Nucleotide base

Thymine and uracil are distinguished by merely the presence or absence of a methyl group on the fifth carbon (C5) of these heterocyclic six-membered rings.

At the sides of nucleic acid structure, phosphate molecules successively connect the two sugar-rings of two adjacent nucleotide monomers, thereby creating a long chain biomolecule.

These chain-joins of phosphates with sugars (ribose or deoxyribose) create the "backbone" strands for a single- or double helix biomolecule.

[citation needed] DNA and RNA also contain other (non-primary) bases that have been modified after the nucleic acid chain has been formed.

The most common applications are used as fluorescent probes, either directly or indirectly, such as aminoallyl nucleotide, which are used to label cRNA or cDNA in microarrays.

Several groups are working on alternative "extra" base pairs to extend the genetic code, such as isoguanine and isocytosine or the fluorescent 2-amino-6-(2-thienyl)purine and pyrrole-2-carbaldehyde.

RNA is composed of purine and pyrimidine nucleotides, both of which are necessary for reliable information transfer, and thus Darwinian evolution.

Nam et al.[16] demonstrated the direct condensation of nucleobases with ribose to give ribonucleosides in aqueous microdroplets, a key step leading to RNA formation.

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.