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5 Primary Structure of Nucleic Acids

5.1 Introduction

Nucleic acids are high-molecular weight compounds - polycondensation products of nucleotides. The monomer units of a nucleic acid molecule usually are either only ribonucleotides or only deoxyribonucleotides. Depending on the type of the constituent monomer units, distinction is made between ribonucleic (RNA with ribonucleotides as the monomer units) and deoxyribonucleic (DNA with deoxyribonucleotides as the monomer units) acids.

Both polynucleotide types, DNA and RNA, are found in all living cellular organisms, except for viruses which contain polymers of one type only. The structural differences between the monomer units in polynucleotides of the two types account for their different three-dimensional structure (or macrostructure). As a consequence, the chemical properties of both polymers differ markedly.

DNA and RNA are also dissimilar biologically. They perform different functions and, consequently, occupy different positions in the cell. It is extremely important to know the structure of nucleic acids if the mechanism of their functioning is to be understood. A distraction is made between primary structure and macromolecular (dimensional) structure. By primary structure is meant the sequence of monomer units linked by covalent bonds into a continuous polynucleotide chain. In determining the primary structure of nucleic acids one is actually trying to establish the type of the intermonomer linkage and the sequence of monomer units in the polymer chain. The macromolecular structure of a nucleic acid is defined as the spatial arrangement of polynucleotide chains in the molecule.