Definition of DNA
Each molecule of DNA consists of two strands coiled round each other to form a double helix. DNA consists of a pair of chemical groups called bases (there are four types G,C, A and T), which combine in specific pairs (G-C and A-T) so that the sequence on one strand of the double helix is complementary to that on the other: it is the specific sequence of bases which codes amino acids which are the building blocks of proteins.