DNA is the single most important molecule found within cells. It is a stable polynucleotide, which contains coded information for inherited characteristics. It is contained in chromosomes in the nucleus of an eukaryot cell. The essential features of the Watson-Crick model are summarised below. 1. The two helical polynucleotide chains are coiled around a common axis. The two chains have opposite polarity i.e. they are antiparrallel. 2. The regular repeating sugar phosphate backbone of each strand lies on the outside of the helix. The purine and pyrimidine bases project inwards at 900 to the axis of the helix.