It is a bit like an incredibly long twisted rope ladder. Each of these bases pairs only with one other. This means that the sequence of bases along one strand of the DNA is a perfect mirror image of the sequence on the other strand. So each can be used, like a mould, to make a copy of the other. These bases are a bit like letters of the alphabet, and the sequence is broken up into 'sentences' called genes that provide the instructions to make a particular protein .
Proteins are the basic material of all living cells.
Proteins are built up from different combinations of chemicals called amino acids . All these amino acids are on hand in each cell, so in order to make a certain protein, the DNA must instruct the cell to make the right combination of amino acids. These groups are called codons , and the sequence of the three bases spells out the name of a particular amino acid. Altogether, there are 64 different ways the four bases can be arranged in groups of three, so there must be 64 different codons. Since there are only 20 amino acids, some codons code for the same amino acid.