Overview

To understand genes, one must first consider nucleic acid, for nucleic acid is the stuff that genes are made of. Inasmuch as function follows structure, a clear picture of nucleic acid will illuminate all genetic processes.

The genetic material of all life forms is nucleic acid, either deoxyribonucleic acid (DNA) or ribonucleic acid (RNA). DNA and RNA are linear chains made of subunits called nucleotides. Two strands of DNA typically associate in a two-stranded helix. This chapter describes the structure of nucleotides, the way nucleotides are connected to make chains, and a bit about the shapes of RNA and DNA, including some properties of double helices.

Polymers

Nucleic acids, the coding molecules of life, are linear polymers. A linear polymer is an unbranched chain made of many subunits connected by covalent chemical bonds; short polymers are known as oligomers (Figure 2.1). The subunit of a nucleic acid polymer is the nucleotide. Energy [typically about 400 kJ/mol] is required to make or to break a covalent bond between subunits of a polymer.

In this simplified representation of an RNA segment, the circles and triangles stand for nucleotides and each line stands for a covalent bond joining adjacent nucleotides. A = adenosine, G = guanidine, U = uridine, and C = cytidine.

Nucleotides

Every nucleotide has three parts: sugar, nitrogenous base, and one to three phosphates; the subunits of nucleic acid are monophosphates. A nucleoside is a nucleotide minus the phosphate (nucleoside = sugar + nitrogenous base) (Figure 2.2).