Pyrimidine Cytosine

Avery s paper prompted other biochemists to rethink their ideas about DNA. One of them, Erwin Chargaff of Columbia University, soon discovered that the distribution of adenine, thymine, cytosine, and guanine differed from species to species, but was the same within a species and within all the cells of a species. Perhaps DNA did have the capacity to cariy genetic information after all. Chargaff also found that regardless of the source of the DNA, half the bases were purines and the other half were pyrimidines. Significantly, the ratio of the purine adenine (A) to the pyrimidine thymine (T) was always close to 1 1. Likewise, the ratio of the purine guanine (G) to the pyrimidine cytosine (C) was also close to 1 1. For human DNA the values are ... 

As indicated in Chapter 11, the base pairing in DNA is very specific the purine adenine pairs with the pyrimidine thymine the purine guanine pairs with the pyrimidine cytosine. Further, the A T pair and G C pair have virtually identical dimensions (Figure 12.10). Watson and Crick realized that units of such similarity could serve as spatially invariant substructures to build a polymer whose exterior dimensions would be uniform along its length, regardless of the sequence of bases. 

The sugar component in RNA is ribose, and the sugar in DNA is 2 -deoxy-ribose. (The prefix 2 -deoxv indicates that oxygen is missing from the 2 position of ribose.) DNA contains four different amine bases, two substituted purines (adenine and guanine) and two substituted pyrimidines (cytosine and thymine). Adenine, guanine, and cytosine also occur in RNA, but thymine is replaced in RNA by a closely related pyrimidine base called uracil. 

C with G the pyrimidine cytosine (C) always pairs with the purine guanine (G). 

Nucleotides can be linked together into oligonucleotides through a phosphate bridge at the 5 position of one ribose unit and the 3 position of another. The purine bases, adenine and guanine, have two heterocyclic rings, while the pyrimidines cytosine, thymine, and uracil have one. The structure of adenosine monophosphate is shown in Figure 11. 

The photochemistry of the polynucleotides has been elucidated primarily by studies of the photochemical behavior of the individual pyrimidine and purine bases (the ribose and phosphate groups would not be expected to undergo photochemical reactions in this wavelength range). These studies have shown the pyrimidines (cytosine and thymine) to be roughly ten times more sensitive to UV than the purines (adenine and guanine.) Thus we would expect most of the photochemistry of the nucleic acids to result from the action of light on the pyrimidines. 

The nitrogen-containing bases that occur in DNA and RNA fall into two structural categories the purines and the pyrimidines. The former contain a five-membered ring fused to a six-membered ring, while the latter contain a six-membered ring only. The two purines are common to both DNA and RNA adenine (A) and guanine (G). The pyrimidine cytosine (C) occurs in both DNA and RNA. The other pyrimidine is thymine (T) in DNA but uracil (U) in RNA. 

The pyrimidines cytosine and thymine both react with hydrazine, which initially attacks the unsaturated carbonyl system and then leads to ring opening. Again, base treatment is used to hydrolyse the phosphodiester bond. This reaction becomes selective for cytosine in the presence of NaCl, which suppresses reaction with thymine. 

Pyrimidine is a six-membered aromatic heterocyclic compound that contains two nitrogen atoms, separated by a carbon atom, in the ring. Nucleic acids, DNA and RNA, contain substituted purines and pyrimidines. Cytosine, uracil, thymine and alloxan are just a few of the biologically significant modified pyrimidine compounds, the first three being the components of the nucleic acids. 

Cytosine, thymine, and uracil are pyrimidines along with adenine and guanine they account for the five nucleic acid bases. Pyrimidines are heterocyclic single-ringed compounds based on the structure of pyrimidine. Cytosine, thymine, and uracil, like adenine and guanine, form nucleosides and nucleotides in RNA and DNA. When the bases combine with ribose, a ribo-nucleoside forms and when it attaches to deoxyribose, a deoxyribosenucleoside is formed. Names of the nucleoside are summarized in Table 29.1. These in turn combine with phospho-ryl groups, in a process called phosphorylation, to form their respective nucleotides that form nucleic acids. The nucleotides can be tri, di, and mono phosphate nucleotides similar to the way in which adenine forms ATP, ADP, and AMP. 

The common pyrimidine ribonucleotides are cytidine 5 -monophosphate (CMP cytidylate) and uridine 5 -monophosphate (UMP uridylate), which contain the pyrimidines cytosine and uracil. De novo pyrimidine nucleotide biosynthesis (Fig. 22-36) proceeds in a somewhat different manner from purine nucleotide synthesis the six-membered pyrimidine ring is made first and then attached to ribose 5-phosphate. Required in this process is carbamoyl phosphate, also an intermediate in the urea cycle (see Fig. 18-10). However, as we noted... 

Both DNA and RNA contain the same purine bases adenine (A) and guanine (G). Both DNA and RNA contain the pyrimidine cytosine (C), but they differ in their second pyrimidine base DNA contains thymine (T), whereas RNA contains uracil (U). T and U differ by only one methyl group, which is present on T but absent on U (Figure 22.1). [Note Unusual bases are occasionally found in some species... 

