Purine bases guanine

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... 

The purine base guanine is also formed in concentrated solutions of ammonium cyanide, i.e., the same substance which became known from Or6 s adenine synthesis. Or6, as well as Stanley Miller, was involved in a new series of experiments (Levi et al., 1999). The yield of guanine is, however, 10 10 times lower than that of adenine surprisingly, the synthesis is just as effective at 253 K as at 353 K. Low temperatures seem conceivable in certain parts of Earth as well as on the Jovian moon Europa (see Sect. 3.1.5) or in the Murchison meteorite. 

The genetic code is composed of four letters —two pyrimidine nitrogenous bases, thymine and cytosine, and two purine bases, guanine and adenine—which can be regarded functionally as arranged in codons (or triplets). Each codon consists of a combination of three letters therefore, 43 (64) different codons are possible. Sixty-one codons code for specific amino acids (three produce stop signals), and as only 20 different amino acids are used to make proteins, one amino acid can be specified by more than one codon. 

AZAGUANINE An analogue of the normal DNA and RNA purine base guanine selection for resistance to the toxic effects of 8-azaguanine is the basis of several mutation-detection systems. 

Figure 7-21. Salvage of the purine bases guanine, adenine, and hypoxanthine occurs in reactions catalyzed by phosphoribosyl transferases.

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. 

This Traube process has been used to synthesize the important purine base guanine (G of the genetic code), as shown in Scheme 4.26. Ethyl formate has also been used instead of formic acid. 

Specifically, we investigate the known TT dimer formation quenching ability of neighboring purine bases [43 6]. The ability of a neighboring purine base to quench the formation of a TT dimer depends on two variables. The first is whether the purine is 3 or 5 to the TT step. It has been shown that a purine only on the 5 side of a TT step quenches TT dimer formation more effectively than does the equivalent purine only on the 3 side of the TT step [44,46]. The second variable is the type of purine base. Guanine and other purines with lower ionization potential than adenine have been shown to be more effective at quenching TT dimer formation than adenine [45,46]. 

The role of purines in the biosynthesis of the imidazole ring of histidine has been investigated in baeteria. The use of labeled purine bases (guanine and adenine) followed by the chemical breakdown of newly formed histidine established that the and the C2 of the purine base were incorporated in the N3 and C2 of the imidazole ring of histidine. The N3 of the imidazole ring was shown to be derived from glutamine. On the basis of further studies on cell-free preparations, the biosynthetic pathway of the imidazole ring of histidine ean now be outlined. Ribose-1-phosphate reacts with ATP to yield phosphoribosyl pyrophospate and AMP PRPP reacts with ATP to form a new complex assumed to be phosphoribosyl ATP. Phosphoribosyl (PR) ATP is then converted to PR AMP by a specific... 

The incorporation in vivo of formate-C into acid soluble adenine and the other purine bases (guanine and hypoxanthine) was taken as an index of the behaviour of purine biosynthesis in different organs (liver, kidney, heart, spleen). 

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... 

With this thought in mind, it is possible to consider, for example, that the high xanthine oxidase activity of rat tissues may be responsible for the demonstrated nonutilization of hypoxanthine, since the base was extensively converted to allantoin in the same experiment. That hypoxanthine can be acted upon by an anabolic enzyme system is demonstrated by Kalckar s nucleoside phosphorylase. " The enzyme has been isolated from rat tissues, and acts on the purine base guanine in addition to hypoxanthine to convert these substances to the corresponding... 

The resultant purine bases, guanine and hypo-xanthine (from AMP and dAMP), may be either salvaged (Section 16.6) or converted to xanthine. The oxidation of xanthine by xanthine oxidase yields urate (uric acid) which is excreted in the urine of man and primates. Other organisms are capable of the synthesis of various enzymes which permit the continuation of the pathway to various end products, e.g. allantoin in mammals, urea and glyoxylate in most fishes (not teleost fishes) and amphibians, and ammonia and COj in crustaceans. 

The Lesch-Nyhan syndrome is a rare, X-linked genetic disease due to a functional absence of the enzyme hypoxanthine-guanine phosphoribosyltransferase (HGPRT) (Seegmiller, Rosenbloom and Kelley, 1967). This enzyme catalyzes the transfer of the 5-phos-phoribosyl moiety of 5-phosphoribosyl-l-pyrophosphate (PP-ribose-P) to the purine bases guanine and hypoxanthine to form the nucleotides inosinic acid and guanylic acid. 

The nueleobases are heterocyclic aromatic organic compounds that are basic parts of the nucleic adds. These heterocycles are, in general, the purine bases guanine (G) and adenine (A), and the pyrimidine bases cytosine (Q, as well as thymine (T, DNA) or uracil (U, RNA) (Fig. 1). 

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