ABSTRACT
In a deoxyribonucleic acid (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. Deoxynucleoside triphosphates are the precursors for DNA synthesis. In DNA the nucleotides are covalently joined together by 3' 5' phosphodiester bonds to form a repetitive sugar–phosphate chain which is the backbone to which the bases are attached. DNA molecules are typically many thousands of nucleotides long so that the number of possible messages is enormous.
