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It differs in having an extra amine group, creating a more stable bond to thymine. [3] Adenine and guanine have a fused-ring skeletal structure derived of purine, hence they are called purine bases. [4] The purine nitrogenous bases are characterized by their single amino group (−NH 2), at the C6 carbon in adenine and C2 in guanine. [5]
It binds to cytosine through three hydrogen bonds. In cytosine, the amino group acts as the hydrogen bond donor and the C-2 carbonyl and the N-3 amine as the hydrogen-bond acceptors. Guanine has the C-6 carbonyl group that acts as the hydrogen bond acceptor, while a group at N-1 and the amino group at C-2 act as the hydrogen bond donors.
Each unit is joined when a covalent bond forms between its phosphate group and the pentose sugar of the next nucleotide, forming a sugar-phosphate backbone. DNA is a complementary, double stranded structure as specific base pairing (adenine and thymine, guanine and cytosine) occurs naturally when hydrogen bonds form between the nucleotide bases.
Cytosine, thymine, and uracil are pyrimidines, hence the glycosidic bonds form between their 1 nitrogen and the 1' -OH of the deoxyribose. For both the purine and pyrimidine bases, the phosphate group forms a bond with the deoxyribose sugar through an ester bond between one of its negatively charged oxygen groups and the 5' -OH of the sugar. [2]
Cytosine can be found as part of DNA, as part of RNA, or as a part of a nucleotide. As cytidine triphosphate (CTP), it can act as a co-factor to enzymes, and can transfer a phosphate to convert adenosine diphosphate (ADP) to adenosine triphosphate (ATP). In DNA and RNA, cytosine is paired with guanine.
This is particularly important in RNA molecules (e.g., transfer RNA), where Watson–Crick base pairs (guanine–cytosine and adenine–uracil) permit the formation of short double-stranded helices, and a wide variety of non–Watson–Crick interactions (e.g., G–U or A–A) allow RNAs to fold into a vast range of specific three-dimensional ...
In the G-C Hoogsteen base pair, like the A-T Hoogsteen base pair, the purine (guanine) is rotated 180° about the glycosidic bond while the pyrimidine (cytosine) remains in place. One hydrogen bond from the Watson-Crick base pair is maintained (guanine O6 and cytosine N4) and the other occurs between guanine N7 and a protonated cytosine N3 ...
Between A and T there are two hydrogen bonds, while there are three between C and G. Right: two complementary strands of DNA. Complementarity is achieved by distinct interactions between nucleobases: adenine, thymine (uracil in RNA), guanine and cytosine. Adenine and guanine are purines, while thymine, cytosine and uracil are pyrimidines ...