ABSTRACT
In DNA there are four bases: adenine (abbreviated A), guanine (G), thymine (T) and cytosine (C). Adenine and guanine are purines; thymine and cytosine are pyrimidines. In a DNA molecule, the different nucleotides are covalently joined to form a long polymer chain by covalent bonding between the phosphates and sugars. In 1953, J. D. Watson and Crick worked out the three-dimensional structure of DNA, starting from X-ray diffraction photographs taken by Franklin and Wilkins. They deduced that DNA is composed of two strands wound round each other to form a double helix, with the bases on the inside and the sugar–phosphate backbones on the outside. The DNA sequence is the sequence of A, C, G and T along the DNA molecule which carries the genetic information. Since the deoxynucleotides in DNA differ only in the bases they carry, this sequence of deoxynucleotides can be recorded simply as a base sequence.
