Life depends upon a complex chemical system to maintain itself. The chemical processes occurring in an organism take place at normal temperatures and pressures because of a system of protein catalysts called enzymes. Protein molecules are made up of a string of amino acids and they fold into their precise three-dimensional shape because of the attraction and repulsion occurring between nearby amino acids, or the water molecules around them. To produce a protein which will act as a catalyst, the string of amino acids must fold into a precise shape, which will allow it to bond with the chemicals it is causing to react. The place where the substrate or substrates, combine with the enzyme is called the active site. The formation of this active site, requires that there is a mechanism instructing the chemical systems in the cell to place the amino acids in the correct positions as the protein is being constructed. The instruction system is in fact made up of Nucleic Acids. The main types of nucleic acid are Deoxyribose Nucleic Acid (DNA) and Ribose Nucleic Acid (RNA). The latter occurs in three forms, messenger RNA (mRNA), transfer RNA (tRNA), and ribosomal RNA (rRNA).

The basic components of a nucleic acid are a Pentose sugar, either ribose or deoxyribose, phosphate groups, and nucleotide bases. There are five nucleotide bases Adenine, Thymine, Guanine, Cytosine and Uracil.

In DNA, a backbone of deoxyribose sugar monomers is built up by connecting the sugar molecules using phosphate groups between carbons 5 and 3. Connected to carbon 1 of each of the sugar molecules we find one of four nucleotide bases, A, T, G or C. The DNA molecule shown in the diagram above is a double a-helix formed from two molecules spiralling around each other and held in place by the hydrogen bonds (electrical attractions) between the pairs of bases. The base pairs A - T, and C - G attract each other because they have similar hydrogen bonding sites. A and T have two sites, while G and C have three sites.

DNA molecules are polynucleotides and can be very long. They are macromolecules. In humans they reach a length of approximately one metre, fortunately they are very thin and so the 46 DNA molecules in each of our cells can be folded into the tiny space inside the nucleus. The 46 human chromosomes contain about 3 billion bases. The bases form a code; every group of three bases represents an amino acid, or a start or stop instruction. So to read a chromosome, you find a start sequence, then each triplet of bases after this tells us which amino acids to place into the sequence and which order they are in. The DNA is always read from the 5' end down towards the 3' end, and can only be read when the two halves of the double spiral are moved apart to expose the bases.

RNA molecules are made up of ribose sugars, connected by phosphate groups, and they have the four bases, A - U and C - G, notice that thymine is replaced in RNA by the base Uracil. Unlike DNA, the RNA molecule is single stranded and may be linear, or folded in shape depending upon its type.