Deoxyribonucleic acid heterocyclic bases

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... 

The storage of genetic information and the transcription and translation of this information are functions of the nucleic acids deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). They are polymers whose building blocks are nucleotides, which are themselves combinations of three parts, i.e. a heterocyclic base, a sugar, and phosphate (see Section 14.1). 

Deoxyribonucleic Acid (DNA) Sugar 2-deoxyribose, a phosphate group and a heterocyclic base. The base is either purine A or G and pyrimidine C or T. 

The nucleic acids, deoxyribonucleic acid (DNA) and ribonucleic acid (RNA), are the chemical carriers of a cell s genetic information. Nucleic acids are biopolymers made of nucleotides joined together to form a long chain. These biopolymers are often found associated with proteins, and in this form they are called nucleoproteins. Each nucleotide comprises a nucleoside bonded to a phosphate group, and each nucleoside is composed of an aldopentose sugar, ribose or 2-deoxyribose, linked to a heterocyclic purine or pyrimidine base (see Section 4.7). 

If the terminal pyrophosphate is removed from a molecule of ATP, the remainder is AMP, adenosine monophosphate, one of the four building blocks of the important biological macromolecules, the nucleic acids. There are two types of nucleic acids (26) ribonucleic acid (RNA), and deoxyribonucleic acid (DNA). RNA is a polymer of four different nucleotides, one of which is AMP, the ribose phosphate of adenine. The other three nucleotides are also ribose phosphates of heterocyclic bases, guanine, cytosine, and uracil. The structure of the four bases is shown in Figure 6. 

The basic monomers of nucleic acids are nucleotides which are made up of heterocyclic nitrogen-containing compounds, purines and pyrimidines, linked to pentose sugars. There are two types of nucleic acids and these can be distinguished on the basis of the sugar moiety of the molecule, Ribonucleic acids (RNA) contain ribose, while deoxyribonucleic acid (DNA) contains deoxyribose. The bases cytosine (C) adenine (A) and guanine (G) are common in both RNA and DNA. However, RNA molecules contain a unique base, uracil (U), while the unique DNA base is thymidine (T). These differences in the base structure markedly affect the secondary structures of these polymers. The structures of DNA and RNA are outlined in Appendix 5.2. 

Common Names of Heterocycles Used Broadly in Biology. The naming of heterocycles by systematic methods is important but cumbersome for designating some of the most commonly occurring heterocycles. In particular, the bases that occur in ribonucleic acids (RNA) and deoxyribonucleic acids (DNA) have specific substitution patterns. Because they occur so commonly, they have been given trivial names that are invariably used when discussed or named in the biological literature. 

Nucleotides in DNA and RNA. Nucleotides are the monomeric units of the nucleic acids, DNA (deoxyribonucleic acid) and RNA (ribonucleic acid). Each nucleotide consists of a heterocyclic nitrogenous base, a sugar, and phosphate DNA contains the purine bases adenine (A) and guanine (G) and the pyrimidine bases cytosine (C) and thymine (T). RNA contains A, G, and C, but it has uracil (U) instead of thymine. In DNA, the sugar is deoxyribose, whereas in RNA it is ribose. 

Figure 1 is a reminder that a nucleic acid molecule consists of a backbone of ribose and phosphate, to which the various heterocyclic bases are attached. The sequence of these bases, of course, constitutes the genetic code. The primes are used in numbered positions of the ribose whereas the unprimed numbers refer to the numbering in the heterocyclic bases. The two diflFerent nucleic add molecules, DNA (deoxyribonucleic acid) and RNA (ribonucleic acid), diflFer by the presence or absence of the 2 -hydroxyl groups. RNA contains the 2 -hydroxyl groups DNA does not. 

Nucleic acids are classified into two categories ribonucleic acid (RNA), found mainly in the cytoplasm of living cells, and deoxyribonucleic acid (DNA), found primarily in the nuclei of cells. Both DNA and RNA are polymers, consisting of long, linear molecules. The repeating structural units, or monomers, of the nucleic acids are called nucleotides. Nucleotides, however, are composed of three simpler components a heterocyclic base, a sugar, and a phosphate (see > Figure 11.2). These components will be discussed individually. 

Like polysaccharides and polypeptides, nucleic acids are condensation polymers. Each monomer in these polymers includes one of two simple sugars, one phosphoric acid group, and one of a group of heterocyclic nitrogen compounds that behave chemically as bases. A particular nucleic acid is a deoxyribonucleic acid (DNA) if it contains the sugar 2-deoxy-D-ribose, and it is a ribonucleic acid (RNA) if it contains the sugar D-ribose. 

Deoxyribonucleic acid (DNA) (Sections 25.1 and 25.4A) One of the two molecules (the other is RNA) that carry genetic information in cells. Two molecular strands held together by hydrogen bonds give DNA a twisted ladder -like structure, with four typ>es of heterocyclic bases (adenine, cytosine, thymine, and guanine) making up the rungs of the ladder. 

Deoxyribonucleic acids consist of a backbone of alternating units of deoxyribose and phosphate in which the 3 -hydroxyl of one deoxyribose unit is joined by a phosphodiester bond to the 5 -hydroxyl of another deoxyribose unit (Figure 20.5). This pentose—phosphodiester backbone is constant throughout an entire DNA molecule. A heterocyclic aromatic amine base—adenine, guanine, thymine, or cytosine—is bonded to each deoxyribose unit by a j8-Atglycosidic bond. The primary structure of a DNA molecule is the order of heterocyclic bases along the pentose-phosphodiester backbone. The sequence of bases is read from the 5 end to the 3 end. 

The nucleic acids were discovered by Miescher in 1868-1869, when he isolated from pus cell nuclei a material which contained phosphorus, was soluble in alkali, but precipitated under acidic conditions. This material was subsequently prepared from other sources and when freed from protein it was called nucleic acid, a term introduced by Altman in 1889. The classical preparations of nucleic acid from yeast yielded a product which we now recognize as ribonucleic acid (RNA). The nucleic acid prepared from thymus glands, thymonucleic acid, was also extensively studied this material [which, in present terms, was deoxyribonucleic acid (DNA)) was different from yeast nucleic acid. From hydrolysates of these preparations the heterocyclic bases were isolated and characterized. At one time, yeast and thymus nucleic acids were thought to be representative of plant and animal nucleic acids, respectively (3). By 1909, it was apparent that yeast nucleic acid contained adenine, guanine, cytosine, uracil, phosphoric acid, and a sugar which Levene showed at that time to be D-ribose. Thymonucleic acid yielded adenine, guanine, cytosine, thymine, phosphoric acid, and a sugar which was not identified correctly until 1929, when it was characterized as 2-deoxy-D-ribose. 

In deoxyribonucleic acid (DNA), 2-deoxy-D-ribose and phosphate units alternate in the backbone. The 3 hydroxyl of one ribose unit is linked to the 5 hydroxyl of the next ribose unit by a phosphodiester bond. The heterocyclic base is connected to the ano-meric carbon of each deoxyribose unit by a /3-N-glycosidic bond. Figure 18.3 shows a schematic drawing of a DNA segment. 

We mentioned earlier that some polycyclic aromatic hydrocarbons are strongly carcinogenic. Although all the detdls of this carcinogenesis are not worked out, the broad outlines are known for some molecules. Deoxyribonucleic acid (DNA) and ribonucleic acid (RNA) are huge biomolecules that contain and help transcribe genetic information (Chapter 23). Both DNA and RNA are phosphate-linked sequences of nucleotides, which are sugar molecules attached to heterocyclic bases. 

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