Pyrimidines bases in DNA

Basic to molecular biology is the concept that DNA and RNA are macromolecules that convey information. The sequence of purine and pyrimidine bases in DNA encodes all the information needed to form and direct the chemical reactions within cells. This information thus encodes all catalytic, regulatory, and structural proteins contained within an organism. The flow of information from DNA to RNA to proteins is... 

All nucleic acids consist of covalently linked nucleotides. Each nucleotide has three characteristic components (1) a purine or pyrimidine base (2) a pentose and (3) a phosphate group. The purine or pyrimidine bases are linked to the C-T carbon of a deoxyribose sugar in DNA or a ribose sugar in RNA. The phosphate groups are linked to the sugar at the C-5 and C-3 positions. The purine bases in both DNA and RNA are always adenine (A) and guanine (G). The pyrimidine bases in DNA are thymine (T) and cyto-... 

Applying the same analysis to pyrimidines (3 and 6) leads to pyrimidones, examples of which are the pyrimidine bases in DNA and RNA. Thus deoxycytidine 46 and deoxythymidine 47 are two of the four 2 -deoxyribonucleosides that are the building blocks of DNA and uridine 48 is one of the four nucleoside building blocks of RNA. As for pyridones, the contribution of dipolar resonance hybrids to pyrimidones and other systems with exocyclic conjugation often helps to understand their properties, including their aromatic character. 

The photobiological activity of the psoralens is related, at least in part, to the ability of these coumarins to undergo [ 2 + 2] photocycloaddition to pyrimidine bases in DNA. The two cis-syn adducts (101) and (102) have now been obtained by irradiation of 2 -deoxycytidine in the presence of 3-carbethoxypsoralen, whereas reversible [ 2 + 2] dimerisation of the coumarin nucleus is preferred on solid state irradiation of the 8-alkoxypsoralen derivative (103). ° [ 2 +, 2] Photoadditions of 7-aminocoumarins... 

Light exposure within the absorption spectrum of the psoralens (300 to 400 nm) leads to covalent binding of the psoralen to pyrimidine bases in DNA. Monofunctional psoralens, such as angelicines, bind to only one of the DNA strands bifunctional ones (linear psoralens) bind to both strands, and therefore cross-link one strand to the other. This will slow down the rate of cell division, which is abnormally high in psoriatic skin. Psoralens may also act on membranes in a type II process. 

The electrophiles or electrophilic intermediates that are or are postulated to be responsible for the carcinogenic action of chemicals include (i) positively charged carbonium, nitrenium, oxonium and episulfonium ions, (ii) free radicals, (iii) polarized double bonds, (iv) aldehydes, (v) strained rings such as epoxide, aziridine, lactones and sultones, and (vi) quinone/ quinoid/quinoneimine structures. Based on their reactivity (Table I), electrophiles may be graded from "soft" to "hard" similar to the concept of "soft" and "hard" acids and bases (18). In general, soft electrophiles react preferentially with soft nucleophiles whereas hard electrophiles react preferentially with hard nucleophiles. Thus, since the nucleophilic sites in the purine and pyrimidine bases in DNA are moderately hard nucleophiles, moderately hard electrophiles tend to have the greatest likelihood of covalent binding to DNA. Soft electrophiles often deplete the cellular pool of noncritical soft nucleophiles (such as GSH) before they can react with DNA. 

Chen G, Han XH, Zhang LY, Ye JN (2002) Determination of purine and pyrimidine bases in DNA by micellar electrokinetic capillary chromatography with electrochemical detection. J Chnunatogr A 954 267-276... 

These are used for the biosynthesis of the purine and pyrimidine bases in DNA and RNA. Hence THF is vital for cell growth and division. 

When DNA is exposed to radiation at wavelengths of about 260 nm, adjacent pyrimidines become covalently linked by the formation of a four-member ring structure resulting from the saturation of their respective C5, C6 double bonds. The structure formed by this photochemical cycloaddition is referred to as a pyrimidine dimer. The pyrimidine bases in DNA are thymine and cytosine. 

Point mutation change in a single purine or pyrimidine base in DNA codon... 

These relationships are general. Hydroxyl-substituted purines and pyrimidines exist in their keto forms fflnino-substituted ones retain structures with an anino group on the ring. The pyrimidine and purine bases in DNA and RNA listed in Table 28.1 follow this general rule. Beginning in Section 28.7 we ll see how critical it is that we know the correct tautomeric forms of the nucleic acid bases. 

GG8, the radical cation must traverse five A/T base pairs. Electrochemical measurements in solution have shown that the purine bases (A and G) have considerably lower Eox than the pyrimidines (C and T), with the Eox of G estimated to be about 0.25 V below that of A [20]. It is not very likely that the Eox of bases in DNA will be the same as they are in solution, but it is generally assumed that the order of Eox will remain the same. Consequently, the radical cation at Gi of AQ-DNA(l) must traverse a bridge of five A bases to reach GG8. The process whereby the radical cation crosses such bridges has been a major point of debate in consideration of long distance radical cation migration mechanisms in DNA this issue will be discussed fully below. 

Nucleotide A subunit of DNA or RNA consisting of a purine (adenine and guanine) or a pyrimidine base [thymine (DNA only), uracil (RNA only) and cytosine], a phosphate molecule, and a sugar molecule (deoxyribose in DNA and ribose in RNA). 

The hydrogen-bond pairing interactions between pyrimidine and purine bases in DNA and RNA. See DNA... 

Free pyrimidines and purines are weakly basic compounds and are thus called bases. They have a variety of chemical properties that affect the structure, and ultimately the function, of nucleic acids. The purines and pyrimidines common in DNA and RNA are highly conjugated molecules (Fig. 8-2), a property with important consequences for the structure, electron distribution, and fight absorption of nucleic acids. Resonance among atoms in the ring gives most of the bonds partial double-bond character. One result is that pyrimidines are planar molecules purines are very nearly... 

Because of the different orientations of the bases in Z DNA, this DNA conformation requires that the sequence of purine and pyrimidine bases in each chain strictly alternate. An alternating sequence of G and C, or T and G or A and C residues can adopt a Z conformation. Of course, in all cases the opposing strand must contain a sequence of bases that is complementary as in all DNA duplex structures. An alternating A-T DNA sequence cannot adopt the Z confer-... 

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