Nucleobases are nitrogen-containing biological compounds found within nucleotides—the basic
building blocks of deoxyribonucleic acid and ribonucleic acid.
Nucleobases are also found in nucleosides.
Often simply called bases in genetics, their ability to form base-pairs and to stack upon
one another lead directly to the helical structure of DNA and RNA.
Use of the word base is historical, in reference to the chemical properties of nucleobases
in acid-base reactions within the test tube, and is not especially relevant or important
for understanding most of their biological functions.
The primary nucleobases are cytosine, guanine, adenine, thymine and uracil, abbreviated as
C, G, A, T, and U, respectively.
Because A, G, C, and T appear in the DNA, these molecules are called DNA-bases; A, G,
C, and U are called RNA-bases.
Uracil and thymine are identical except that uracil lacks the 5' methyl group.
Adenine and guanine belong to the double-ringed class of molecules called purines.
Cytosine, thymine, and uracil are all pyrimidines.
In normal spiral DNA the bases form pairs between the two strands: A with T and C with
G. Purines pair with pyrimidines mainly for dimensional reasons - only this combination
fits the constant width geometry of the DNA spiral.
The A-T and C-G pairings are required to match the hydrogen bonds between the amine and carbonyl
groups on the complementary bases.
The compound formed when a nucleobase forms a glycosidic bond with the 1' anomeric carbon
of a ribose or deoxyribose is called a nucleoside, and a nucleoside with one or more phosphate
groups attached at the 5' carbon is called a nucleotide.
Apart from adenine, cytosine, guanine, thymine and uracil, DNA and RNA also contain bases
that have been modified after the nucleic acid chain has been formed.
In DNA, the most common modified base is 5-methylcytosine.
In RNA, there are many modified bases, including those contained in the nucleosides pseudouridine,
dihydrouridine, inosine, and 7-methylguanosine.
Hypoxanthine and xanthine are two of the many bases created through mutagen presence, both
of them through deamination.
Hypoxanthine is produced from adenine, xanthine from guanine.
In similar manner, deamination of cytosine results in uracil.
In August 2011, a report, based on NASA studies with meteorites found on Earth, was published
suggesting nucleobases may have been formed in outer space.
Structure The "skeleton" of adenine and guanine is purine,
hence the name purine-bases.
The "skeleton" of cytosine, uracil, and thymine is pyrimidine, hence pyrimidine-bases.
Primary bases The following are nucleobases and ribonucleoside,
a type of nucleoside including ribose as a component.
Modified purine bases These are examples of modified adenosine or
guanosine.
Modified pyrimidine bases These are examples of modified cytidine, thymidine
or uridine.
Novel bases
A vast number of nucleobase analogues exist.
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.
In medicine, several nucleoside analogues are used as anticancer and antiviral agents.
The viral polymerase incorporates these compounds with non-canon bases.
These compounds are activated in the cells by being converted into nucleotides; they
are administered as nucleosides as charged nucleotides cannot easily cross cell membranes.
See also Nucleoside
Nucleotide Nucleic acid notation
References
External links Base pairing in DNA Double Helix
Bridged Nucleic Acid