Purine and pyrimidine bases

Nucleotides are the phosphoric acid ester of nucleoside, while nucleosides are compounds in which nitrogenous bases (purines and pyrimidines) are conjugated to the pentose sugar (ribose or deoxyribose) by a b-glycosidic linkage. AMP, ATP, ADP, GMP, CMP, UMP are the examples of mononucleotides. 

The nitrogenous bases (purines and pyrimidines) which are components of the nucleic acids (RNA and DNA-used for information storage and processing)... 

Figure 4-6. A part ot ihc double lielix of deoxynbonudeicacid. The frame formed by the heterocyclic bases, purine and pyrimidine, as well as the deoxyribose residues, are sliown in monotone. Only the phosphate chain H atoms are shown (after M. Eigen).

A transformational set is a set of homogeneous transformation matrices associated with a molecular contact type defined by the nature of the nucleotides in contact. For instance, there are four types of RNA bases determining ten different types of pairs by considering that symmetric pairs are likely to share the same types of molecular contacts, and two main types of molecular contacts paired and connected. Coimected nucleotides can be either stacked or not. In practice, we consider only two types of bases (purines and pyrimidines) for contact defined by a phosphodiester bond. This partition of the different molecular contacts gives a possibility of (10 + 6 = 16) different transformational sets. 

The accurate historic origin of the expression "functional polymers," as in most cases in the naming of a scientific discipline, cannot be reconstructed unequivocally. Takemoto used "functional monomers and polymers" as the general theme for the publication of his work on poljnnerizable derivatives of nucleic bases, purines, and pyrimidines. Probably simultaneously, the expression "functional pol3nners" was used by our research group as a common expression for pol3nneric stabilizers and polymeric drugs. 

Genetic code The sequence of triplets of the nucleic acid bases (purines and pyrimidines) that specifies the individual amino acids. 

Nucleic acids are acidic substances present m the nuclei of cells and were known long before anyone suspected they were the primary substances involved m the storage transmission and processing of genetic information There are two kinds of nucleic acids ribonucleic acid (RNA) and deoxyribonucleic acid (DNA) Both are complicated biopolymers based on three structural units a carbohydrate a phosphate ester linkage between carbohydrates and a heterocyclic aromatic compound The heterocyclic aro matic compounds are referred to as purine and pyrimidine bases We 11 begin with them and follow the structural thread... 

Purine and pyrimidine bases Nucleosides Nucleotides Nucleic acids... 

These relationships are general Hydroxyl substituted purines and pyrimidines exist in their keto forms ammo substituted ones retain structures with an ammo group on the ring The pyrimidine and punne bases m DNA and RNA listed m Table 28 1 follow this general rule Beginning m Section 28 7 we 11 see how critical it is that we know the cor rect tautomeric forms of the nucleic acid bases... 

FIGURE 28 5 (a) Tube and (b) space filling models of a DNA double helix The carbohydrate-phosphate backbone is on the out side and can be roughly traced in (b) by the red oxygen atoms The blue atoms belong to the purine and pyrimidine bases and he on the inside The base pairing is more clearly seen in (a)... 

Section 28.8 The most common fonn of DNA is B-DNA, which exists as a right-handed double helix. The car bohydrate-phosphate backbone lies on the outside, the purine and pyrimidine bases on the inside. The double helix is stabilized by complementary hydrogen bonding (base pauing) between adenine (A) and thymine (T), and guanine (G) and cytosine (C). 

The DNA double heUx illustrates the contribution of multiple forces to the structure of biomolecules. While each individual DNA strand is held together by covalent bonds, the two strands of the helix are held together exclusively by noncovalent interactions. These noncovalent interactions include hydrogen bonds between nucleotide bases (Watson-Crick base pairing) and van der Waals interactions between the stacked purine and pyrimidine bases. The hehx presents the charged phosphate groups and polar ribose sugars of... 

Synthetic analogs of purine and pyrimidine bases and their derivatives serve as anticancer dmgs either by inhibiting an enzyme of nucleotide biosynthesis or by being incorporated into DNA or RNA. 

Human tissues can synthesize purines and pyrimidines from amphibolic intermediates. Ingested nucleic acids and nucleotides, which therefore are dietarily nonessential, are degraded in the intestinal tract to mononucleotides, which may be absorbed or converted to purine and pyrimidine bases. The purine bases are then oxidized to uric acid, which may be absorbed and excreted in the urine. While little or no dietary purine or pyrimidine is incorporated into tissue nucleic acids, injected compounds are incorporated. The incorporation of injected [ H] thymidine into newly synthesized DNA thus is used to measure the rate of DNA synthesis. 

Figure 35-1. A segment of one strand of a DNA molecule in which the purine and pyrimidine bases guanine (G), cytosine (C), thymine (T), and adenine (A) are held together by a phosphodiester backbone between 2 -de-oxyribosyl moieties attached to the nucleobases by an W-glycosidic bond. Note that the backbone has a polarity (ie,a direction). Convention dictates that a single-stranded DNA sequence is written in the 5 to 3 direction (ie, pGpCpTpA, where G, C,T, and A represent the four bases and p represents the interconnecting phosphates).

The double-stranded structure of DNA can be separated into two component strands (melted) in solution by increasing the temperature or decreasing the salt concentration. Not only do the two stacks of bases puU apart but the bases themselves unstack while still connected in the polymer by the phosphodiester backbone. Concomitant with this denaturation of the DNA molecule is an increase in the optical absorbance of the purine and pyrimidine bases—a phenomenon referred to as hyperchromicity of denaturation. Because of the... 

In RNA, the sugar moiety to which the phosphates and purine and pyrimidine bases are attached is ribose rather than the 2 -deoxyribose of DNA. 

An alteration in the sequence of purine and pyrimidine bases in a gene due to a change—a removal or an insertion—of one or more bases may result in an altered gene product. Such alteration in the genetic material results in a mutation whose consequences are discussed in detail in Chapter 38. 

FapyGua from isolated DNA, no further chemical modifications to the purine and pyrimidine bases have been observed (Halliwell and Aruoma, 1992). 

---