These are examples of modified cytosine, thymine or uridine.Ī vast number of nucleobase analogues exist. These are examples of modified adenosine or guanosine. ![]() Hypoxanthine is produced from adenine, xanthine from guanine, and uracil results from deamination of cytosine. 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). In RNA, there are many modified bases, including those contained in the nucleosides pseudouridine (Ψ), dihydrouridine (D), inosine (I), and 7-methylguanosine (m 7G). In DNA, the most common modified base is 5-methylcytosine (m 5C). 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.ĭNA and RNA also contain other (non-primary) bases that have been modified after the nucleic acid chain has been formed. These chain-joins of phosphates with sugars ( ribose or deoxyribose) create the "backbone" strands for a single- or double helix biomolecule. The strands twist around each other to form a double helix structure.Īt 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. 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. + This structure also shows the directionality of each of the two phosphate-deoxyribose backbones, or strands. 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). 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. 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. In both cases, the hydrogen bonds are between the amine and carbonyl groups on the complementary bases. The A–T pairing is based on two hydrogen bonds, while the C–G pairing is based on three. Only pairing purine with pyrimidine ensures a constant width for the DNA. 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. Similarly, the simple-ring structure of cytosine, uracil, and thymine is derived of pyrimidine, so those three bases are called the pyrimidine bases.Įach 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. The purine nitrogenous bases are characterized by their single amino group ( −NH 2), at the C6 carbon in adenine and C2 in guanine. Īdenine and guanine have a fused-ring skeletal structure derived of purine, hence they are called purine bases. It differs in having an extra amine group, creating a more stable bond to thymine. ![]() ![]() In addition, some viruses have aminoadenine (Z) instead of adenine. 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. 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. Five nucleobases- adenine (A), cytosine (C), guanine (G), thymine (T), and uracil (U)-are called primary or canonical. 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). Nucleobases ( nitrogenous bases or 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. Pyrimidine nucleobases are simple ring molecules. ![]() Purine nucleobases are fused-ring molecules. Adenine (A) is paired with uracil (U) via two hydrogen bonds, in red. Guanine (G) is paired with cytosine (C) via three hydrogen bonds, in red. For the Japanese animation production company, see A.C.G.T.īase pairing: Two base pairs are produced by four nucleotide monomers, nucleobases are in blue.
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