Pyrimidine bases cytosine

FIGURE 11.3 The common pyrimidine bases—cytosine, nracil, and thymine—in the tantomeric forms predominant at pH 7. 

Hydrolytic cleavage of the glycosidic bond holding the DNA bases to the sugar-phosphate backbone is typically a very slow process under physiological conditions (pH 7.4 37°C). Loss of the pyrimidine bases cytosine and thymine occurs with a rate constant of 1.5 X 10 s (ty2 = 14,700 years), while loss of the purine bases guanine and adenine proceeds slightly faster, with a rate constant of 3.0 X... 

Figure 11-3. Molecular orbitals for the pyrimidine bases cytosine and uracil...

An introduction to the method of in vitro evolution is given by Wilson and Stoszak (1999). The RNA lipase ribozyme, with about 140 nucleotides (but without the pyrimidine base cytosine), folded in a defined structure and was able to reach a reaction rate 105 times higher than in the uncatalysed reaction. This result certainly surprised those biogenesis researchers who were critical of the RNA world but we do not know whether the result changed their attitude to it ... 

The bases that occur in nucleic acids are aromatic heterocyclic compounds derived from either pyrimidine or purine. Five of these bases are the main components of nucleic acids in all living creatures. The purine bases adenine (abbreviation Ade, not A ) and guanine (Gua) and the pyrimidine base cytosine (Cyt) are present in both RNA and DNA. In contrast, uracil (Ura) is only found in RNA. In DNA, uracil is replaced by thymine (Thy), the 5-methyl derivative of uracil. 5-methylcyto-sine also occurs in small amounts in the DNA of the higher animals. A large number of other modified bases occur in tRNA (see p. 82) and in other types of RNA. 

The sugar component in RNA is ribose, whereas in DNA it is 2-dexoyribose. In deoxyribonucleotides, the heterocyclic bases are purine bases, adenine and guanine, and pyrimidine bases, cytosine and thymine. In ribonucleotides, adenine, guanine and cytosine are present, but not thymine, which is replaced by uracil, another pyrimidine base. 

The purine bases adenine and guanine and the pyrimidine bases cytosine, thymine, and uracil are all heterocycles. They typically have strong 7uT UV absorption bands and, due to the lone electron pairs on the heteroatoms, have additional low lying nn transitions. Furthermore, for some bases Tier states are... 

Among pyrimidine bases, cytosine shows significant tautomeric acitivity. In argon and nitrogen matrices, it exists as a combination of amino-oxo (N1H) and amino-hydroxy forms the tautomeric equilibrium being shifted towards the latter form [66, 67], In microwave studies, the amino-oxo, imino-oxo, and amino-hydroxy forms of cytosine are revealed [68] and in the aqueous solution only amino-oxo forms (N1H and N3H) are present [69], In a recent REMPI study of laser desorbed jet cooled... 

Fig. 9.5 The molecular structures of DNA and RNA components the purine bases adenine and guanine, the pyrimidine bases cytosine, thymine and uracil and the structure of riboso-monophosphates.

The bases found in nucleic acids are shown in Figure 28-17. Each type of nucleic acid contains two pyrimidine bases and two purine bases. The two purine bases adenine (A) and guanine (G) and the pyrimidine base cytosine (C) are found in both RNA and DNA. 

The biopolymers DNA (deoxyribonucleic acid) and RNA (ribonucleic acid) are present in any living cell and contain or transfer genetic information. They consist ofpurine (adenine, guanine) and pyrimidine bases (cytosine, uracil, thymine) linked to either 2-deoxy-D-ribose (DNA) or D-ribose (RNA). The sugar units are linked together via phosphates (Figure 23). 

RNA is similar to DNA. Like DNA, it is composed of nucleotides joined by 3 - to 5 -phosphodiester bonds, the purine bases adenine and guanine, and the pyrimidine base cytosine. However, its other pyrimidine base is uracil rather than thymine. Uracil and thymine are identical bases except that thymine has a methyl group at position 5 of the ring (Fig. 12.17). In RNA, the sugar is ribose, which contains a hydroxyl group on the 2"-carbon (see Fig 12.3. The prime refers to the position on the ribose ring). (The presence of this hydroxyl group allows RNA to be cleaved to its constituent nucleotides in alkaline solutions.)... 

The pyrimidine nucleotides are dephosphorylated, and the nucleosides are cleaved to produce ribose 1-phosphate and the free pyrimidine bases cytosine, uracil, and thymine. Cytosine is deaminated, forming uracil, which is converted to CO2, NH/,... 

