In DNA there are four types of nucleotides that differ by the nitrogen base they have: adenine (A), guanine (G), cytosine (C) and thymine (T). These molecules are placed one behind the other and form a very long chain. DNA molecule is formed by two long polynucleotide chains, the two chains are complementary but not identical, that is, the adenine nucleotides of one chain pair with the thymine nucleotides of the other chain, that is, the adenine bases found in a strand interact with the thymine bases of the opposite strand, thus forming the double helix of the DNA.
