Nucleotides and nucleic acids

Chapter 9

Nucleotides and nucleic acids

The nucleotides which consist of three parts, namely a nitrogenous base, a pentose sugar and a phosphate radical, are a very important group of compounds since one or more of them is involved in virtually every biochemical process. The adenosine di- and tri-phosphates which play an essential part in cellular energy exchanges have a nucleotide-type structure as do many of the coenzymes. Furthermore, nucleotides constitute the monomeric units of which the nucleic acids are composed; that is to say nucleic acids are polynucleotides. The nucleic acids which are of two types, deoxyribonucleic acid (DNA) and ribonucleic acid (RNA), are responsible for directing the synthesis of proteins. They specify the unique sequence of amino acids in any particular protein and consequently should be regarded as primordial molecules on whose existence that of the proteins depends. However, since the synthesis of the nucleic acids depends on enzymes which are themselves proteins, this poses the fundamental evolutionary question as to which came first – the biochemical version of the problem of the chicken and the egg!

In addition to their role in protein synthesis nucleic acids transmit genetic information from parent to offspring and the entire range of inherited characteristics of any organism is believed to be defined in terms of the deoxyribonucleic acid that its cells contain.

Nucleic acid components

Nucleic acids may be broken down into their constituents by acid hydrolysis which produces a mixture of nitrogenous bases, pentose sugars and inorganic phosphate.

The nitrogenous bases are planar aromatic heterocyclic ring compounds which absorb ultraviolet light. They are of two different types, derived from pyrimidine and purine respectively.

The structure of pyrimidine is shown below. It is a six-membered unsaturated ring compound containing four carbon and two nitrogen atoms.

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Three different substituted pyrimidines are found in the nucleic acids, namely cytosine (2-oxy-4-aminopyrimidine) which is present in both DNA and RNA, uracil (2,4-dioxypyrimidine) which is present in RNA but not in DNA and thymine (5-methyluracil) which is present in DNA but not in RNA. They all exhibit keto-enol tautomerism.

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Purine has a double ring structure in which one of the rings is similar to that of pyrimidine (although it is numbered differently!) and shares two of its carbon atoms with the second five-membered ring which also contains two nitrogen atoms and is similar to the imidazole ring of histidine.

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DNA and RNA both contain the two substituted purines adenine (6-aminopurine) and guanine (2-amino-6-oxypurine). Small amounts of other bases including hypoxanthine and a variety of methylated purines and pyrimidines are found in certain types of RNA (page 299).

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The pentose sugars obtained by complete hydrolysis of the nucleic acids are of two types. The pentose derived from RNA is D-ribose and that from DNA is D-2-deoxyribose. When present in nucleotide combination they are found in the furanose form, i.e. as D-ribofuranose and D-2-deoxyribofuranose respectively.

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Phosphoric acid is released on acid hydrolysis of both DNA and RNA.

Nucleosides and nucleotides

Compounds in which a purine or pyrimidine base is combined with a sugar are known as nucleosides and addition of a phosphoric acid moiety converts them into nucleotides. Deoxyribonucleosides and deoxyribonucleotides are of less general occurrence in the body than the corresponding ribose derivatives and in order to distinguish them it is usual to prefix the normal abbreviation for the ribonucleosides and ribonucleotides with d. Thus dA, dG, dC and dT specify the deoxyribonucleosides of adenine, guanine, cytosine and thymine and dAMP, dGMP, dCMP and TMP the corresponding deoxyribonucleotides. (No prefix is necessary to TMP because it does not exist in the ribonucleotide form.) A list of the most common nucleosides and nucleotides is given in Table 9.1.

Table 9.1

Some nitrogenous bases and their derivatives

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Nitrogenous base Type Substituents Nucleoside Nucleotide
Adenine Purine 6-amino Adenosine Adenylic acid
Guanine Purine 2-amino-6-hydroxy Guanosine Guanylic acid
Hypoxanthine Purine 6-hydroxy Inosine Inosinic acid
Cytosine Pyrimidine 2-hydroxy-4-amino Cytidine Cytidylic acid
Thymine Pyrimidine 2,4-dihydroxy-5-methyl Thymidine Thymidylic acid
Uracil

Dec 10, 2015 | Posted by in General Dentistry | Comments Off on Nucleotides and nucleic acids
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