Nucleobase - wikipedia Nucleobase `` ACGT '' redirects here. For the Japanese animation production company, see A.C.G.T. 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. 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. 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 ). Five nucleobases -- adenine ( A ), cytosine ( C ), guanine ( G ), thymine ( T ), and uracil ( U ) -- are called primary or canonical. They function as the fundamental units of the genetic code, with the bases A, G, C, and T being found in DNA while A, G, C, and U are found in RNA. Thymine and uracil are identical excepting that T includes a methyl group that U lacks. Adenine and guanine have a fused - ring skeletal structure derived of purine, hence they are called purine bases. Similarly, the simple - ring structure of cytosine, uracil, and thymine is derived of pyrimidine, so those three bases are called the pyrimidine bases. Each of the base pairs in a typical double - helix DNA comprises a purine and a pyrimidine : either an A paired with a T or a C paired with a G. These purine - pyrimidine pairs, which are called base complements, connect the two strands of the helix and are often compared to the rungs of a ladder. The pairing of purines and pyrimidines may result, in part, from dimensional constraints, as this combination enables a geometry of constant width for the DNA spiral helix. The A-T and C-G pairings function to form double or triple hydrogen bonds between the amine and carbonyl groups on the complementary bases. In August 2011, a report based on NASA studies of meteorites suggested that nucleobases such as adenine, guanine, xanthine, hypoxanthine, purine, 2, 6 - diaminopurine, and 6, 8 - diaminopurine may have formed in outer space as well as on earth. The origin of the term base reflects these compounds ' chemical properties in acid - base reactions, but those properties are not especially important for understanding most of the biological functions of nucleobases. Contents 1 Structure 2 Modified nucleobases 2.1 Modified purine nucleobases 2.2 Modified pyrimidine nucleobases 3 Artificial nucleobases 4 See also 5 References 6 External links Structure 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. 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. In the double helix of DNA, the two strands are oriented chemically in opposite directions, which permits base pairing by providing complementarity between the two bases, and which is essential for replication of or transcription of the encoded information found in DNA. Modified nucleobases DNA and RNA also contain other ( non-primary ) bases that have been modified after the nucleic acid chain has been formed. In DNA, the most common modified base is 5 - methylcytosine ( m C ). In RNA, there are many modified bases, including those contained in the nucleosides pseudouridine ( Ψ ), dihydrouridine ( D ), inosine ( I ), and 7 - methylguanosine ( m G ). Hypoxanthine and xanthine are two of the many bases created through mutagen presence, both of them through deamination ( replacement of the amine - group with a carbonyl - group ). Hypoxanthine is produced from adenine, xanthine from guanine., and uracil results from deamination of cytosine. Modified purine nucleobases These are examples of modified adenosine or guanosine. Nucleobase Hypoxanthine Xanthine 7 - Methylguanine Nucleoside Inosine Xanthosine X 7 - Methylguanosine m G Modified pyrimidine nucleobases These are examples of modified cytosine, thymine or uridine. Nucleobase 5, 6 - Dihydrouracil 5 - Methylcytosine 5 - Hydroxymethylcytosine Nucleoside Dihydrouridine 5 - Methylcytidine m C Artificial nucleobases Main article : Nucleic acid analogue 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-canonical bases. These compounds are activated in the cells by being converted into nucleotides ; they are administered as nucleosides as charged nucleotides can not easily cross cell membranes. At least one set of new base pairs has been announced as of May 2014. See also Nucleoside Nucleotide Nucleic acid notation Nucleic acid sequence References Jump up ^ Callahan ; Smith, K.E. ; Cleaves, H.J. ; Ruzica, J. ; Stern, J.C. ; Glavin, D.P. ; House, C.H. ; Dworkin, J.P. ( 11 August 2011 ). `` Carbonaceous meteorites contain a wide range of extraterrestrial nucleobases ''. PNAS. doi : 10.1073 / pnas. 1106493108. Retrieved 2011 - 08 - 15. Jump up ^ Steigerwald, John ( 8 August 2011 ). `` NASA Researchers : DNA Building Blocks Can Be Made in Space ''. NASA. Retrieved 2011 - 08 - 10. Jump up ^ ScienceDaily Staff ( 9 August 2011 ). `` DNA Building Blocks Can Be Made in Space, NASA Evidence Suggests ''. ScienceDaily. Retrieved 2011 - 08 - 09. Jump up ^ BIOL2060 : Translation Jump up ^ `` Role of 5 ' mRNA and 5 ' U snRNA cap structures in regulation of gene expression '' -- Research -- Retrieved 13 December 2010. Jump up ^ T Nguyen, D Brunson, CL Crespi, BW Penman, JS Wishnok, and SR Tannenbaum, DNA damage and mutation in human cells exposed to nitric oxide in vitro, Proc Natl Acad Sci US A. 1992 April 1 ; 89 ( 7 ) : 3030 -- 3034 Jump up ^ Denis A. Malyshev ; Kirandeep Dhami ; Thomas Lavergne ; Tingjian Chen ; Nan Dai ; Jeremy M. Foster ; Ivan R. Corrêa & Floyd E. Romesberg ( 2014 ). `` A semi-synthetic organism with an expanded genetic alphabet ''. Nature. 509 : 385 -- 388. doi : 10.1038 / nature13314. PMC 4058825. PMID 24805238. External links Base pairing in DNA Double Helix ( shows specific hydrogen bonds ) hide Nucleic acid constituents Nucleobase Purine Adenine Guanine Hypoxanthine Xanthine Purine analogue Pyrimidine Uracil Thymine Cytosine Pyrimidine analogue Unnatural base pair ( UBP ) Nucleoside Ribonucleoside Adenosine Guanosine 5 - Methyluridine Uridine 5 - Methylcytidine Cytidine Inosine Xanthosine Wybutosine Deoxyribonucleoside Deoxyadenosine Deoxyguanosine Thymidine Deoxyuridine Deoxycytidine Deoxyinosine Deoxyxanthosine Nucleotide ( Nucleoside monophosphate ) Ribonucleotide AMP GMP m UMP UMP CMP IMP XMP Deoxyribonucleotide dAMP dGMP dTMP dUMP dCMP dIMP dXMP Cyclic nucleotide cAMP cGMP c - di - GMP c - di - AMP cADPR Nucleoside diphosphate ADP GDP m UDP UDP CDP dADP dGDP dTDP dUDP dCDP Nucleoside triphosphate ATP GTP m UTP UTP CTP ITP XTP dATP dGTP dTTP dUTP dCTP dITP dXTP Retrieved from `` https://en.wikipedia.org/w/index.php?title=Nucleobase&oldid=837733160 '' Categories : Nucleobases DNA RNA Hidden categories : All articles with unsourced statements Articles with unsourced statements from April 2012 Use dmy dates from December 2012 Talk Contents About Wikipedia Български Bosanski Čeština Dansk Deutsch Esperanto Euskara فارسی Français 한국어 Հայերեն Hrvatski Bahasa Indonesia ქართული Latina Lietuvių Magyar Македонски Nederlands Norsk Polski Simple English Slovenčina Српски / srpski Srpskohrvatski / српскохрватски Svenska Tagalog Türkçe Tiếng Việt 中文 22 more Edit links This page was last edited on 22 April 2018, at 18 : 09 ( UTC ). 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Question: adenine guanine cytosine and thymine are the four in dna
Answer: nucleobases