Purine and pyrimidine base

Adenine (A)

Adenine is one of the purines. The two rings are formed by a 5-membered ring fused with a 6-membered ring. Conjugated double bonds are found between positions 1 and 6, 2 and 3, 4 and 5, 7 and 8. The Nitrogen atom at position 9 is covalently bonded to ribose or deoxyribose to form nucleosides called adenosine and deoxyadenosine respectively. Nucleotides, which are the main constituents in DNA and RNA, are formed after adding a phosphate group to nucleosides. In DNA, adenine pairs with Thymine through two hydrogen bonds. In RNA, adenine forms hydrogen bonds with Uracil instead of Thymine. The Nitrogen atom at position 1 and one of the Hydrogen atoms of the amino group attached to Carbon number 6 are involved in bond formation. In addition, many biomolecules which take part in important biochemical pathways are derived from Adenine. For example, adenosine-triphosphate (ATP) is an energy carrier that is involved in cellular respiration and photosynthesis. Other derivatives of adenine include cofactors nicotinamide adenine dinucleotide (NAD) and flavin adenine dinucleotide (FAD).

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Guanine (G)

Guanine is an example of purine. The structure of guanine consists of two rings fused together – one 6-membered ring and one 5-membered ring. Double bonds are found in the rings between positions 2 and 3, 4 and 5, 7 and 8. The Nitrogen atom at position 9 forms covalent bond with Carbon 1 of ribose or deoxyribose to form nucleosides called guanosine and deoxyguanosine respectively. Guanine is one of the nitrogenous bases found in both DNA and RNA. Guanine is complementary to cytosine in nucleic acids and they are held together by three hydrogen bonds. The atoms involved in hydrogen bond formation include Oxygen atom, Nitrogen atom at position 1 and one of the hydrogen atoms in the amino group at position 2.

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Cytosine (C)

Cytosine is one of the pyrimidines and the ring structure is six-membered. Double bonds are formed between N-3 and C-4, C-5 and C-6, as well as in keto group at C-2. An amino group at C-4 is also present in cytosine. N-1 is the position where ribose or deoxyribose are attached to form nucleosides cytidine and deoxycytidine respectively. Cytosine is a constituent present in both DNA and RNA. It is complementary to guanine and held by three hydrogen bonds. Oxygen atom in the keto group, N-3 and one of the Hydrogen atoms in the amino group are involved in hydrogen bond formation with guanine. Cytosine is not chemically stable. Point mutation occurs when cytosine is spontaneously converted to uracil by deamination. Tautomerization can also occur, changing cytosine from amino form to imino form due to proton movements between the amino group and the double bond between N-3 and C-4.

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Thymine (T)

Thymine is a pyrimidine, which has a six-membered ring in its structure. Double bonds can be found between positions 5 and 6 as well as in the two keto groups. It is structurally similar to another pyrimidine, namely uracil. Thymine has a methyl group at position 5 whereas –CH3 is replaced by a hydrogen atom in uracil. The Nitrogen atom at position 1 is covalently bonded to deoxyribose. It is different from adenine, guanine and cytosine that it is only present in DNA only but not in RNA. As one of the nitrogenous bases in DNA, thymine pairs up with adenine by two hydrogen bonds. The bond formation involves Hydrogen and Oxygen atoms attached to N-3 and C-4 respectively.

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Uracil (U)

Uracil is an example of pyrimidine, with a six-membered ring. It can only be found in RNA. There are double bonds between C-5 and C-6, as well as in the two keto groups. N-1 is where uracil links with ribose. Its structure is similar to thymine. A hydrogen atom is bonded to C-5 in uracil whereas methyl group rather than a hydrogen atom is found at the same position in thymine. RNA, therefore, has less steric hindrance due to the absence of the methyl group. Thymine is complementary to adenine in DNA but it is replaced by uracil in RNA. Uracil pairs with adenine via two hydrogen bonds during transcription. This kind of pairing also exists in RNA with secondary structure, for example, tRNA. The Hydrogen atom bonded to N-3 and the Oxygen atom of the keto group at C-4 are involved in hydrogen bond formation.

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