DNA is a double stranded helix. Each strand is held together by complementary base pairs or nucleotides. A nucleotide consists of a base, a deoxyribose sugar and a phosphate. The bases are A, T, C and G. G and C are a complementary base pair and A and T form the other. These pairs are held together by hydrogen bonds. A sugar-phosphate backbone is also formed by bonding between the deoxyribose of one nucleotide and the phosphate of the next nucleotide. In a eukaryotic cell, DNA is found in linear chromosomes within the nucleus. Small sections of DNA within these chromosomes are called genes and code for proteins.