Uracil U and Thymine T

Fig. 4,2. Separation of mono- (MP), di (DP) and triphosphate (TP) nucleotides of adenine (A), guanine (G), hypoxanthine (I), xanthine (X), cytosine (C), uracil (U) and thymine (T). Chromatographic conditions column, Partisil 10-SAX mobile phase, linear gradient of 0.007 M KHjPO, pH 4.0 to 0.25 M KHjPO, 0.5 M KCl, pH 4.5 in 45 min flow rate, 1.5 ml/min temperature, ambient detection, UV at 260 nm. Reproduced from Zakaria and Brown (1981), with permission.

Fig. 26. Nucleic acid structures. A The structure of the four bases in DNA, guanine (G), cytosine (C) adenine (A) and thymine (T). Uracil (U) replaces thymine (T) in RNA. B The spontaneous attraction of A for T and C for G allows the recognition of homologous sequences in aqueous solutions and the strong and specific hybridization of one sequence with its homologous sequence. C DNA forms a double helix at body temperature, which can be denatured to separate the strands by heating. D single stranded mRNA structure.

Fig. 4.6. A short string of single-strand DNA giving the formulae of four bases. In RNA, thymine (T) is replaced by uracil (U) and deoxyribose is replaced by ribose.

There are five common bases found in nucleic acids. Adenine (A), guanine (G) and cytosine (C) are found in both DNA and RNA. Uracil (U) is found only in RNA and thymine (T) only in DNA. The structures of these bases are shown in Figure 13.2. Adenine and guanine are purine bases while uracil, thymine and cytosine are the pyrimidine bases. 

Only five common nitrogen heterocycles are used to form these nucleosides. Three compounds have one ring, and are derived from a nitrogen heterocycle called pyrimidine. Two are bicyclic, and are derived from a nitrogen heterocycle called purine. These five amines are referred to as bases. Each base is designated by a one-letter abbreviation, as shown in the names and structures drawn. Note that uracil (U) occurs only in ribonucleosides and thymine (T) occurs only in deoxyribonucleosides. 

A nitrogenous heterocyclic base (either a purine or a pyrimidine) attached to the 1 -carbon atom of the sugar by an N-glycosidic bond. In DNA the purine bases are adenine (A) and guanine (G) and the pyrimidine bases are cytosine (C) and thymine (T). The bases in RNA are the same except that uracil (U), a pyrimidine, replaces thymine. 

All nncleic acids are polynucleotides, with each nucleotide being made np of a base, a sugar unit, and a phosphate. The composition of DNA differs from that of RNA in two major ways (see Figure 1). Whereas DNA contains the bases gnanine (G), cytosine (C), adenine (A), and thymine (T), RNA contains G, C, and A, but it contains uracil (U) in place of thymine. Both DNA and RNA contain a five-membered cyclic sngar (a pentose). RNA contains a ribose sngar. The sugar in DNA, however, is 2 -deoxyribose. 

Ribonucleic acid (RNA) is a nucleic acid, like DNA, and is also made up of sugars, phosphates, and nitrogenous bases (or just a base). It contains a ri-bose sugar, whereas DNA contains a deoxyribose sugar. The four main bases found in RNA are guanine (G), cytosine (C), uracil (U), and adenine (A). DNA contains thymine (T) instead of uracil. 

The heterocyclic bases in RNA are adenine, guanine, cytosine, and uracil (A, G, C, and U) the heterocyclic bases in DNA are adenine, guanine, cytosine, and thymine (A, G, C, and T). In other words, the heterocyclic bases in RNA and DNA are the same, except that RNA contains U s, whereas DNA contains T s (Sections 27.1 and 27.14). The T s used for the biosynthesis of DNA are synthesized from U s by thymidylate synthase, an enzyme that requires A, A °-methylene-THF as a coenzyme. Even though the only structural difference between a U and a T is a methyl group, a T is synthesized by first transferring a methylene group to a U. 

Nucleic acids are made up form three components nucleobases (usually referred to as bases), sugars and phosphoric acid. The nucleobases are derivatives of purine and pyrimidine (Figs. 1.20 and 1.21). Both DNA and RNA contain the purines Adenine (A) and Guanine (G). Of the pyrimidines. Thymine (T) and Cytosine (C) are components of DNA whereas Uracil (U) and Cytosine (C) are components of RNA. The sugar component of DNA is /3-D-deoxyribose, while RNA contains y3-D-ribose, (Fig. 1.22). These components are summarised in Table 1.4. 

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