DNA contains two purine bases, guanine and adenine, and two pyrimidine bases, cytosine and thymine. In RNA thymine is replaced by uracil and in another form, t-RNA, other bases including small amounts of A-alkylated derivatives are present. 

The pyrimidine bases cytosine, uracil, and thymine cannot be directly alkylated by halo sugars. The nucleophilicity of the pyridone-type ring nitrogens is much reduced relative to the pyridine-type nitrogens present in the purine bases. The dialkoxypyrimidines are reactive, and dealkylation occurs at the alkylated nitrogen. This is known as the Hilbert-Johnson procedure A recent modification employs... 

RNA contains purine bases (adenine and guanine) as well as pyrimidine bases (cytosine and uracil) (Figure 29-1). The bases are bound to the sugar by... 

In the case of DNA, a D-2-deoxyribose molecule is combined to each of the bases to form a nucleoside, and the nucleosides are then combined with each other with a phosphoric acid to form a polymer (DNA). On the other hand, in the case of RNA, D-ribose, instead of D-2-deoxyribose, is combined to each of the bases to form a nucleoside, and as in the case of DNA, these nucleosides are combined with each other to form a polymer (RNA). Among the bases within DNA and RNA, adenine and guanine have been described in the preceding section. In this section, cytosine, thymine, and uracil, which are pyrimidine bases, will be described. Purine derivatives exist as a constituent unit of nucleic acids and as many kinds of monomers, and these are also present in natural products, such as caffeine, inosinic acid, and cytokinin. On the other hand, as natural products, pyrimidine derivatives are rather rare. Nucleosides composed of pyrimidine bases cytosine, thymine, and uracil coupled with D-ribose are known as cytidine, thymidine, and uridine, respectively. Among these alkaloids, cytidine was first isolated from the nucleic acid of yeast [1,2], and thymidine was isolated from thymonucleic acid [3,4]. In the meantime, uridine was obtained by the weak alkali hydrolysis [5] of the nucleic acids originating from yeast. 

Albomycin an antibiotic synthesized by Actinomyces subtropicus. A. is a cyclic polypeptide containing a pyrimidine base (cytosine) and 4.16% Fe in the form of a hydroxamate-Fe(IIl) complex (Fig.). It is one of the sideromycins (similar to, and possibly identical to, grisein), and it interferes with iron metabolism as an antimetabolite of the sideramines. A. is effective against both Gram-positive and Gram-negative bacteria, and inhibits the aerobic metabolism of Staphylococcus aureus and E. colt... 

Cytidine, Cyd, cytosine riboside, 3-D-ribofumno-syt-cylosine a 3-glycosidic Nucleoside (see) consisting of D-ribose and the pyrimidine base cytosine. M, 243.22, m,p. 220-230 °C (d,), [a]g +34.2°, (c = 2 in water). Cytidine phosphates (see) are metabolically important in all living organisms. 

Pyrimidine 1,3-diazine, a heterocyclic compound, consisting of a six-membered ring with 2 nitrogen atoms (Fig.l), M, 80.1, m.p. 20-22°C, b.p. 124°C. The P. ring system is present in many natural compounds, e. g. antibiotics (nucleoside antibiotics), pterins, purines and vitamins, it is especially important in the pyrimidine bases. Cytosine (see). Uracil (see) and Thymine (see), which are constituents of nucleic acids. Pyrimidine itself does not occur naturally. Pyrimidine analogs (see) can also be incorporated into nucleic acids. 

Examine the structures of adenine and guanine (Figure 18.1). Do you expect their rings to be planar or puckered Explain. What about the pyrimidine bases, cytosine and thymine ... 

Nucleosides are formed from mononucleotides (sometimes referred to as simple nucleic acids ) by elimination of the phosphoric acid moiety. Those derived from deoxyribonucleic acid contain the purine bases guanine and adenine and the pyrimidine bases cytosine and th37mine the bases are linked to deoxyribose via a N-glycosidic bond. 5-Methylcytosine, 5-hydroxymethylcytosine and 5-hydroxymethyluracil also occur in some deoxyribonucleic acids. The four nucleosides which... 

They demonstrated that N -labeled ammonium salts were rapidly incorporated into the nucleic acids of the internal organs of the rat, and that the extent of labeling of the pyrimidines was approximately equal to that of the purines. Since the pyrimidine ring is structurally part of the completed purine molecule, the question naturally arose at the time as to whether pyrimidines might be the precursors of the purine molecule. The later studies of Plentl and Schoenheimer showed that the N -labeled pyrimidines, uracil and thymine (Fig. 8), when fed to rats, did not appear in either the pyrimidine or purine bases of the nucleic acids. The other naturally occurring pyrimidine base, cytosine (Fig. 8), was investigated... 

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