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Chapter: 11th Biochemistry : Nucleic Acids

Composition of Nucleic acids

Nucleotides have three characteristic components (i) Base (ii) Sugar (iii) Phosphate group.

Composition

 

Nucleic acids are very long, thread-like polymers, made up of a linear array of monomers called nucleotides held together by phosphodiester bridges. Nucleotides have three characteristic components (i) Base (ii) Sugar (iii) Phosphate group.

 

1. Common Bases of Nucleic acids

 

The bases of nucleic acids are heterocyclic, containing aromatic ring in their structures. They may be monocyclic pyrimidines or bicyclic purines.

 

i. Pyrimidine bases

 

Pyrimidinesare six-membered heterocyclic aromatic rings containing two nitrogen atoms (Figure 7.1) Pyrimidine ring is numbered in the clock-wise fashion. The common naturally occurring pyrimidines are cytosine, uracil, and thymine (5-methyluracil) (Figure 7.2). Cytosine and thymine are the pyrimidines typically found in DNA, whereas cytosine and uracil are common in RNA.


 

Properties of Pyrimidine bases

 

·     Pyrimidines are basic in nature.  They are less soluble in water.

 

·     They absorb UV light at 260 nm. This property is used to detect and estimate DNA and RNA in biological solutions.

 

·     They are capable of forming hydrogen bonds with purine nucleotides in nucleic acids.

 

·     They exhibit keto–enol tautomerism.

 

ii. Purine bases

 

Purine is a bicyclic ring formed by fusion of pyrimidine ring with imidazole ring. Purine ring is numbered in the anti-clockwise fashion. Adenine (6-amino purine) and guanine (2-amino-6-oxy purine), the two common purines, are found in both DNA and RNA (Figure 7.2 ). Other naturally occurring purine derivatives include hypoxanthine, xanthine, and uric acid. Hypoxanthine and xanthine are found only rarely as constituents of RNA, while they are intermediates (compounds formed in the synthetic pathway) in the synthesis of nucleic acids Uric acid is the catabolic end product of nucleic acids.


 

Properties of Purine bases

 

·     Purines are basic in nature. They are sparingly soluble in water.

 

·     They also absorb UV light at 260 nm. This property is used to detect and estimate DNA and RNA in biological solutions.

 

·     They are capable of forming hydrogen bonds with pyrimidine nucleotides in nucleic acids.

 

·     They exhibit keto –enol tautomerism.

 

2. Sugars

 

There are two types of sugars present in nucleic acids. They are ribose and deoxyribose (Fig.7.3). Based on the sugar moiety present in the nucleic acids, they are classified as Deoxy ribonucleic acid (DNA) and Ribonucleic acid (RNA). DNA is present in the nucleus, mitochondria and chloroplasts. RNA is present in the nucleus, nucleolus, ribosome and cytoplasm. Ribose and deoxy ribose have differences in their properties. Deoxy ribose is less reactive in nature, when compared to ribose.


 

3. Phosphate

 

Phosphoric acid forms phospho-diester linkage between nucleosides. Based on the number of phosphate group present in the nucleotide, they are classified as monophosphates, diphosphates and triphosphates.


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