Nucleic Acid Metabolism

Nucleic Acid Metabolism

Introduction

Nucleic acids are the chemical basis of life and heredity. They serve as transmitters of genetic information. As the name implies, their location is mainly in nuclei. However, it is also found to be present in other intracellular organelles. Nucleic acids are present both in the free state as well as conjucated with proteins (Nucleoproteins). Like amino acids in proteins, nucleotides are the basic units of nucleic acids. There are two types of nucleic acids namely,

1.           Ribonucleic acid

2.           Deoxyribonucleic acid

Structural Components of Nucleic acids

Phosphoric acid

The molecular formula of phosphoric acid is H₃PO₄. It contains 3 monovalent hydroxyl groups and a divalent oxygen atom, all linked to a pentavalent phosphorus atom.

Pentose Sugar

The two types of nucleic acids are distinguished primarily on the basis of the 5 carbon sugar pentose which they possess. One possesses D-2-deoxyribose, (deoxyribonucleic acid) while the other contains D-ribose (hence called ribonucleic acid). Both these sugars in nucleic acids are present in the furanose form and are of β configuration.

Nitrogenous Bases

Two types of nitrogenous bases are found in all nucleic acids. These are derivatives of purine and pyrimidine.

i. Purine Bases

These are all derived from their parent compound purine, which contains a six membered pyrimidine ring fused to the 5 membered imidazole ring, the purine derivatives found in nucleic acids are adenine and guanine.

ii. Pyrimidine Bases

These are all derived from their parent heterocyclic compound pyrimidine. The common pyrimidine derivatives found in nucleic acids are Uracil, Thymine and Cytosine.

Minor bases in nucleic acid

Apart from the above four bases, certain minor, unusual bases are also found in DNA and RNA. They are 5 methylcytosine, N4 acetyl cytosine, N6 methyladenine, N6, N6 dimethyladenine and pseudouracil etc.,

Base Pairing

Base pairing is an essential feature not only to maintain the double helical structure of DNA, but also plays an important role in DNA, RNA and protein biosynthesis.

In DNA

Adenine (A) pairs with thymine (T)           (A=T)

Guanine (G) pairs with cytosine (C)           (G≡C)

In RNA

Adenine (A) pairs with uracil (U)               (A=U)

Guanine (G) pairs with cytosine (C)          (G≡C)

Chargaffs rule of DNA composition

DNA has equal number of adenine and thymine residues (A=T) and equal number of guanine and cytosine (G≡C) residues. This is known as Chargaffs rule of molar equivalence between purine and pyrimidines in DNA.

Structure of Nucleotides

Nucleotides are the fundamental units of nucleic acids. Each nucleotide is comprising of a

i)             Phosphate group ii) Pentose Sugar and iii) Nitrogenous base.

Sugar + Base ------------>  Nucleoside

Nucleoside + Phosphate ----------->Nucleotide