expression, giving rise to the unpigmented white areas of the flower.

RNA interference (RNAi) is a biological process in which RNA molecules are involved in sequence-specific suppression of gene expression by double-stranded RNA, through translational or transcriptional repression.


Nucleobase (a purine derivative).

Adenine structure, with standard numbering of positions in red.
Adenine on Crick and Watson's DNA molecular model, 1953. The picture is shown upside down compared to most modern drawings of adenine, such as those used in this article.

It also has functions in protein synthesis and as a chemical component of DNA and RNA.