A nucleotide is the basic structural unit and building block for DNA. The nucleotides are hooked together to form a chain of DNA (single strand). Each nucleotide has a fixed part (a five-sided sugar and a phosphate group) and a base (containing nitrogen) which is different in every nucleotide type. In fact in DNA there are 4 bases: Adenine (A), Cytosine (C), Guanine (G) and Thymine (T); and consequently 4 types of nucleotides. Their sequence along the DNA single strand is responsible to store the information in DNA, thanks to a code based on 4 elements.
Two chains of DNA are held together into a double helix (double strand) by chemical bonds between the bases, forming a base pair. Only a few bases are compatible (complementary) allowing the formation of only two couples (CG and AT).