The principal tautomeric properties of the fundamental biological pyrimidines—cytosine, uracil, and thymine—are due to the presence in these N-heteroaromatic compounds of electron-donor substituents such as NH2 and OH and of SH in some important analogs. The labile hydrogen may remain attached at the exocyclic 0, N, or S atom or migrate to one of the ring nitrogens, giving rise to three principal types of tautomerism (Scheme 1) ... 

The prebiotic synthesis of the pyrimidine cytosine involves cyanoac-etylene, which is synthesized in good yield by sparking mixtures of CH4 + N2. Cyanoacetylene reacts with cyanate to give cytosine,29... 

The electrochemical behaviour and the adsorption of nucleic acid molecules and DNA constituents have been extensively studied over recent decades [1-6]. Electrochemical studies demonstrated that all DNA bases can be electrochemically oxidized on carbon electrodes [7-13], following a pH-dependent mechanism. The purines, guanine (G) and adenine (A), are oxidized at much lower positive potentials than the pyrimidines, cytosine (C) and thymine (T), the oxidation of which occurs only at very high positive potentials near the potential corresponding to oxygen evolution, and consequently are more difficult to detect. Also, for the same concentrations, the oxidation currents observed for pyrimidine bases are much smaller than those observed for the purine bases. Consequently, the electrochemical detection of oxidative changes occurring in DNA has been based on the detection of purine base oxidation peaks or of the major... 

G. Dryhurst and P.J. Elving, Electrochemical oxidation-reduction paths for pyrimidine, cytosine, purine and adenine. Correlation and application, Talanta, 16 (1969) 855-874. 

The DNA molecule is composed of two parts. One is a repeating sequence of a 5-carbon sugar (deoxyribose) and phosphate that makes up the backbone of the molecule. The second part is one of four bases associated with each repeat unit in the backbone two purines, adenine (A) and guanine (G), and two pyrimidines, cytosine (C) and thymine (T). DNA consists of two vertical backbones bonded horizontally by a purine-pyrimidine pair at each sugar-phosphate unit. A purine or pyrimidine with its associated sugar and phosphate is called a nucleotide. ... 

As much of the terminology used in molecular biology may be unfamiliar to some readers, it is appropriate to define some of the vocabulary and this is given in an appendix to this chapter. There are two types of nucleic acids, the ribonucleic acids (RNA) and the deoxyribonucleic acids (DNA). Genetic information is carried in the linear sequence of nucleotides in DNA. Each molecule of DNA contains two complementary strands of deoxyribonucleotides which contain the purine bases, adenine and guanine and the pyrimidines, cytosine and thymine. RNA is single-stranded, being composed of a linear sequence of ribonucleotides the bases are the same as in DNA with the exception that thymine is replaced by the closely related base uracil. DNA replication occurs by the polymerisation of a new complementary strand on to each of the old strands. 

The basic components of these macromolecules are the orthophosphor ic acid, a sugar (ribose or deoxyribose), and purines [adenine (A) and guanine ( )] and Pyrimidines [cytosine (C), thymine (T), and uracil (I/)]. The puric and pyrimidinic bases are the active chemical components, while the orthophosphoric acid and the sugar constitute the skeleton of these macromolecules. 

In several platinated NpN adducts with. vyn-residues and N = DNA base, the 3 residue was found to be syn [87-89], In these cases, N was a purine. This observation of a syn orientation for only the 3 residue could be related to the 5 residue having an N-sugar in these adducts the N pucker favors an anH-orientation [90]. Also, in our AHT model of (S,R,R,S)-BipPt-(d(GpG)), there is a 3,-G NH2-phosphate hydrogen bond. This interaction could help stabilize the xyn-orientation for the 3 -G residue. In the one instance of a platinated NpN complex with a syn 5 residue, the 5 sugar pucker was N, but the 5 base was a pyrimidine, cytosine [48],... 

DNA is composed of three chemical functions A deoxyribose (a pentose, i.e., a sugar with five carbons), organic (nitrogenous) bases (pyrimidines cytosine and thymine purines adenine and guanine), and a phosphoric acid. 

Nucleic acids are polymers containing nitrogenous bases attached to sugar-phosphate backbones. The common nitrogenous bases of nucleic acids are the bicyclic purines, adenine and guanine, and the monocyclic pyrimidines, cytosine, uracil, and thymine (Fig. V-l). 

Nitrogenous bases of nucleotides are derivatives of either purine (adenine, A or guanine, G) or pyrimidine (cytosine, C thymine, T or uracil, U) (see Figure 1.4). 

Figure 2 The components of RNA. (a) The primary bases of RNA, which includes the purines adenine and guanine and the pyrimidines cytosine and uracil, (b) A purine nucleoside (adenosine) and a pyrimidine nucleoside (uridine), (c) The nucleotide adenosine 5 -triphosphate. (d) An RNA strand with the sequence AGCU.

Classic Watson-Crick base pairs are formed by unique hydrogen-bonding interactions between the nitrogenous bases of DNA and RNA. The purine adenine associates specifically with the pyrimidine thymine in DNA (or the related unmethylated analog, macil, in RNA), and the pmine guanine interacts with the pyrimidine cytosine. These complementarity rules. 

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